Merge #6112 from ZyX-I/split-eval'/buf_get_changedtick

Better b:changedtick support

27 files changed, 689 insertions, 364 deletions

diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 1105601a0e..88ab502ab6 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -8606,9 +8606,12 @@ This does NOT work: >
 				:lockvar v
 				:let v = 'asdf'		" fails!
 				:unlet v
-<							*E741*
+<							*E741* *E940*
 			If you try to change a locked variable you get an
-			error message: "E741: Value is locked: {name}"
+			error message: "E741: Value is locked: {name}".
+			If you try to lock or unlock a built-in variable you 
+			will get an error message "E940: Cannot lock or unlock 
+			variable {name}".
 
 			[depth] is relevant when locking a |List| or
 			|Dictionary|.  It specifies how deep the locking goes:
diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index 738a5ae091..7f6e1093ed 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -411,10 +411,10 @@ end:
 
 /// Gets a buffer-scoped (b:) variable.
 ///
-/// @param buffer     Buffer handle
-/// @param name       Variable name
-/// @param[out] err   Error details, if any
-/// @return Variable value
+/// @param buffer The buffer handle
+/// @param name The variable name
+/// @param[out] err Details of an error that may have occurred
+/// @return The variable value
 Object nvim_buf_get_var(Buffer buffer, String name, Error *err)
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
@@ -426,6 +426,22 @@ Object nvim_buf_get_var(Buffer buffer, String name, Error *err)
   return dict_get_value(buf->b_vars, name, err);
 }
 
+/// Gets a changed tick of a buffer
+///
+/// @param[in]  buffer  The buffer handle.
+///
+/// @return `b:changedtick` value.
+Integer nvim_buf_get_changedtick(Buffer buffer, Error *err)
+{
+  const buf_T *const buf = find_buffer_by_handle(buffer, err);
+
+  if (!buf) {
+    return -1;
+  }
+
+  return buf->b_changedtick;
+}
+
 /// Sets a buffer-scoped (b:) variable
 ///
 /// @param buffer     Buffer handle
@@ -440,7 +456,7 @@ void nvim_buf_set_var(Buffer buffer, String name, Object value, Error *err)
     return;
   }
 
-  dict_set_value(buf->b_vars, name, value, false, false, err);
+  dict_set_var(buf->b_vars, name, value, false, false, err);
 }
 
 /// Removes a buffer-scoped (b:) variable
@@ -456,7 +472,7 @@ void nvim_buf_del_var(Buffer buffer, String name, Error *err)
     return;
   }
 
-  dict_set_value(buf->b_vars, name, NIL, true, false, err);
+  dict_set_var(buf->b_vars, name, NIL, true, false, err);
 }
 
 /// Sets a buffer-scoped (b:) variable
@@ -479,7 +495,7 @@ Object buffer_set_var(Buffer buffer, String name, Object value, Error *err)
     return (Object) OBJECT_INIT;
   }
 
-  return dict_set_value(buf->b_vars, name, value, false, true, err);
+  return dict_set_var(buf->b_vars, name, value, false, true, err);
 }
 
 /// Removes a buffer-scoped (b:) variable
@@ -498,7 +514,7 @@ Object buffer_del_var(Buffer buffer, String name, Error *err)
     return (Object) OBJECT_INIT;
   }
 
-  return dict_set_value(buf->b_vars, name, NIL, true, true, err);
+  return dict_set_var(buf->b_vars, name, NIL, true, true, err);
 }
 
 
diff --git a/src/nvim/api/private/helpers.c b/src/nvim/api/private/helpers.c
index ba4d005e9a..7efa086af2 100644
--- a/src/nvim/api/private/helpers.c
+++ b/src/nvim/api/private/helpers.c
@@ -98,7 +98,7 @@ Object dict_get_value(dict_T *dict, String key, Error *err)
   return vim_to_object(&di->di_tv);
 }
 
-/// Set a value in a dict. Objects are recursively expanded into their
+/// Set a value in a scope dict. Objects are recursively expanded into their
 /// vimscript equivalents.
 ///
 /// @param dict The vimscript dict
@@ -109,8 +109,8 @@ Object dict_get_value(dict_T *dict, String key, Error *err)
 /// @param retval If true the old value will be converted and returned.
 /// @param[out] err Details of an error that may have occurred
 /// @return The old value if `retval` is true and the key was present, else NIL
-Object dict_set_value(dict_T *dict, String key, Object value, bool del,
-                      bool retval, Error *err)
+Object dict_set_var(dict_T *dict, String key, Object value, bool del,
+                    bool retval, Error *err)
 {
   Object rv = OBJECT_INIT;
 
@@ -120,7 +120,7 @@ Object dict_set_value(dict_T *dict, String key, Object value, bool del,
   }
 
   if (key.size == 0) {
-    api_set_error(err, Validation, _("Empty dictionary keys aren't allowed"));
+    api_set_error(err, Validation, _("Empty variable names aren't allowed"));
     return rv;
   }
 
@@ -129,7 +129,20 @@ Object dict_set_value(dict_T *dict, String key, Object value, bool del,
     return rv;
   }
 
-  dictitem_T *di = dict_find(dict, (uint8_t *)key.data, (int)key.size);
+  dictitem_T *di = dict_find(dict, (char_u *)key.data, (int)key.size);
+
+  if (di != NULL) {
+    if (di->di_flags & DI_FLAGS_RO) {
+      api_set_error(err, Exception, _("Key is read-only: %s"), key.data);
+      return rv;
+    } else if (di->di_flags & DI_FLAGS_FIX) {
+      api_set_error(err, Exception, _("Key is fixed: %s"), key.data);
+      return rv;
+    } else if (di->di_flags & DI_FLAGS_LOCK) {
+      api_set_error(err, Exception, _("Key is locked: %s"), key.data);
+      return rv;
+    }
+  }
 
   if (del) {
     // Delete the key
diff --git a/src/nvim/api/tabpage.c b/src/nvim/api/tabpage.c
index 9e61ec1871..d24250d032 100644
--- a/src/nvim/api/tabpage.c
+++ b/src/nvim/api/tabpage.c
@@ -71,7 +71,7 @@ void nvim_tabpage_set_var(Tabpage tabpage,
     return;
   }
 
-  dict_set_value(tab->tp_vars, name, value, false, false, err);
+  dict_set_var(tab->tp_vars, name, value, false, false, err);
 }
 
 /// Removes a tab-scoped (t:) variable
@@ -87,7 +87,7 @@ void nvim_tabpage_del_var(Tabpage tabpage, String name, Error *err)
     return;
   }
 
-  dict_set_value(tab->tp_vars, name, NIL, true, false, err);
+  dict_set_var(tab->tp_vars, name, NIL, true, false, err);
 }
 
 /// Sets a tab-scoped (t:) variable
@@ -110,7 +110,7 @@ Object tabpage_set_var(Tabpage tabpage, String name, Object value, Error *err)
     return (Object) OBJECT_INIT;
   }
 
-  return dict_set_value(tab->tp_vars, name, value, false, true, err);
+  return dict_set_var(tab->tp_vars, name, value, false, true, err);
 }
 
 /// Removes a tab-scoped (t:) variable
@@ -129,7 +129,7 @@ Object tabpage_del_var(Tabpage tabpage, String name, Error *err)
     return (Object) OBJECT_INIT;
   }
 
-  return dict_set_value(tab->tp_vars, name, NIL, true, true, err);
+  return dict_set_var(tab->tp_vars, name, NIL, true, true, err);
 }
 
 /// Gets the current window in a tabpage
diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index fd0f98c017..bf11795d9e 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -368,7 +368,7 @@ Object nvim_get_var(String name, Error *err)
 /// @param[out] err Error details, if any
 void nvim_set_var(String name, Object value, Error *err)
 {
-  dict_set_value(&globvardict, name, value, false, false, err);
+  dict_set_var(&globvardict, name, value, false, false, err);
 }
 
 /// Removes a global (g:) variable
@@ -377,7 +377,7 @@ void nvim_set_var(String name, Object value, Error *err)
 /// @param[out] err Error details, if any
 void nvim_del_var(String name, Error *err)
 {
-  dict_set_value(&globvardict, name, NIL, true, false, err);
+  dict_set_var(&globvardict, name, NIL, true, false, err);
 }
 
 /// Sets a global variable
@@ -393,7 +393,7 @@ void nvim_del_var(String name, Error *err)
 ///                  or if previous value was `v:null`.
 Object vim_set_var(String name, Object value, Error *err)
 {
-  return dict_set_value(&globvardict, name, value, false, true, err);
+  return dict_set_var(&globvardict, name, value, false, true, err);
 }
 
 /// Removes a global variable
@@ -405,7 +405,7 @@ Object vim_set_var(String name, Object value, Error *err)
 /// @return Old value
 Object vim_del_var(String name, Error *err)
 {
-  return dict_set_value(&globvardict, name, NIL, true, true, err);
+  return dict_set_var(&globvardict, name, NIL, true, true, err);
 }
 
 /// Gets a v: variable
diff --git a/src/nvim/api/window.c b/src/nvim/api/window.c
index 1f555a6a05..13294e6bf2 100644
--- a/src/nvim/api/window.c
+++ b/src/nvim/api/window.c
@@ -210,7 +210,7 @@ void nvim_win_set_var(Window window, String name, Object value, Error *err)
     return;
   }
 
-  dict_set_value(win->w_vars, name, value, false, false, err);
+  dict_set_var(win->w_vars, name, value, false, false, err);
 }
 
 /// Removes a window-scoped (w:) variable
@@ -226,7 +226,7 @@ void nvim_win_del_var(Window window, String name, Error *err)
     return;
   }
 
-  dict_set_value(win->w_vars, name, NIL, true, false, err);
+  dict_set_var(win->w_vars, name, NIL, true, false, err);
 }
 
 /// Sets a window-scoped (w:) variable
@@ -249,7 +249,7 @@ Object window_set_var(Window window, String name, Object value, Error *err)
     return (Object) OBJECT_INIT;
   }
 
-  return dict_set_value(win->w_vars, name, value, false, true, err);
+  return dict_set_var(win->w_vars, name, value, false, true, err);
 }
 
 /// Removes a window-scoped (w:) variable
@@ -268,7 +268,7 @@ Object window_del_var(Window window, String name, Error *err)
     return (Object) OBJECT_INIT;
   }
 
-  return dict_set_value(win->w_vars, name, NIL, true, true, err);
+  return dict_set_var(win->w_vars, name, NIL, true, true, err);
 }
 
