vim-patch:9.1.0810: cannot easily adjust the |:find| command

Problem:  cannot easily adjust the |:find| command
Solution: Add support for the 'findexpr' option (Yegappan Lakshmanan)

closes: vim/vim#15901
closes: vim/vim#15905

https://github.com/vim/vim/commit/aeb1c97db5b9de4f4903e7f288f2aa5ad6c49440

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

19 files changed, 472 insertions, 33 deletions

diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 90f7f56ca2..17b2d22173 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -2598,6 +2598,50 @@ A jump table for the options with a short description can be found at |Q_op|.
 	  eob		EndOfBuffer		|hl-EndOfBuffer|
 	  lastline	NonText			|hl-NonText|
 
+						*'findexpr'* *'fexpr'*
+'findexpr' 'fexpr'	string	(default "")
+			global or local to buffer |global-local|
+	Expression that is evaluated to obtain the filename(s) for the |:find|
+	command.  When this option is empty, the internal |file-searching|
+	mechanism is used.
+
+	While evaluating the expression, the |v:fname| variable is set to the
+	argument of the |:find| command.
+
+	The expression is evaluated only once per |:find| command invocation.
+	The expression can process all the directories specified in 'path'.
+
+	If a match is found, the expression should return a |List| containing
+	one or more file names.  If a match is not found, the expression
+	should return an empty List.
+
+	If any errors are encountered during the expression evaluation, an
+	empty List is used as the return value.
+
+	Using a function call without arguments is faster |expr-option-function|
+
+	It is not allowed to change text or jump to another window while
+	evaluating 'findexpr' |textlock|.
+
+	This option cannot be set from a |modeline| or in the |sandbox|, for
+	security reasons.
+
+	Examples:
+	>vim
+	    " Use glob()
+	    func FindExprGlob()
+		return glob(v:fname, v:false, v:true)
+	    endfunc
+	    set findexpr=FindExprGlob()
+
+	    " Use the 'git ls-files' output
+	    func FindGitFiles()
+		let fnames = systemlist('git ls-files')
+		return fnames->filter('v:val =~? v:fname')
+	    endfunc
+	    set findexpr=FindGitFiles()
+<
+
 		*'fixendofline'* *'fixeol'* *'nofixendofline'* *'nofixeol'*
 'fixendofline' 'fixeol'	boolean	(default on)
 			local to buffer
diff --git a/runtime/doc/quickref.txt b/runtime/doc/quickref.txt
index d77750b485..f64865a031 100644
--- a/runtime/doc/quickref.txt
+++ b/runtime/doc/quickref.txt
@@ -705,6 +705,7 @@ Short explanation of each option:		*option-list*
 'fileignorecase'  'fic'     ignore case when using file names
 'filetype'	  'ft'	    type of file, used for autocommands
 'fillchars'	  'fcs'     characters to use for displaying special items
+'findexpr'	  'fexpr'   expression to evaluate for |:find|
 'fixendofline'	  'fixeol'  make sure last line in file has <EOL>
 'foldclose'	  'fcl'     close a fold when the cursor leaves it
 'foldcolumn'	  'fdc'     width of the column used to indicate folds
diff --git a/runtime/doc/vvars.txt b/runtime/doc/vvars.txt
index 15d836a83d..3f9ce8cc96 100644
--- a/runtime/doc/vvars.txt
+++ b/runtime/doc/vvars.txt
@@ -254,7 +254,8 @@ v:fcs_reason
 					*v:fname* *fname-variable*
 v:fname
 		When evaluating 'includeexpr': the file name that was
-		detected.  Empty otherwise.
+		detected.  When evaluating 'findexpr': the argument passed to
+		the |:find| command.  Empty otherwise.
 