 /// Gets a window option value
diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index a471ebf06f..083ed43047 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -686,8 +686,18 @@ free_buffer_stuff (
     free_buf_options(buf, true);
     ga_clear(&buf->b_s.b_langp);
   }
+  {
+    // Avoid loosing b:changedtick when deleting buffer: clearing variables
+    // implies using clear_tv() on b:changedtick and that sets changedtick to
+    // zero.
+    hashitem_T *const changedtick_hi = hash_find(
+        &buf->b_vars->dv_hashtab, (const char_u *)"changedtick");
+    assert(changedtick_hi != NULL);
+    hash_remove(&buf->b_vars->dv_hashtab, changedtick_hi);
+  }
   vars_clear(&buf->b_vars->dv_hashtab);   // free all internal variables
   hash_init(&buf->b_vars->dv_hashtab);
+  buf_init_changedtick(buf);
   uc_clear(&buf->b_ucmds);              // clear local user commands
   buf_delete_signs(buf);                // delete any signs
   bufhl_clear_all(buf);                // delete any highligts
@@ -1436,6 +1446,26 @@ void do_autochdir(void)
 
 static int top_file_num = 1;            ///< highest file number
 
+/// Initialize b:changedtick and changedtick_val attribute
+///
+/// @param[out]  buf  Buffer to intialize for.
+static inline void buf_init_changedtick(buf_T *const buf)
+  FUNC_ATTR_ALWAYS_INLINE FUNC_ATTR_NONNULL_ALL
+{
+  STATIC_ASSERT(sizeof("changedtick") <= sizeof(buf->changedtick_di.di_key),
+                "buf->changedtick_di cannot hold large enough keys");
+  buf->changedtick_di = (dictitem16_T) {
+    .di_flags = DI_FLAGS_RO|DI_FLAGS_FIX,  // Must not include DI_FLAGS_ALLOC.
+    .di_tv = (typval_T) {
+      .v_type = VAR_NUMBER,
+      .v_lock = VAR_FIXED,
+      .vval.v_number = buf->b_changedtick,
+    },
+    .di_key = "changedtick",
+  };
+  dict_add(buf->b_vars, (dictitem_T *)&buf->changedtick_di);
+}
+
 /// Add a file name to the buffer list.
 /// If the same file name already exists return a pointer to that buffer.
 /// If it does not exist, or if fname == NULL, a new entry is created.
@@ -1530,6 +1560,7 @@ buf_T * buflist_new(char_u *ffname, char_u *sfname, linenr_T lnum, int flags)
     // init b: variables
     buf->b_vars = dict_alloc();
     init_var_dict(buf->b_vars, &buf->b_bufvar, VAR_SCOPE);
+    buf_init_changedtick(buf);
   }
 
   if (ffname != NULL) {
diff --git a/src/nvim/buffer.h b/src/nvim/buffer.h
index 193c9f73d8..ed3e6ab6cc 100644
--- a/src/nvim/buffer.h
+++ b/src/nvim/buffer.h
@@ -5,6 +5,8 @@
 #include "nvim/pos.h"  // for linenr_T
 #include "nvim/ex_cmds_defs.h"  // for exarg_T
 #include "nvim/screen.h"  // for StlClickRecord
+#include "nvim/func_attr.h"
+#include "nvim/eval.h"
 
 // Values for buflist_getfile()
 enum getf_values {
@@ -79,6 +81,31 @@ static inline void restore_win_for_buf(win_T *save_curwin,
   }
 }
 
+static inline void buf_set_changedtick(buf_T *const buf, const int changedtick)
+  REAL_FATTR_NONNULL_ALL REAL_FATTR_ALWAYS_INLINE;
+
+/// Set b_changedtick and corresponding variable
+///
+/// @param[out]  buf  Buffer to set changedtick in.
+/// @param[in]  changedtick  New value.
+static inline void buf_set_changedtick(buf_T *const buf, const int changedtick)
+{
+#ifndef NDEBUG
+  dictitem_T *const changedtick_di = dict_find(
+      buf->b_vars, (char_u *)"changedtick", sizeof("changedtick") - 1);
+  assert(changedtick_di != NULL);
+  assert(changedtick_di->di_tv.v_type == VAR_NUMBER);
+  assert(changedtick_di->di_tv.v_lock == VAR_FIXED);
+  // For some reason formatc does not like the below.
+# ifndef UNIT_TESTING_LUA_PREPROCESSING
+  assert(changedtick_di->di_flags == (DI_FLAGS_RO|DI_FLAGS_FIX));
+# endif
+  assert(changedtick_di == (dictitem_T *)&buf->changedtick_di);
+  assert(&buf->b_changedtick == &buf->changedtick_di.di_tv.vval.v_number);
+#endif
+  buf->b_changedtick = changedtick;
+}
+
 #define WITH_BUFFER(b, code) \
   do { \
     win_T *save_curwin = NULL; \
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index ccdab16ca1..a1b5633c32 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -489,7 +489,9 @@ struct file_buffer {
 
   int b_changed;                // 'modified': Set to true if something in the
                                 // file has been changed and not written out.
-  int b_changedtick;            // incremented for each change, also for undo
+/// Change identifier incremented for each change, including undo
+#define b_changedtick changedtick_di.di_tv.vval.v_number
+  dictitem16_T changedtick_di;  // b:changedtick dictionary item.
 
   bool b_saving;                /* Set to true if we are in the middle of
                                    saving the buffer. */
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index a582c56208..942a82a040 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -171,6 +171,8 @@ static char *e_letwrong = N_("E734: Wrong variable type for %s=");
 static char *e_nofunc = N_("E130: Unknown function: %s");
 static char *e_illvar = N_("E461: Illegal variable name: %s");
 static char *e_float_as_string = N_("E806: using Float as a String");
+static const char *e_readonlyvar = N_(
+    "E46: Cannot change read-only variable \"%.*s\"");
 
 static char_u * const empty_string = (char_u *)"";
 static char_u * const namespace_char = (char_u *)"abglstvw";
@@ -201,15 +203,22 @@ static garray_T ga_scripts = {0, 0, sizeof(scriptvar_T *), 4, NULL};
 
 static int echo_attr = 0;   /* attributes used for ":echo" */
 
-// Values for trans_function_name() argument:
-#define TFN_INT         1       // internal function name OK
-#define TFN_QUIET       2       // no error messages
-#define TFN_NO_AUTOLOAD 4       // do not use script autoloading
-#define TFN_NO_DEREF    8       // do not dereference a Funcref
-
-// Values for get_lval() flags argument:
-#define GLV_QUIET       TFN_QUIET        // no error messages
-#define GLV_NO_AUTOLOAD TFN_NO_AUTOLOAD  // do not use script autoloading
+/// trans_function_name() flags
+typedef enum {
+  TFN_INT = 1,  ///< May use internal function name
+  TFN_QUIET = 2,  ///< Do not emit error messages.
+  TFN_NO_AUTOLOAD = 4,  ///< Do not use script autoloading.
+  TFN_NO_DEREF = 8,  ///< Do not dereference a Funcref.
+  TFN_READ_ONLY = 16,  ///< Will not change the variable.
+} TransFunctionNameFlags;
+
+/// get_lval() flags
+typedef enum {
+  GLV_QUIET = TFN_QUIET,  ///< Do not emit error messages.
+  GLV_NO_AUTOLOAD = TFN_NO_AUTOLOAD,  ///< Do not use script autoloading.
+  GLV_READ_ONLY = TFN_READ_ONLY,  ///< Indicates that caller will not change
+                                  ///< the value (prevents error message).
+} GetLvalFlags;
 
 // function flags
 #define FC_ABORT    0x01          // abort function on error
@@ -541,8 +550,8 @@ void eval_init(void)
     list_T *const type_list = list_alloc();
     type_list->lv_lock = VAR_FIXED;
     type_list->lv_refcount = 1;
-    dictitem_T *const di = dictitem_alloc((char_u *) msgpack_type_names[i]);
-    di->di_flags = DI_FLAGS_RO | DI_FLAGS_FIX;
+    dictitem_T *const di = dictitem_alloc((char_u *)msgpack_type_names[i]);
+    di->di_flags |= DI_FLAGS_RO|DI_FLAGS_FIX;
     di->di_tv = (typval_T) {
       .v_type = VAR_LIST,
       .vval = { .v_list = type_list, },
@@ -1646,13 +1655,7 @@ static void list_glob_vars(int *first)
  */
 static void list_buf_vars(int *first)
 {
-  char numbuf[NUMBUFLEN];
-
   list_hashtable_vars(&curbuf->b_vars->dv_hashtab, "b:", true, first);
-
-  snprintf(numbuf, sizeof(numbuf), "%d", curbuf->b_changedtick);
-  list_one_var_a("b:", "changedtick", sizeof("changedtick") - 1, VAR_NUMBER,
-                 numbuf, first);
 }
 
 /*
@@ -1949,46 +1952,33 @@ ex_let_one (
   return arg_end;
 }
 
-/*
- * If "arg" is equal to "b:changedtick" give an error and return TRUE.
- */
-static int check_changedtick(char_u *arg)
-{
-  if (STRNCMP(arg, "b:changedtick", 13) == 0 && !eval_isnamec(arg[13])) {
-    EMSG2(_(e_readonlyvar), arg);
-    return TRUE;
-  }
-  return FALSE;
-}
-
-/*
- * Get an lval: variable, Dict item or List item that can be assigned a value
- * to: "name", "na{me}", "name[expr]", "name[expr:expr]", "name[expr][expr]",
- * "name.key", "name.key[expr]" etc.
- * Indexing only works if "name" is an existing List or Dictionary.
- * "name" points to the start of the name.
- * If "rettv" is not NULL it points to the value to be assigned.
- * "unlet" is TRUE for ":unlet": slightly different behavior when something is
- * wrong; must end in space or cmd separator.
- *
- * flags:
- *  GLV_QUIET:       do not give error messages
- *  GLV_NO_AUTOLOAD: do not use script autoloading
- *
- * Returns a pointer to just after the name, including indexes.
- * When an evaluation error occurs "lp->ll_name" is NULL;
- * Returns NULL for a parsing error.  Still need to free items in "lp"!
- */
-static char_u *
-get_lval (
-    char_u *name,
-    typval_T *rettv,
-    lval_T *lp,
-    int unlet,
-    int skip,
-    int flags,                  /* GLV_ values */
-    int fne_flags              /* flags for find_name_end() */
-)
+/// Get an lvalue
+///
+/// Lvalue may be
+/// - variable: "name", "na{me}"
+/// - dictionary item: "dict.key", "dict['key']"
+/// - list item: "list[expr]"
+/// - list slice: "list[expr:expr]"
+///
+/// Indexing only works if trying to use it with an existing List or Dictionary.
+///
+/// @param[in]  name  Name to parse.
+/// @param  rettv  Pointer to the value to be assigned or NULL.
+/// @param[out]  lp  Lvalue definition. When evaluation errors occur `->ll_name`
+///                  is NULL.
+/// @param[in]  unlet  True if using `:unlet`. This results in slightly
+///                    different behaviour when something is wrong; must end in
+///                    space or cmd separator.
+/// @param[in]  skip  True when skipping.
+/// @param[in]  flags  @see GetLvalFlags.
+/// @param[in]  fne_flags  Flags for find_name_end().
+///
+/// @return A pointer to just after the name, including indexes. Returns NULL
+///         for a parsing error, but it is still needed to free items in lp.
+static char_u *get_lval(char_u *const name, typval_T *const rettv,
+                        lval_T *const lp, const bool unlet, const bool skip,
+                        const int flags, const int fne_flags)
+  FUNC_ATTR_NONNULL_ARG(1, 3)
 {
   char_u      *p;
   char_u      *expr_start, *expr_end;
@@ -2207,8 +2197,13 @@ get_lval (
         if (len == -1)
           clear_tv(&var1);
         break;
-      } else if (var_check_ro(lp->ll_di->di_flags, name, false)) {
-        // existing variable, need to check if it can be changed
+      // existing variable, need to check if it can be changed
+      } else if (!(flags & GLV_READ_ONLY) && var_check_ro(lp->ll_di->di_flags,
+                                                          (const char *)name,
+                                                          (size_t)(p - name))) {
+        if (len == -1) {
+          clear_tv(&var1);
+        }
         return NULL;
       }
 
@@ -2299,32 +2294,33 @@ static void set_var_lval(lval_T *lp, char_u *endp, typval_T *rettv, int copy, ch
   dictitem_T  *di;
 
   if (lp->ll_tv == NULL) {
-    if (!check_changedtick(lp->ll_name)) {
-      cc = *endp;
-      *endp = NUL;
-      if (op != NULL && *op != '=') {
-        typval_T tv;
-
-        // handle +=, -= and .=
-        di = NULL;
-        if (get_var_tv((const char *)lp->ll_name, (int)STRLEN(lp->ll_name),
-                       &tv, &di, true, false) == OK) {
-          if ((di == NULL
-               || (!var_check_ro(di->di_flags, lp->ll_name, false)
-                   && !tv_check_lock(di->di_tv.v_lock, lp->ll_name, false)))
-              && tv_op(&tv, rettv, op) == OK) {
-            set_var(lp->ll_name, &tv, false);
-          }
-          clear_tv(&tv);
+    cc = *endp;
+    *endp = NUL;
+    if (op != NULL && *op != '=') {
+      typval_T tv;
+
+      // handle +=, -= and .=
+      di = NULL;
+      if (get_var_tv((const char *)lp->ll_name, (int)STRLEN(lp->ll_name),
+                     &tv, &di, true, false) == OK) {
+        if ((di == NULL
+             || (!var_check_ro(di->di_flags, (const char *)lp->ll_name,
+                               STRLEN(lp->ll_name))
+                 && !tv_check_lock(di->di_tv.v_lock, (const char *)lp->ll_name,
+                                   STRLEN(lp->ll_name))))
+            && tv_op(&tv, rettv, op) == OK) {
+          set_var(lp->ll_name, &tv, false);
         }
-      } else
-        set_var(lp->ll_name, rettv, copy);
-      *endp = cc;
+        clear_tv(&tv);
+      }
+    } else {
+      set_var(lp->ll_name, rettv, copy);
     }
+    *endp = cc;
   } else if (tv_check_lock(lp->ll_newkey == NULL
                            ? lp->ll_tv->v_lock
                            : lp->ll_tv->vval.v_dict->dv_lock,
-                           lp->ll_name, false)) {
+                           (const char *)lp->ll_name, STRLEN(lp->ll_name))) {
   } else if (lp->ll_range) {
     listitem_T *ll_li = lp->ll_li;
     int ll_n1 = lp->ll_n1;
@@ -2332,7 +2328,8 @@ static void set_var_lval(lval_T *lp, char_u *endp, typval_T *rettv, int copy, ch
     // Check whether any of the list items is locked
     for (listitem_T *ri = rettv->vval.v_list->lv_first;
          ri != NULL && ll_li != NULL; ) {
-      if (tv_check_lock(ll_li->li_tv.v_lock, lp->ll_name, false)) {
+      if (tv_check_lock(ll_li->li_tv.v_lock, (const char *)lp->ll_name,
+                        STRLEN(lp->ll_name))) {
         return;
       }
       ri = ri->li_next;
@@ -2938,16 +2935,18 @@ static int do_unlet_var(lval_T *lp, char_u *name_end, int forceit)
     cc = *name_end;
     *name_end = NUL;
 
-    /* Normal name or expanded name. */
-    if (check_changedtick(lp->ll_name))
-      ret = FAIL;
-    else if (do_unlet(lp->ll_name, forceit) == FAIL)
+    // Normal name or expanded name.
+    if (do_unlet(lp->ll_name, forceit) == FAIL) {
       ret = FAIL;
+    }
     *name_end = cc;
   } else if ((lp->ll_list != NULL
-              && tv_check_lock(lp->ll_list->lv_lock, lp->ll_name, false))
+              && tv_check_lock(lp->ll_list->lv_lock, (const char *)lp->ll_name,
+                               STRLEN(lp->ll_name)))
              || (lp->ll_dict != NULL
-                 && tv_check_lock(lp->ll_dict->dv_lock, lp->ll_name, false))) {
+                 && tv_check_lock(lp->ll_dict->dv_lock,
+                                  (const char *)lp->ll_name,
+                                  STRLEN(lp->ll_name)))) {
     return FAIL;
   } else if (lp->ll_range) {
     listitem_T    *li;
@@ -2956,7 +2955,8 @@ static int do_unlet_var(lval_T *lp, char_u *name_end, int forceit)
 
     while (ll_li != NULL && (lp->ll_empty2 || lp->ll_n2 >= ll_n1)) {
       li = ll_li->li_next;
-      if (tv_check_lock(ll_li->li_tv.v_lock, lp->ll_name, false)) {
+      if (tv_check_lock(ll_li->li_tv.v_lock, (const char *)lp->ll_name,
+                        STRLEN(lp->ll_name))) {
         return false;
       }
       ll_li = li;
@@ -3038,13 +3038,14 @@ int do_unlet(char_u *name, int forceit)
     }
     if (hi != NULL && !HASHITEM_EMPTY(hi)) {
       di = HI2DI(hi);
-      if (var_check_fixed(di->di_flags, name, false)
-          || var_check_ro(di->di_flags, name, false)
-          || tv_check_lock(d->dv_lock, name, false)) {
+      if (var_check_fixed(di->di_flags, (const char *)name, STRLEN(name))
+          || var_check_ro(di->di_flags, (const char *)name, STRLEN(name))
+          || tv_check_lock(d->dv_lock, (const char *)name, STRLEN(name))) {
         return FAIL;
       }
 
-      if (d == NULL || tv_check_lock(d->dv_lock, name, false)) {
+      if (d == NULL
+          || tv_check_lock(d->dv_lock, (const char *)name, STRLEN(name))) {
         return FAIL;
       }
 
@@ -3078,33 +3079,33 @@ int do_unlet(char_u *name, int forceit)
 static int do_lock_var(lval_T *lp, char_u *name_end, int deep, int lock)
 {
   int ret = OK;
-  int cc;
-  dictitem_T  *di;
 
-  if (deep == 0)        /* nothing to do */
+  if (deep == 0) {  // Nothing to do.
     return OK;
+  }
 
   if (lp->ll_tv == NULL) {
-    cc = *name_end;
-    *name_end = NUL;
-
     // Normal name or expanded name.
-    if (check_changedtick(lp->ll_name)) {
+    const size_t name_len = (size_t)(name_end - lp->ll_name);
+    dictitem_T *const di = find_var(
+        (const char *)lp->ll_name, name_len, NULL,
+        true);
+    if (di == NULL) {
       ret = FAIL;
+    } else if ((di->di_flags & DI_FLAGS_FIX)
+               && di->di_tv.v_type != VAR_DICT
+               && di->di_tv.v_type != VAR_LIST) {
+      // For historical reasons this error is not given for Lists and
+      // Dictionaries. E.g. b: dictionary may be locked/unlocked.
+      emsgf(_("E940: Cannot lock or unlock variable %s"), lp->ll_name);
     } else {
-      di = find_var((const char *)lp->ll_name, STRLEN(lp->ll_name), NULL, true);
-      if (di == NULL) {
-        ret = FAIL;
+      if (lock) {
+        di->di_flags |= DI_FLAGS_LOCK;
       } else {
-        if (lock) {
-          di->di_flags |= DI_FLAGS_LOCK;
-        } else {
-          di->di_flags &= ~DI_FLAGS_LOCK;
-        }
-        item_lock(&di->di_tv, deep, lock);
+        di->di_flags &= ~DI_FLAGS_LOCK;
       }
+      item_lock(&di->di_tv, deep, lock);
     }
-    *name_end = cc;
   } else if (lp->ll_range) {
     listitem_T    *li = lp->ll_li;
 
@@ -3131,68 +3132,72 @@ static int do_lock_var(lval_T *lp, char_u *name_end, int deep, int lock)
 static void item_lock(typval_T *tv, int deep, int lock)
 {
   static int recurse = 0;
-  list_T      *l;
-  listitem_T  *li;
-  dict_T      *d;
-  hashitem_T  *hi;
-  int todo;
 
   if (recurse >= DICT_MAXNEST) {
     EMSG(_("E743: variable nested too deep for (un)lock"));
     return;
   }
-  if (deep == 0)
+  if (deep == 0) {
     return;
-  ++recurse;
+  }
+  recurse++;
 