 				*v:fname_diff* *fname_diff-variable*
 v:fname_diff
diff --git a/runtime/lua/vim/_meta/options.lua b/runtime/lua/vim/_meta/options.lua
index 00f7554832..8d5601ff6a 100644
--- a/runtime/lua/vim/_meta/options.lua
+++ b/runtime/lua/vim/_meta/options.lua
@@ -2294,6 +2294,57 @@ vim.wo.fcs = vim.wo.fillchars
 vim.go.fillchars = vim.o.fillchars
 vim.go.fcs = vim.go.fillchars
 
+--- Expression that is evaluated to obtain the filename(s) for the `:find`
+--- command.  When this option is empty, the internal `file-searching`
+--- mechanism is used.
+---
+--- While evaluating the expression, the `v:fname` variable is set to the
+--- argument of the `:find` command.
+---
+--- The expression is evaluated only once per `:find` command invocation.
+--- The expression can process all the directories specified in 'path'.
+---
+--- If a match is found, the expression should return a `List` containing
+--- one or more file names.  If a match is not found, the expression
+--- should return an empty List.
+---
+--- If any errors are encountered during the expression evaluation, an
+--- empty List is used as the return value.
+---
+--- Using a function call without arguments is faster `expr-option-function`
+---
+--- It is not allowed to change text or jump to another window while
+--- evaluating 'findexpr' `textlock`.
+---
+--- This option cannot be set from a `modeline` or in the `sandbox`, for
+--- security reasons.
+---
+--- Examples:
+---
+--- ```vim
+---     " Use glob()
+---     func FindExprGlob()
+--- 	return glob(v:fname, v:false, v:true)
+---     endfunc
+---     set findexpr=FindExprGlob()
+---
+---     " Use the 'git ls-files' output
+---     func FindGitFiles()
+--- 	let fnames = systemlist('git ls-files')
+--- 	return fnames->filter('v:val =~? v:fname')
+---     endfunc
+---     set findexpr=FindGitFiles()
+--- ```
+---
+---
+--- @type string
+vim.o.findexpr = ""
+vim.o.fexpr = vim.o.findexpr
+vim.bo.findexpr = vim.o.findexpr
+vim.bo.fexpr = vim.bo.findexpr
+vim.go.findexpr = vim.o.findexpr
+vim.go.fexpr = vim.go.findexpr
+
 --- When writing a file and this option is on, <EOL> at the end of file
 --- will be restored if missing.  Turn this option off if you want to
 --- preserve the situation from the original file.
diff --git a/runtime/lua/vim/_meta/vvars.lua b/runtime/lua/vim/_meta/vvars.lua
index e00402ab3f..cba200101b 100644
--- a/runtime/lua/vim/_meta/vvars.lua
+++ b/runtime/lua/vim/_meta/vvars.lua
@@ -267,7 +267,8 @@ vim.v.fcs_choice = ...
 vim.v.fcs_reason = ...
 
 --- When evaluating 'includeexpr': the file name that was
---- detected.  Empty otherwise.
+--- detected.  When evaluating 'findexpr': the argument passed to
+--- the `:find` command.  Empty otherwise.
 --- @type string
 vim.v.fname = ...
 
diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index 2142b5b298..42cc745fe6 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -2049,6 +2049,7 @@ void free_buf_options(buf_T *buf, bool free_p_ff)
   clear_string_option(&buf->b_p_indk);
   clear_string_option(&buf->b_p_fp);
   clear_string_option(&buf->b_p_fex);
+  clear_string_option(&buf->b_p_fexpr);
   clear_string_option(&buf->b_p_kp);
   clear_string_option(&buf->b_p_mps);
   clear_string_option(&buf->b_p_fo);
diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 134d69de96..88e8d59faa 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -608,6 +608,7 @@ struct file_buffer {
   char *b_p_mp;                 ///< 'makeprg' local value
   char *b_p_efm;                ///< 'errorformat' local value
   char *b_p_ep;                 ///< 'equalprg' local value
+  char *b_p_fexpr;              ///< 'findexpr' local value
   char *b_p_path;               ///< 'path' local value
   int b_p_ar;                   ///< 'autoread' local value
   char *b_p_tags;               ///< 'tags' local value
diff --git a/src/nvim/errors.h b/src/nvim/errors.h
index 39095db952..bea56541a2 100644
--- a/src/nvim/errors.h
+++ b/src/nvim/errors.h
@@ -156,6 +156,11 @@ EXTERN const char e_luv_api_disabled[] INIT(= N_("E5560: %s must not be called i
 EXTERN const char e_floatonly[] INIT(= N_("E5601: Cannot close window, only floating window would remain"));
 EXTERN const char e_floatexchange[] INIT(= N_("E5602: Cannot exchange or rotate float"));
 