-  /* lock/unlock the item itself */
-  if (lock)
-    tv->v_lock |= VAR_LOCKED;
-  else
-    tv->v_lock &= ~VAR_LOCKED;
+  // lock/unlock the item itself
+#define CHANGE_LOCK(var, lock) \
+  do { \
+    var = ((VarLockStatus[]) { \
+      [VAR_UNLOCKED] = (lock ? VAR_LOCKED : VAR_UNLOCKED), \
+      [VAR_LOCKED] = (lock ? VAR_LOCKED : VAR_UNLOCKED), \
+      [VAR_FIXED] = VAR_FIXED, \
+    })[var]; \
+  } while (0)
+  CHANGE_LOCK(tv->v_lock, lock);
 
   switch (tv->v_type) {
-  case VAR_LIST:
-    if ((l = tv->vval.v_list) != NULL) {
-      if (lock)
-        l->lv_lock |= VAR_LOCKED;
-      else
-        l->lv_lock &= ~VAR_LOCKED;
-      if (deep < 0 || deep > 1)
-        /* recursive: lock/unlock the items the List contains */
-        for (li = l->lv_first; li != NULL; li = li->li_next)
-          item_lock(&li->li_tv, deep - 1, lock);
+    case VAR_LIST: {
+      list_T *const l = tv->vval.v_list;
+      if (l != NULL) {
+        CHANGE_LOCK(l->lv_lock, lock);
+        if (deep < 0 || deep > 1) {
+          // Recursive: lock/unlock the items the List contains.
+          for (listitem_T *li = l->lv_first; li != NULL; li = li->li_next) {
+            item_lock(&li->li_tv, deep - 1, lock);
+          }
+        }
+      }
+      break;
     }
-    break;
-  case VAR_DICT:
-    if ((d = tv->vval.v_dict) != NULL) {
-      if (lock)
-        d->dv_lock |= VAR_LOCKED;
-      else
-        d->dv_lock &= ~VAR_LOCKED;
-      if (deep < 0 || deep > 1) {
-        /* recursive: lock/unlock the items the List contains */
-        todo = (int)d->dv_hashtab.ht_used;
-        for (hi = d->dv_hashtab.ht_array; todo > 0; ++hi) {
-          if (!HASHITEM_EMPTY(hi)) {
-            --todo;
-            item_lock(&HI2DI(hi)->di_tv, deep - 1, lock);
+    case VAR_DICT: {
+      dict_T *const d = tv->vval.v_dict;
+      if (d != NULL) {
+        CHANGE_LOCK(d->dv_lock, lock);
+        if (deep < 0 || deep > 1) {
+          // Recursive: lock/unlock the items the List contains.
+          int todo = (int)d->dv_hashtab.ht_used;
+          for (hashitem_T *hi = d->dv_hashtab.ht_array; todo > 0; hi++) {
+            if (!HASHITEM_EMPTY(hi)) {
+              todo--;
+              item_lock(&HI2DI(hi)->di_tv, deep - 1, lock);
+            }
           }
         }
       }
+      break;
+    }
+    case VAR_NUMBER:
+    case VAR_FLOAT:
+    case VAR_STRING:
+    case VAR_FUNC:
+    case VAR_PARTIAL:
+    case VAR_SPECIAL: {
+      break;
+    }
+    case VAR_UNKNOWN: {
+      assert(false);
     }
-    break;
-  case VAR_NUMBER:
-  case VAR_FLOAT:
-  case VAR_STRING:
-  case VAR_FUNC:
-  case VAR_PARTIAL:
-  case VAR_SPECIAL:
-    break;
-  case VAR_UNKNOWN:
-    assert(false);
   }
-  --recurse;
+#undef CHANGE_LOCK
+  recurse--;
 }
 
 /*
@@ -3302,10 +3307,6 @@ char_u *get_user_var_name(expand_T *xp, int idx)
       ++hi;
     return cat_prefix_varname('b', hi->hi_key);
   }
-  if (bdone == ht->ht_used) {
-    ++bdone;
-    return (char_u *)"b:changedtick";
-  }
 
   /* w: variables */
   ht = &curwin->w_vars->dv_hashtab;
@@ -6373,7 +6374,7 @@ dict_T *dict_alloc(void) FUNC_ATTR_NONNULL_RET
   first_dict = d;
 
   hash_init(&d->dv_hashtab);
-  d->dv_lock = 0;
+  d->dv_lock = VAR_UNLOCKED;
   d->dv_scope = 0;
   d->dv_refcount = 0;
   d->dv_copyID = 0;
@@ -6446,9 +6447,8 @@ static void dict_free_contents(dict_T *d) {
        * something recursive causing trouble. */
       di = HI2DI(hi);
       hash_remove(&d->dv_hashtab, hi);
-      clear_tv(&di->di_tv);
-      xfree(di);
-      --todo;
+      dictitem_free(di);
+      todo--;
     }
   }
 
@@ -7820,9 +7820,10 @@ static void f_add(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 
   rettv->vval.v_number = 1;   /* Default: Failed */
   if (argvars[0].v_type == VAR_LIST) {
+    const char *const arg_errmsg = _("add() argument");
+    const size_t arg_errmsg_len = strlen(arg_errmsg);
     if ((l = argvars[0].vval.v_list) != NULL
-        && !tv_check_lock(l->lv_lock,
-                          (char_u *)N_("add() argument"), true)) {
+        && !tv_check_lock(l->lv_lock, arg_errmsg, arg_errmsg_len)) {
       list_append_tv(l, &argvars[1]);
       copy_tv(&argvars[0], rettv);
     }
@@ -9399,7 +9400,8 @@ void dict_extend(dict_T *d1, dict_T *d2, char_u *action)
   hashitem_T  *hi2;
   int todo;
   bool watched = is_watched(d1);
-  char_u *arg_errmsg = (char_u *)N_("extend() argument");
+  const char *const arg_errmsg = _("extend() argument");
+  const size_t arg_errmsg_len = strlen(arg_errmsg);
 
   todo = (int)d2->dv_hashtab.ht_used;
   for (hi2 = d2->dv_hashtab.ht_array; todo > 0; ++hi2) {
@@ -9433,8 +9435,8 @@ void dict_extend(dict_T *d1, dict_T *d2, char_u *action)
       } else if (*action == 'f' && HI2DI(hi2) != di1) {
         typval_T oldtv;
 
-        if (tv_check_lock(di1->di_tv.v_lock, arg_errmsg, true)
-            || var_check_ro(di1->di_flags, arg_errmsg, true)) {
+        if (tv_check_lock(di1->di_tv.v_lock, arg_errmsg, arg_errmsg_len)
+            || var_check_ro(di1->di_flags, arg_errmsg, arg_errmsg_len)) {
           break;
         }
 
@@ -9460,7 +9462,8 @@ void dict_extend(dict_T *d1, dict_T *d2, char_u *action)
  */
 static void f_extend(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
-  char_u *arg_errmsg = (char_u *)N_("extend() argument");
+  const char *const arg_errmsg = N_("extend() argument");
+  const size_t arg_errmsg_len = strlen(arg_errmsg);
 
   if (argvars[0].v_type == VAR_LIST && argvars[1].v_type == VAR_LIST) {
     list_T          *l1, *l2;
@@ -9470,7 +9473,7 @@ static void f_extend(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 
     l1 = argvars[0].vval.v_list;
     l2 = argvars[1].vval.v_list;
-    if (l1 != NULL && !tv_check_lock(l1->lv_lock, arg_errmsg, true)
+    if (l1 != NULL && !tv_check_lock(l1->lv_lock, arg_errmsg, arg_errmsg_len)
         && l2 != NULL) {
       if (argvars[2].v_type != VAR_UNKNOWN) {
         before = get_tv_number_chk(&argvars[2], &error);
@@ -9500,7 +9503,7 @@ static void f_extend(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 
     d1 = argvars[0].vval.v_dict;
     d2 = argvars[1].vval.v_dict;
-    if (d1 != NULL && !tv_check_lock(d1->dv_lock, arg_errmsg, true)
+    if (d1 != NULL && !tv_check_lock(d1->dv_lock, arg_errmsg, arg_errmsg_len)
         && d2 != NULL) {
       /* Check the third argument. */
       if (argvars[2].v_type != VAR_UNKNOWN) {
@@ -9643,20 +9646,22 @@ static void filter_map(typval_T *argvars, typval_T *rettv, int map)
   typval_T save_key;
   int rem = false;
   int todo;
-  char_u      *ermsg = (char_u *)(map ? "map()" : "filter()");
-  char_u      *arg_errmsg = (char_u *)(map ? N_("map() argument")
-                                       : N_("filter() argument"));
+  char_u *ermsg = (char_u *)(map ? "map()" : "filter()");
+  const char *const arg_errmsg = (map
+                                  ? _("map() argument")
+                                  : _("filter() argument"));
+  const size_t arg_errmsg_len = strlen(arg_errmsg);
   int save_did_emsg;
   int idx = 0;
 
   if (argvars[0].v_type == VAR_LIST) {
     if ((l = argvars[0].vval.v_list) == NULL
-        || (!map && tv_check_lock(l->lv_lock, arg_errmsg, true))) {
+        || (!map && tv_check_lock(l->lv_lock, arg_errmsg, arg_errmsg_len))) {
       return;
     }
   } else if (argvars[0].v_type == VAR_DICT) {
     if ((d = argvars[0].vval.v_dict) == NULL
-        || (!map && tv_check_lock(d->dv_lock, arg_errmsg, true))) {
+        || (!map && tv_check_lock(d->dv_lock, arg_errmsg, arg_errmsg_len))) {
       return;
     }
   } else {
@@ -9689,8 +9694,8 @@ static void filter_map(typval_T *argvars, typval_T *rettv, int map)
 
           di = HI2DI(hi);
           if (map
-              && (tv_check_lock(di->di_tv.v_lock, arg_errmsg, true)
-                  || var_check_ro(di->di_flags, arg_errmsg, true))) {
+              && (tv_check_lock(di->di_tv.v_lock, arg_errmsg, arg_errmsg_len)
+                  || var_check_ro(di->di_flags, arg_errmsg, arg_errmsg_len))) {
             break;
           }
 
@@ -9700,8 +9705,8 @@ static void filter_map(typval_T *argvars, typval_T *rettv, int map)
           if (r == FAIL || did_emsg)
             break;
           if (!map && rem) {
-            if (var_check_fixed(di->di_flags, arg_errmsg, true)
-                || var_check_ro(di->di_flags, arg_errmsg, true)) {
+            if (var_check_fixed(di->di_flags, arg_errmsg, arg_errmsg_len)
+                || var_check_ro(di->di_flags, arg_errmsg, arg_errmsg_len)) {
               break;
             }
             dictitem_remove(d, di);
@@ -9713,7 +9718,8 @@ static void filter_map(typval_T *argvars, typval_T *rettv, int map)
       vimvars[VV_KEY].vv_type = VAR_NUMBER;
 
       for (li = l->lv_first; li != NULL; li = nli) {
-        if (map && tv_check_lock(li->li_tv.v_lock, arg_errmsg, true)) {
+        if (map
+            && tv_check_lock(li->li_tv.v_lock, arg_errmsg, arg_errmsg_len)) {
           break;
         }
         nli = li->li_next;
@@ -10524,10 +10530,6 @@ static void f_getbufvar(typval_T *argvars, typval_T *rettv, FunPtr fptr)
         // buffer-local-option
         done = true;
       }
-    } else if (STRCMP(varname, "changedtick") == 0) {
-      rettv->v_type = VAR_NUMBER;
-      rettv->vval.v_number = curbuf->b_changedtick;
-      done = true;
     } else {
       // Look up the variable.
       // Let getbufvar({nr}, "") return the "b:" dictionary.
@@ -12395,13 +12397,14 @@ static void f_insert(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   long before = 0;
   listitem_T  *item;
   list_T      *l;
-  int error = FALSE;
+  int error = false;
+  const char *const arg_errmsg = _("insert() argument");
+  const size_t arg_errmsg_len = strlen(arg_errmsg);
 
   if (argvars[0].v_type != VAR_LIST) {
     EMSG2(_(e_listarg), "insert()");
   } else if ((l = argvars[0].vval.v_list) != NULL
-             && !tv_check_lock(l->lv_lock,
-                               (char_u *)N_("insert() argument"), true)) {
+             && !tv_check_lock(l->lv_lock, arg_errmsg, arg_errmsg_len)) {
     if (argvars[2].v_type != VAR_UNKNOWN) {
       before = get_tv_number_chk(&argvars[2], &error);
     }
@@ -12452,38 +12455,33 @@ static void f_islocked(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   dictitem_T  *di;
 
   rettv->vval.v_number = -1;
-  end = get_lval(get_tv_string(&argvars[0]), NULL, &lv, FALSE, FALSE,
-      GLV_NO_AUTOLOAD, FNE_CHECK_START);
+  end = get_lval(get_tv_string(&argvars[0]), NULL, &lv, false, false,
+                 GLV_NO_AUTOLOAD|GLV_READ_ONLY, FNE_CHECK_START);
   if (end != NULL && lv.ll_name != NULL) {
     if (*end != NUL)
       EMSG(_(e_trailing));
     else {
       if (lv.ll_tv == NULL) {
-        if (check_changedtick(lv.ll_name)) {
-          rettv->vval.v_number = 1;  // Always locked.
-        } else {
-          di = find_var((const char *)lv.ll_name, STRLEN(lv.ll_name), NULL,
-                        true);
-          if (di != NULL) {
-            /* Consider a variable locked when:
-             * 1. the variable itself is locked
-             * 2. the value of the variable is locked.
-             * 3. the List or Dict value is locked.
-             */
-            rettv->vval.v_number = ((di->di_flags & DI_FLAGS_LOCK)
-                                    || tv_islocked(&di->di_tv));
-          }
+        di = find_var((const char *)lv.ll_name, STRLEN(lv.ll_name), NULL, true);
+        if (di != NULL) {
+          // Consider a variable locked when:
+          // 1. the variable itself is locked
+          // 2. the value of the variable is locked.
+          // 3. the List or Dict value is locked.
+          rettv->vval.v_number = ((di->di_flags & DI_FLAGS_LOCK)
+                                  || tv_islocked(&di->di_tv));
         }
-      } else if (lv.ll_range)
+      } else if (lv.ll_range) {
         EMSG(_("E786: Range not allowed"));
-      else if (lv.ll_newkey != NULL)
+      } else if (lv.ll_newkey != NULL) {
         EMSG2(_(e_dictkey), lv.ll_newkey);
-      else if (lv.ll_list != NULL)
-        /* List item. */
+      } else if (lv.ll_list != NULL) {
+        // List item.
         rettv->vval.v_number = tv_islocked(&lv.ll_li->li_tv);
-      else
-        /* Dictionary item. */
+      } else {
+        // Dictionary item.
         rettv->vval.v_number = tv_islocked(&lv.ll_di->di_tv);
+      }
     }
   }
 
@@ -14430,20 +14428,21 @@ static void f_remove(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   char_u      *key;
   dict_T      *d;
   dictitem_T  *di;
-  char_u      *arg_errmsg = (char_u *)N_("remove() argument");
+  const char *const arg_errmsg = _("remove() argument");
+  const size_t arg_errmsg_len = strlen(arg_errmsg);
 
   if (argvars[0].v_type == VAR_DICT) {
     if (argvars[2].v_type != VAR_UNKNOWN) {
       EMSG2(_(e_toomanyarg), "remove()");
     } else if ((d = argvars[0].vval.v_dict) != NULL
-               && !tv_check_lock(d->dv_lock, arg_errmsg, true)) {
+               && !tv_check_lock(d->dv_lock, arg_errmsg, arg_errmsg_len)) {
       key = get_tv_string_chk(&argvars[1]);
       if (key != NULL) {
         di = dict_find(d, key, -1);
         if (di == NULL) {
           EMSG2(_(e_dictkey), key);
-        } else if (!var_check_fixed(di->di_flags, arg_errmsg, true)
-                   && !var_check_ro(di->di_flags, arg_errmsg, true)) {
+        } else if (!var_check_fixed(di->di_flags, arg_errmsg, arg_errmsg_len)
+                   && !var_check_ro(di->di_flags, arg_errmsg, arg_errmsg_len)) {
           *rettv = di->di_tv;
           init_tv(&di->di_tv);
           dictitem_remove(d, di);
@@ -14456,7 +14455,7 @@ static void f_remove(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   } else if (argvars[0].v_type != VAR_LIST) {
     EMSG2(_(e_listdictarg), "remove()");
   } else if ((l = argvars[0].vval.v_list) != NULL
-             && !tv_check_lock(l->lv_lock, arg_errmsg, true)) {
+             && !tv_check_lock(l->lv_lock, arg_errmsg, arg_errmsg_len)) {
     int error = (int)false;
 
     idx = get_tv_number_chk(&argvars[1], &error);
@@ -14727,19 +14726,19 @@ fail:
  */
 static void f_reverse(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
-  list_T      *l;
-  listitem_T  *li, *ni;
+  const char *const arg_errmsg = _("reverse() argument");
+  const size_t arg_errmsg_len = strlen(arg_errmsg);
 
+  list_T *l;
   if (argvars[0].v_type != VAR_LIST) {
     EMSG2(_(e_listarg), "reverse()");
   } else if ((l = argvars[0].vval.v_list) != NULL
-             && !tv_check_lock(l->lv_lock,
-                               (char_u *)N_("reverse() argument"), true)) {
-    li = l->lv_last;
+             && !tv_check_lock(l->lv_lock, arg_errmsg, arg_errmsg_len)) {
+    listitem_T *li = l->lv_last;
     l->lv_first = l->lv_last = NULL;
     l->lv_len = 0;
     while (li != NULL) {
-      ni = li->li_prev;
+      listitem_T *const ni = li->li_prev;
       list_append(l, li);
       li = ni;
     }
@@ -16384,16 +16383,17 @@ static void do_sort_uniq(typval_T *argvars, typval_T *rettv, bool sort)
   sortinfo_T *old_sortinfo = sortinfo;
   sortinfo = &info;
 
+  const char *const arg_errmsg = (sort
+                                  ? _("sort() argument")
+                                  : _("uniq() argument"));
+  const size_t arg_errmsg_len = strlen(arg_errmsg);
+
   if (argvars[0].v_type != VAR_LIST) {
     EMSG2(_(e_listarg), sort ? "sort()" : "uniq()");
   } else {
     l = argvars[0].vval.v_list;
     if (l == NULL
-        || tv_check_lock(l->lv_lock,
-                         (char_u *)(sort
-                                    ? N_("sort() argument")
-                                    : N_("uniq() argument")),
-                         true)) {
+        || tv_check_lock(l->lv_lock, arg_errmsg, arg_errmsg_len)) {
         goto theend;
     }
     rettv->vval.v_list = l;
@@ -17732,10 +17732,10 @@ static void f_termopen(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   (void)setfname(curbuf, (uint8_t *)buf, NULL, true);
   // Save the job id and pid in b:terminal_job_{id,pid}
   Error err;
-  dict_set_value(curbuf->b_vars, cstr_as_string("terminal_job_id"),
-                 INTEGER_OBJ(rettv->vval.v_number), false, false, &err);
-  dict_set_value(curbuf->b_vars, cstr_as_string("terminal_job_pid"),
-                 INTEGER_OBJ(pid), false, false, &err);
+  dict_set_var(curbuf->b_vars, cstr_as_string("terminal_job_id"),
+               INTEGER_OBJ(rettv->vval.v_number), false, false, &err);
+  dict_set_var(curbuf->b_vars, cstr_as_string("terminal_job_pid"),
+               INTEGER_OBJ(pid), false, false, &err);
 
   Terminal *term = terminal_open(topts);
   data->term = term;
@@ -19382,23 +19382,13 @@ static int get_var_tv(
 {
   int ret = OK;
   typval_T    *tv = NULL;
-  typval_T atv;
   dictitem_T  *v;
 
-  // Check for "b:changedtick".
-  if (sizeof("b:changedtick") - 1 == len
-      && STRNCMP(name, "b:changedtick", len) == 0) {
-    atv.v_type = VAR_NUMBER;
-    atv.vval.v_number = curbuf->b_changedtick;
-    tv = &atv;
-  } else {
-    // Check for user-defined variables.
-    v = find_var(name, (size_t)len, NULL, no_autoload);
-    if (v != NULL) {
-      tv = &v->di_tv;
-      if (dip != NULL) {
-        *dip = v;
-      }
+  v = find_var(name, (size_t)len, NULL, no_autoload);
+  if (v != NULL) {
+    tv = &v->di_tv;
+    if (dip != NULL) {
+      *dip = v;
     }
   }
 
@@ -20381,7 +20371,7 @@ void new_script_vars(scid_T id)
 void init_var_dict(dict_T *dict, dictitem_T *dict_var, int scope)
 {
   hash_init(&dict->dv_hashtab);
-  dict->dv_lock = 0;
+  dict->dv_lock = VAR_UNLOCKED;
   dict->dv_scope = scope;
   dict->dv_refcount = DO_NOT_FREE_CNT;
   dict->dv_copyID = 0;
@@ -20557,8 +20547,8 @@ set_var (
 
   if (v != NULL) {
     // existing variable, need to clear the value
-    if (var_check_ro(v->di_flags, name, false)
-        || tv_check_lock(v->di_tv.v_lock, name, false)) {
+    if (var_check_ro(v->di_flags, (const char *)name, name_len)
+        || tv_check_lock(v->di_tv.v_lock, (const char *)name, name_len)) {
       return;
     }
 
@@ -20631,28 +20621,47 @@ set_var (
   }
 }
 
-// Return true if di_flags "flags" indicates variable "name" is read-only.
-// Also give an error message.
-static bool var_check_ro(int flags, char_u *name, bool use_gettext)
+/// Check whether variable is read-only (DI_FLAGS_RO, DI_FLAGS_RO_SBX)
+///
+/// Also gives an error message.
+///
+/// @param[in]  flags  di_flags attribute value.
+/// @param[in]  name  Variable name, for use in error message.
+/// @param[in]  name_len  Variable name length.
+///
+/// @return True if variable is read-only: either always or in sandbox when
+///         sandbox is enabled, false otherwise.
+static bool var_check_ro(const int flags, const char *const name,
+                         const size_t name_len)
+  FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_NONNULL_ALL
 {
   if (flags & DI_FLAGS_RO) {
-    EMSG2(_(e_readonlyvar), use_gettext ? (char_u *)_(name) : name);
+    emsgf(_(e_readonlyvar), (int)name_len, name);
     return true;
   }
   if ((flags & DI_FLAGS_RO_SBX) && sandbox) {
-    EMSG2(_(e_readonlysbx), use_gettext ? (char_u *)_(name) : name);
+    emsgf(_("E794: Cannot set variable in the sandbox: \"%.*s\""),
+          (int)name_len, name);
     return true;
   }
   return false;
 }
 
-// Return true if di_flags "flags" indicates variable "name" is fixed.
-// Also give an error message.
-static bool var_check_fixed(int flags, char_u *name, bool use_gettext)
+/// Check whether variable is fixed (DI_FLAGS_FIX)
+///
+/// Also gives an error message.
+///
+/// @param[in]  flags  di_flags attribute value.
+/// @param[in]  name  Variable name, for use in error message.
+/// @param[in]  name_len  Variable name length.
+///
+/// @return True if variable is fixed, false otherwise.
+static bool var_check_fixed(const int flags, const char *const name,
+                            const size_t name_len)
+  FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_NONNULL_ALL
 {
   if (flags & DI_FLAGS_FIX) {
-    EMSG2(_("E795: Cannot delete variable %s"),
-          use_gettext ? (char_u *)_(name) : name);
+    emsgf(_("E795: Cannot delete variable %.*s"), (int)name_len, name);
     return true;
   }
   return false;
@@ -20703,28 +20712,42 @@ static int valid_varname(char_u *varname)
   return TRUE;
 }
 
-// Return true if typeval "tv" is set to be locked (immutable).
-// Also give an error message, using "name" or _("name") when use_gettext is
-// true.
-static bool tv_check_lock(int lock, char_u *name, bool use_gettext)
+/// Check whether typval is locked
+///
+/// Also gives an error message.
+///
+/// @param[in]  lock  Lock status.
+/// @param[in]  name  Value name, for use in error message.
+/// @param[in]  name_len  Value name length.
+///
+/// @return True if value is locked.
+static bool tv_check_lock(const VarLockStatus lock,
+                          const char *const name,
+                          const size_t name_len)
+  FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  if (lock & VAR_LOCKED) {
-    EMSG2(_("E741: Value is locked: %s"),
-          name == NULL
-          ? (char_u *)_("Unknown")
-          : use_gettext ? (char_u *)_(name)
-          : name);
-    return true;
-  }
-  if (lock & VAR_FIXED) {
-    EMSG2(_("E742: Cannot change value of %s"),
-          name == NULL
-          ? (char_u *)_("Unknown")
-          : use_gettext ? (char_u *)_(name)
-          : name);
-    return true;
+  const char *error_message = NULL;
+  switch (lock) {
+    case VAR_UNLOCKED: {
+      return false;
+    }
+    case VAR_LOCKED: {
+      error_message = N_("E741: Value is locked: %.*s");
+      break;
+    }
+    case VAR_FIXED: {
+      error_message = N_("E742: Cannot change value of %.*s");
+      break;
+    }
   }
-  return false;
+  assert(error_message != NULL);
+
+  const char *const unknown_name = _("Unknown");
+
+  emsgf(_(error_message), (name != NULL ? name_len : strlen(unknown_name)),
+        (name != NULL ? name : unknown_name));
+
+  return true;
 }
 
 /*
@@ -21523,11 +21546,13 @@ void ex_function(exarg_T *eap)
       goto erret;
     }
     if (fudi.fd_di == NULL) {
-      if (tv_check_lock(fudi.fd_dict->dv_lock, eap->arg, false)) {
+      if (tv_check_lock(fudi.fd_dict->dv_lock, (const char *)eap->arg,
+                        STRLEN(eap->arg))) {
         // Can't add a function to a locked dictionary
         goto erret;
       }
-    } else if (tv_check_lock(fudi.fd_di->di_tv.v_lock, eap->arg, false)) {
+    } else if (tv_check_lock(fudi.fd_di->di_tv.v_lock, (const char *)eap->arg,
+                             STRLEN(eap->arg))) {
       // Can't change an existing function if it is locked
       goto erret;
     }
diff --git a/src/nvim/eval_defs.h b/src/nvim/eval_defs.h
index a61ddb7605..fb2822b851 100644
--- a/src/nvim/eval_defs.h
+++ b/src/nvim/eval_defs.h
@@ -30,8 +30,8 @@ typedef enum {
 /// Variable lock status for typval_T.v_lock
 typedef enum {
   VAR_UNLOCKED = 0,  ///< Not locked.
-  VAR_LOCKED,        ///< User lock, can be unlocked.
-  VAR_FIXED,         ///< Locked forever.
+  VAR_LOCKED = 1,    ///< User lock, can be unlocked.
+  VAR_FIXED = 2,     ///< Locked forever.
 } VarLockStatus;
 
 /// VimL variable types, for use in typval_T.v_type
@@ -93,18 +93,18 @@ struct listwatch_S {
  * Structure to hold info about a list.
  */
 struct listvar_S {
-  listitem_T  *lv_first;        /* first item, NULL if none */
-  listitem_T  *lv_last;         /* last item, NULL if none */
-  int lv_refcount;              /* reference count */
-  int lv_len;                   /* number of items */
-  listwatch_T *lv_watch;        /* first watcher, NULL if none */
-  int lv_idx;                   /* cached index of an item */
-  listitem_T  *lv_idx_item;     /* when not NULL item at index "lv_idx" */
-  int lv_copyID;                /* ID used by deepcopy() */
-  list_T      *lv_copylist;     /* copied list used by deepcopy() */
-  char lv_lock;                 /* zero, VAR_LOCKED, VAR_FIXED */
-  list_T      *lv_used_next;    /* next list in used lists list */
-  list_T      *lv_used_prev;    /* previous list in used lists list */
+  listitem_T *lv_first;  ///< First item, NULL if none.
+  listitem_T *lv_last;  ///< Last item, NULL if none.
+  int lv_refcount;  ///< Reference count.
+  int lv_len;  ///< Number of items.
+  listwatch_T *lv_watch;  ///< First watcher, NULL if none.
+  int lv_idx;  ///< Index of a cached item, used for optimising repeated l[idx].
+  listitem_T *lv_idx_item;  ///< When not NULL item at index "lv_idx".
+  int lv_copyID;  ///< ID used by deepcopy().
+  list_T *lv_copylist;  ///< Copied list used by deepcopy().
+  VarLockStatus lv_lock;  ///< Zero, VAR_LOCKED, VAR_FIXED.
+  list_T *lv_used_next;  ///< next list in used lists list.
+  list_T *lv_used_prev;  ///< Previous list in used lists list.
 };
 
 // Static list with 10 items. Use init_static_list() to initialize.
diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index 0fda9a8ae6..159e027793 100644
--- a/src/nvim/ex_cmds.c
+++ b/src/nvim/ex_cmds.c
@@ -6139,7 +6139,7 @@ void ex_substitute(exarg_T *eap)
     // restore it so that undotree() is identical before/after the preview.
     curbuf->b_u_newhead = save_b_u_newhead;
     curbuf->b_u_time_cur = save_b_u_time_cur;
-    curbuf->b_changedtick = save_changedtick;
+    buf_set_changedtick(curbuf, save_changedtick);
   }
   if (buf_valid(preview_buf)) {
     // XXX: Must do this *after* u_undo_and_forget(), why?
diff --git a/src/nvim/globals.h b/src/nvim/globals.h
index ba4f6e2b3b..07ea045c13 100644
--- a/src/nvim/globals.h
+++ b/src/nvim/globals.h
@@ -1171,11 +1171,7 @@ EXTERN char_u e_loclist[] INIT(= N_("E776: No location list"));
 EXTERN char_u e_re_damg[] INIT(= N_("E43: Damaged match string"));
 EXTERN char_u e_re_corr[] INIT(= N_("E44: Corrupted regexp program"));
 EXTERN char_u e_readonly[] INIT(= N_(
-        "E45: 'readonly' option is set (add ! to override)"));
-EXTERN char_u e_readonlyvar[] INIT(= N_(
-        "E46: Cannot change read-only variable \"%s\""));
-EXTERN char_u e_readonlysbx[] INIT(= N_(
-        "E794: Cannot set variable in the sandbox: \"%s\""));
+    "E45: 'readonly' option is set (add ! to override)"));
 EXTERN char_u e_readerrf[] INIT(= N_("E47: Error while reading errorfile"));
 EXTERN char_u e_sandbox[] INIT(= N_("E48: Not allowed in sandbox"));
 EXTERN char_u e_secure[] INIT(= N_("E523: Not allowed here"));
diff --git a/src/nvim/main.c b/src/nvim/main.c
index fab968836c..d7baa7acfa 100644
--- a/src/nvim/main.c
+++ b/src/nvim/main.c
@@ -576,9 +576,9 @@ void getout(int exitval)
         buf_T *buf = wp->w_buffer;
         if (buf->b_changedtick != -1) {
           apply_autocmds(EVENT_BUFWINLEAVE, buf->b_fname,
-              buf->b_fname, FALSE, buf);
-          buf->b_changedtick = -1;            /* note that we did it already */
-          /* start all over, autocommands may mess up the lists */
+                         buf->b_fname, false, buf);
+          buf_set_changedtick(buf, -1);  // note that we did it already
+          // start all over, autocommands may mess up the lists
           next_tp = first_tabpage;
           break;
         }
diff --git a/src/nvim/memline.c b/src/nvim/memline.c
index 91dab16e27..b67f550358 100644
--- a/src/nvim/memline.c
+++ b/src/nvim/memline.c
@@ -1180,7 +1180,7 @@ void ml_recover(void)
      * empty.  Don't set the modified flag then. */
     if (!(curbuf->b_ml.ml_line_count == 2 && *ml_get(1) == NUL)) {
       changed_int();
-      ++curbuf->b_changedtick;
+      buf_set_changedtick(curbuf, curbuf->b_changedtick + 1);
     }
   } else {
     for (idx = 1; idx <= lnum; ++idx) {
@@ -1190,7 +1190,7 @@ void ml_recover(void)
       xfree(p);
       if (i != 0) {
         changed_int();
-        ++curbuf->b_changedtick;
+        buf_set_changedtick(curbuf, curbuf->b_changedtick + 1);
         break;
       }
     }
diff --git a/src/nvim/memory.c b/src/nvim/memory.c
index 51e0dd926e..b593936d7b 100644
--- a/src/nvim/memory.c
+++ b/src/nvim/memory.c
@@ -625,18 +625,6 @@ void free_all_mem(void)
   /* Destroy all windows.  Must come before freeing buffers. */
   win_free_all();
 
-  /* Free all buffers.  Reset 'autochdir' to avoid accessing things that
-   * were freed already. */
-  p_acd = false;
-  for (buf = firstbuf; buf != NULL; ) {
-    bufref_T bufref;
-    set_bufref(&bufref, buf);
-    nextbuf = buf->b_next;
-    close_buffer(NULL, buf, DOBUF_WIPE, false);
-    // Didn't work, try next one.
-    buf = bufref_valid(&bufref) ? nextbuf : firstbuf;
-  }
-
   free_cmdline_buf();
 
   /* Clear registers. */
@@ -660,6 +648,20 @@ void free_all_mem(void)
 
   eval_clear();
 
+  // Free all buffers.  Reset 'autochdir' to avoid accessing things that
+  // were freed already.
+  // Must be after eval_clear to avoid it trying to access b:changedtick after
+  // freeing it.
+  p_acd = false;
+  for (buf = firstbuf; buf != NULL; ) {
+    bufref_T bufref;
+    set_bufref(&bufref, buf);
+    nextbuf = buf->b_next;
+    close_buffer(NULL, buf, DOBUF_WIPE, false);
+    // Didn't work, try next one.
+    buf = bufref_valid(&bufref) ? nextbuf : firstbuf;
+  }
+
   /* screenlines (can't display anything now!) */
   free_screenlines();
 
diff --git a/src/nvim/misc1.c b/src/nvim/misc1.c
index db34159f24..36087ac59c 100644
--- a/src/nvim/misc1.c
+++ b/src/nvim/misc1.c
@@ -51,6 +51,7 @@
 #include "nvim/os/input.h"
 #include "nvim/os/time.h"
 #include "nvim/event/stream.h"
+#include "nvim/buffer.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "misc1.c.generated.h"
@@ -1788,7 +1789,7 @@ void changed(void)
     }
     changed_int();
   }
-  ++curbuf->b_changedtick;
+  buf_set_changedtick(curbuf, curbuf->b_changedtick + 1);
 }
 
 /*
@@ -2147,7 +2148,7 @@ unchanged (
     redraw_tabline = TRUE;
     need_maketitle = TRUE;          /* set window title later */
   }
-  ++buf->b_changedtick;
+  buf_set_changedtick(buf, buf->b_changedtick + 1);
 }
 
 /*
diff --git a/src/nvim/terminal.c b/src/nvim/terminal.c
index 3fd2814070..e23c2e5748 100644
--- a/src/nvim/terminal.c
+++ b/src/nvim/terminal.c
@@ -611,12 +611,12 @@ static void buf_set_term_title(buf_T *buf, char *title)
     FUNC_ATTR_NONNULL_ALL
 {
   Error err;
-  dict_set_value(buf->b_vars,
-                 cstr_as_string("term_title"),
-                 STRING_OBJ(cstr_as_string(title)),
-                 false,
-                 false,
-                 &err);
+  dict_set_var(buf->b_vars,
+               STATIC_CSTR_AS_STRING("term_title"),
+               STRING_OBJ(cstr_as_string(title)),
+               false,
+               false,
+               &err);
 }
 
 static int term_settermprop(VTermProp prop, VTermValue *val, void *data)
diff --git a/test/functional/api/buffer_spec.lua b/test/functional/api/buffer_spec.lua
index 3d3a2bb046..e7e2168238 100644
--- a/test/functional/api/buffer_spec.lua
+++ b/test/functional/api/buffer_spec.lua
@@ -2,8 +2,11 @@ local helpers = require('test.functional.helpers')(after_each)
 local clear, nvim, buffer = helpers.clear, helpers.nvim, helpers.buffer
 local curbuf, curwin, eq = helpers.curbuf, helpers.curwin, helpers.eq
 local curbufmeths, ok = helpers.curbufmeths, helpers.ok
-local funcs, request = helpers.funcs, helpers.request
+local funcs = helpers.funcs
+local request = helpers.request
 local NIL = helpers.NIL
+local meth_pcall = helpers.meth_pcall
+local command = helpers.command
 
 describe('api/buf', function()
   before_each(clear)
@@ -249,6 +252,24 @@ describe('api/buf', function()
       eq(1, funcs.exists('b:lua'))
       curbufmeths.del_var('lua')
       eq(0, funcs.exists('b:lua'))
+      eq({false, 'Key "lua" doesn\'t exist'}, meth_pcall(curbufmeths.del_var, 'lua'))
+      curbufmeths.set_var('lua', 1)
+      command('lockvar b:lua')
+      eq({false, 'Key is locked: lua'}, meth_pcall(curbufmeths.del_var, 'lua'))
+      eq({false, 'Key is locked: lua'}, meth_pcall(curbufmeths.set_var, 'lua', 1))
+      eq({false, 'Key is read-only: changedtick'},
+         meth_pcall(curbufmeths.del_var, 'changedtick'))
+      eq({false, 'Key is read-only: changedtick'},
+         meth_pcall(curbufmeths.set_var, 'changedtick', 1))
+    end)
+  end)
+
+  describe('get_changedtick', function()
+    it('works', function()
+      eq(2, curbufmeths.get_changedtick())
+      curbufmeths.set_lines(0, 1, false, {'abc\0', '\0def', 'ghi'})
+      eq(3, curbufmeths.get_changedtick())
+      eq(3, curbufmeths.get_var('changedtick'))
     end)
 
     it('buffer_set_var returns the old value', function()
diff --git a/test/functional/api/tabpage_spec.lua b/test/functional/api/tabpage_spec.lua
index e10f30085f..d7ef53a88f 100644
--- a/test/functional/api/tabpage_spec.lua
+++ b/test/functional/api/tabpage_spec.lua
@@ -6,6 +6,8 @@ local curtabmeths = helpers.curtabmeths
 local funcs = helpers.funcs
 local request = helpers.request
 local NIL = helpers.NIL
+local meth_pcall = helpers.meth_pcall
+local command = helpers.command
 
 describe('api/tabpage', function()
   before_each(clear)
@@ -32,6 +34,11 @@ describe('api/tabpage', function()
       eq(1, funcs.exists('t:lua'))
       curtabmeths.del_var('lua')
       eq(0, funcs.exists('t:lua'))
+      eq({false, 'Key "lua" doesn\'t exist'}, meth_pcall(curtabmeths.del_var, 'lua'))
+      curtabmeths.set_var('lua', 1)
+      command('lockvar t:lua')
+      eq({false, 'Key is locked: lua'}, meth_pcall(curtabmeths.del_var, 'lua'))
+      eq({false, 'Key is locked: lua'}, meth_pcall(curtabmeths.set_var, 'lua', 1))
     end)
 
     it('tabpage_set_var returns the old value', function()
diff --git a/test/functional/api/vim_spec.lua b/test/functional/api/vim_spec.lua
index ce6c52e334..3348368a36 100644
--- a/test/functional/api/vim_spec.lua
+++ b/test/functional/api/vim_spec.lua
@@ -7,6 +7,8 @@ local os_name = helpers.os_name
 local meths = helpers.meths
 local funcs = helpers.funcs
 local request = helpers.request
+local meth_pcall = helpers.meth_pcall
+local command = helpers.command
 
 describe('api', function()
   before_each(clear)
@@ -43,7 +45,7 @@ describe('api', function()
     it('works', function()
       nvim('command', 'let g:v1 = "a"')
       nvim('command', 'let g:v2 = [1, 2, {"v3": 3}]')
-      eq({v1 = 'a', v2 = {1, 2, {v3 = 3}}}, nvim('eval', 'g:'))
+      eq({v1 = 'a', v2 = { 1, 2, { v3 = 3 } } }, nvim('eval', 'g:'))
     end)
 
     it('handles NULL-initialized strings correctly', function()
@@ -65,7 +67,7 @@ describe('api', function()
 
   describe('nvim_call_function', function()
     it('works', function()
-      nvim('call_function', 'setqflist', {{{ filename = 'something', lnum = 17}}, 'r'})
+      nvim('call_function', 'setqflist', { { { filename = 'something', lnum = 17 } }, 'r' })
       eq(17, nvim('call_function', 'getqflist', {})[1].lnum)
       eq(17, nvim('call_function', 'eval', {17}))
       eq('foo', nvim('call_function', 'simplify', {'this/./is//redundant/../../../foo'}))
@@ -117,6 +119,11 @@ describe('api', function()
       eq(1, funcs.exists('g:lua'))
       meths.del_var('lua')
       eq(0, funcs.exists('g:lua'))
+      eq({false, 'Key "lua" doesn\'t exist'}, meth_pcall(meths.del_var, 'lua'))
+      meths.set_var('lua', 1)
+      command('lockvar lua')
+      eq({false, 'Key is locked: lua'}, meth_pcall(meths.del_var, 'lua'))
+      eq({false, 'Key is locked: lua'}, meth_pcall(meths.set_var, 'lua', 1))
     end)
 
     it('vim_set_var returns the old value', function()
@@ -396,7 +403,7 @@ describe('api', function()
       eq(1, meths.get_var('avar'))
 
       req = {
-        {'nvim_set_var', {'bvar', {2,3}}},
+        { 'nvim_set_var', { 'bvar', { 2, 3 } } },
         12,
       }
       status, err = pcall(meths.call_atomic, req)
diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index 465bda6bc9..deffc68994 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -8,6 +8,8 @@ local curwinmeths = helpers.curwinmeths
 local funcs = helpers.funcs
 local request = helpers.request
 local NIL = helpers.NIL
+local meth_pcall = helpers.meth_pcall
+local command = helpers.command
 
 -- check if str is visible at the beginning of some line
 local function is_visible(str)
@@ -137,6 +139,11 @@ describe('api/win', function()
       eq(1, funcs.exists('w:lua'))
       curwinmeths.del_var('lua')
       eq(0, funcs.exists('w:lua'))
+      eq({false, 'Key "lua" doesn\'t exist'}, meth_pcall(curwinmeths.del_var, 'lua'))
+      curwinmeths.set_var('lua', 1)
+      command('lockvar w:lua')
+      eq({false, 'Key is locked: lua'}, meth_pcall(curwinmeths.del_var, 'lua'))
+      eq({false, 'Key is locked: lua'}, meth_pcall(curwinmeths.set_var, 'lua', 1))
     end)
 
     it('window_set_var returns the old value', function()
diff --git a/test/functional/eval/changedtick_spec.lua b/test/functional/eval/changedtick_spec.lua
new file mode 100644
index 0000000000..60ea9fa12b
--- /dev/null
+++ b/test/functional/eval/changedtick_spec.lua
@@ -0,0 +1,142 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local eq = helpers.eq
+local eval = helpers.eval
+local feed = helpers.feed
+local clear = helpers.clear
+local funcs = helpers.funcs
+local meths = helpers.meths
+local command = helpers.command
+local exc_exec = helpers.exc_exec
+local redir_exec = helpers.redir_exec
+local meth_pcall = helpers.meth_pcall
+local curbufmeths = helpers.curbufmeths
+
+before_each(clear)
+
+local function changedtick()
+  local ct = curbufmeths.get_changedtick()
+  eq(ct, curbufmeths.get_var('changedtick'))
+  eq(ct, curbufmeths.get_var('changedtick'))
+  eq(ct, eval('b:changedtick'))
+  eq(ct, eval('b:["changedtick"]'))
+  eq(ct, eval('b:.changedtick'))
+  eq(ct, funcs.getbufvar('%', 'changedtick'))
+  eq(ct, funcs.getbufvar('%', '').changedtick)
+  eq(ct, eval('b:').changedtick)
+  return ct
+end
+
+describe('b:changedtick', function()
+  -- Ported tests from Vim-8.0.333
+  it('increments', function()  -- Test_changedtick_increments
+    -- New buffer has an empty line, tick starts at 2
+    eq(2, changedtick())
+    funcs.setline(1, 'hello')
+    eq(3, changedtick())
+    eq(0, exc_exec('undo'))
+    -- Somehow undo counts as two changes
+    eq(5, changedtick())
+  end)
+  it('is present in b: dictionary', function()
+    eq(2, changedtick())
+    command('let d = b:')
+    eq(2, meths.get_var('d').changedtick)
+  end)
+  it('increments at bdel', function()
+    command('new')
+    eq(2, changedtick())
+    local bnr = curbufmeths.get_number()
+    eq(2, bnr)
+    command('bdel')
+    eq(3, funcs.getbufvar(bnr, 'changedtick'))
+    eq(1, curbufmeths.get_number())
+  end)
+  it('fails to be changed by user', function()
+    local ct = changedtick()
+    local ctn = ct + 100500
+    eq(0, exc_exec('let d = b:'))
+    eq('\nE46: Cannot change read-only variable "b:changedtick"',
+       redir_exec('let b:changedtick = ' .. ctn))
+    eq('\nE46: Cannot change read-only variable "b:["changedtick"]"',
+       redir_exec('let b:["changedtick"] = ' .. ctn))
+    eq('\nE46: Cannot change read-only variable "b:.changedtick"',
+       redir_exec('let b:.changedtick = ' .. ctn))
+    eq('\nE46: Cannot change read-only variable "d.changedtick"',
+       redir_exec('let d.changedtick = ' .. ctn))
+    eq({false, 'Key is read-only: changedtick'},
+       meth_pcall(curbufmeths.set_var, 'changedtick', ctn))
+
+    eq('\nE795: Cannot delete variable b:changedtick',
+       redir_exec('unlet b:changedtick'))
+    eq('\nE46: Cannot change read-only variable "b:.changedtick"',
+       redir_exec('unlet b:.changedtick'))
+    eq('\nE46: Cannot change read-only variable "b:["changedtick"]"',
+       redir_exec('unlet b:["changedtick"]'))
+    eq('\nE46: Cannot change read-only variable "d.changedtick"',
+       redir_exec('unlet d.changedtick'))
+    eq({false, 'Key is read-only: changedtick'},
+       meth_pcall(curbufmeths.del_var, 'changedtick'))
+    eq(ct, changedtick())
+
+    eq('\nE46: Cannot change read-only variable "b:["changedtick"]"',
+       redir_exec('let b:["changedtick"] += ' .. ctn))
+    eq('\nE46: Cannot change read-only variable "b:["changedtick"]"',
+       redir_exec('let b:["changedtick"] -= ' .. ctn))
+    eq('\nE46: Cannot change read-only variable "b:["changedtick"]"',
+       redir_exec('let b:["changedtick"] .= ' .. ctn))
+
+    eq(ct, changedtick())
+
+    funcs.setline(1, 'hello')
+
+    eq(ct + 1, changedtick())
+  end)
+  it('is listed in :let output', function()
+    eq('\nb:changedtick         #2',
+       redir_exec(':let b:'))
+  end)
+  it('fails to unlock b:changedtick', function()
+    eq(0, exc_exec('let d = b:'))
+    eq(0, funcs.islocked('b:changedtick'))
+    eq(0, funcs.islocked('d.changedtick'))
+    eq('\nE940: Cannot lock or unlock variable b:changedtick',
+       redir_exec('unlockvar b:changedtick'))
+    eq('\nE46: Cannot change read-only variable "d.changedtick"',
+       redir_exec('unlockvar d.changedtick'))
+    eq(0, funcs.islocked('b:changedtick'))
+    eq(0, funcs.islocked('d.changedtick'))
+    eq('\nE940: Cannot lock or unlock variable b:changedtick',
+       redir_exec('lockvar b:changedtick'))
+    eq('\nE46: Cannot change read-only variable "d.changedtick"',
+       redir_exec('lockvar d.changedtick'))
+    eq(0, funcs.islocked('b:changedtick'))
+    eq(0, funcs.islocked('d.changedtick'))
+  end)
+  it('is being completed', function()
+    feed(':echo b:<Tab><Home>let cmdline="<End>"<CR>')
+    eq('echo b:changedtick', meths.get_var('cmdline'))
+  end)
+  it('cannot be changed by filter() or map()', function()
+    eq(2, changedtick())
+    eq('\nE795: Cannot delete variable filter() argument',
+       redir_exec('call filter(b:, 0)'))
+    eq('\nE742: Cannot change value of map() argument',
+       redir_exec('call map(b:, 0)'))
+    eq('\nE742: Cannot change value of map() argument',
+       redir_exec('call map(b:, "v:val")'))
+    eq(2, changedtick())
+  end)
+  it('cannot be remove()d', function()
+    eq(2, changedtick())
+    eq('\nE795: Cannot delete variable remove() argument',
+       redir_exec('call remove(b:, "changedtick")'))
+    eq(2, changedtick())
+  end)
+  it('does not inherit VAR_FIXED when copying dictionary over', function()
+    eq(2, changedtick())
+    eq('', redir_exec('let d1 = copy(b:)|let d1.changedtick = 42'))
+    eq('', redir_exec('let d2 = copy(b:)|unlet d2.changedtick'))
+    eq(2, changedtick())
+  end)
+end)
diff --git a/test/functional/helpers.lua b/test/functional/helpers.lua
index a894fa9328..65d1ad76ef 100644
--- a/test/functional/helpers.lua
+++ b/test/functional/helpers.lua
@@ -544,6 +544,14 @@ local function skip_fragile(pending_fn, cond)
   return false
 end
 
+local function meth_pcall(...)
+  local ret = {pcall(...)}
+  if type(ret[2]) == 'string' then
+    ret[2] = ret[2]:gsub('^[^:]+:%d+: ', '')
+  end
+  return ret
+end
+
 local funcs = create_callindex(nvim_call)
 local meths = create_callindex(nvim)
 local uimeths = create_callindex(ui)
@@ -615,6 +623,7 @@ local M = {
   skip_fragile = skip_fragile,
   set_shell_powershell = set_shell_powershell,
   tmpname = tmpname,
+  meth_pcall = meth_pcall,
   NIL = mpack.NIL,
 }
 
diff --git a/test/functional/legacy/091_context_variables_spec.lua b/test/functional/legacy/091_context_variables_spec.lua
index edf497d397..c08a58e14b 100644
--- a/test/functional/legacy/091_context_variables_spec.lua
+++ b/test/functional/legacy/091_context_variables_spec.lua
@@ -13,6 +13,14 @@ describe('context variables', function()
     -- Test for getbufvar().
     -- Use strings to test for memory leaks.
     source([[
+      function Getbufscope(buf, ...)
+        let ret = call('getbufvar', [a:buf, ''] + a:000)
+        if type(ret) == type({})
+          return filter(copy(ret), 'v:key isnot# "changedtick"')
+        else
+          return ret
+        endif
+      endfunction
       let t:testvar='abcd'
       $put =string(gettabvar(1, 'testvar'))
       $put =string(gettabvar(1, 'testvar'))
@@ -20,14 +28,14 @@ describe('context variables', function()
       let def_num = '5678'
       $put =string(getbufvar(1, 'var_num'))
       $put =string(getbufvar(1, 'var_num', def_num))
-      $put =string(getbufvar(1, ''))
-      $put =string(getbufvar(1, '', def_num))
+      $put =string(Getbufscope(1))
+      $put =string(Getbufscope(1, def_num))
       unlet b:var_num
       $put =string(getbufvar(1, 'var_num', def_num))
-      $put =string(getbufvar(1, ''))
-      $put =string(getbufvar(1, '', def_num))
-      $put =string(getbufvar(9, ''))
-      $put =string(getbufvar(9, '', def_num))
+      $put =string(Getbufscope(1))
+      $put =string(Getbufscope(1, def_num))
+      $put =string(Getbufscope(9))
+      $put =string(Getbufscope(9, def_num))
       unlet def_num
       $put =string(getbufvar(1, '&autoindent'))
       $put =string(getbufvar(1, '&autoindent', 1))
diff --git a/test/unit/helpers.lua b/test/unit/helpers.lua
index 1bfdd32739..4af078b486 100644
--- a/test/unit/helpers.lua
+++ b/test/unit/helpers.lua
@@ -79,6 +79,13 @@ local function cimport(...)
 
   -- format it (so that the lines are "unique" statements), also filter out
   -- Objective-C blocks
+  if os.getenv('NVIM_TEST_PRINT_I') == '1' then
+    local lnum = 0
+    for line in body:gmatch('[^\n]+') do
+      lnum = lnum + 1
+      print(lnum, line)
+    end
+  end
   body = formatc(body)
   body = filter_complex_blocks(body)
 
diff --git a/test/unit/preprocess.lua b/test/unit/preprocess.lua
index 4d819e5f23..363358d134 100644
--- a/test/unit/preprocess.lua
+++ b/test/unit/preprocess.lua
@@ -124,6 +124,7 @@ function Gcc:init_defines()
   self:define('_GNU_SOURCE')
   self:define('INCLUDE_GENERATED_DECLARATIONS')
   self:define('UNIT_TESTING')
+  self:define('UNIT_TESTING_LUA_PREPROCESSING')
   -- Needed for FreeBSD
   self:define('_Thread_local', nil, '')
   -- Needed for macOS Sierra