+EXTERN const char e_cant_find_directory_str_in_cdpath[] INIT(= N_("E344: Can't find directory \"%s\" in cdpath"));
+EXTERN const char e_cant_find_file_str_in_path[] INIT(= N_("E345: Can't find file \"%s\" in path"));
+EXTERN const char e_no_more_directory_str_found_in_cdpath[] INIT(= N_("E346: No more directory \"%s\" found in cdpath"));
+EXTERN const char e_no_more_file_str_found_in_path[] INIT(= N_("E347: No more file \"%s\" found in path"));
+
 EXTERN const char e_cannot_define_autocommands_for_all_events[] INIT(= N_("E1155: Cannot define autocommands for ALL events"));
 
 EXTERN const char e_resulting_text_too_long[] INIT(= N_("E1240: Resulting text too long"));
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 93cff80bd4..6a455f70a6 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -2631,7 +2631,7 @@ static int may_call_simple_func(const char *arg, typval_T *rettv)
 
 /// Handle zero level expression with optimization for a simple function call.
 /// Same arguments and return value as eval0().
-static int eval0_simple_funccal(char *arg, typval_T *rettv, exarg_T *eap, evalarg_T *const evalarg)
+int eval0_simple_funccal(char *arg, typval_T *rettv, exarg_T *eap, evalarg_T *const evalarg)
 {
   int r = may_call_simple_func(arg, rettv);
 
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 293aaac036..5c61399003 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -5165,6 +5165,90 @@ static void ex_wrongmodifier(exarg_T *eap)
   eap->errmsg = _(e_invcmd);
 }
 
+/// Evaluate the 'findexpr' expression and return the result.  When evaluating
+/// the expression, v:fname is set to the ":find" command argument.
+static list_T *eval_findexpr(const char *ptr, size_t len)
+{
+  const sctx_T saved_sctx = current_sctx;
+  bool use_sandbox = false;
+
+  char *findexpr;
+  if (*curbuf->b_p_fexpr == NUL) {
+    use_sandbox = was_set_insecurely(curwin, kOptFindexpr, OPT_GLOBAL);
+    findexpr = p_fexpr;
+  } else {
+    use_sandbox = was_set_insecurely(curwin, kOptFindexpr, OPT_LOCAL);
+    findexpr = curbuf->b_p_fexpr;
+  }
+
+  set_vim_var_string(VV_FNAME, ptr, (ptrdiff_t)len);
+  current_sctx = curbuf->b_p_script_ctx[BV_FEXPR].script_ctx;
+
+  char *arg = skipwhite(findexpr);
+
+  if (use_sandbox) {
+    sandbox++;
+  }
+  textlock++;
+
+  // Evaluate the expression.  If the expression is "FuncName()" call the
+  // function directly.
+  typval_T tv;
+  list_T *retlist = NULL;
+  if (eval0_simple_funccal(arg, &tv, NULL, &EVALARG_EVALUATE) == FAIL) {
+    retlist = NULL;
+  } else {
+    if (tv.v_type == VAR_LIST) {
+      retlist = tv_list_copy(NULL, tv.vval.v_list, true, get_copyID());
+    }
+    tv_clear(&tv);
+  }
+  if (use_sandbox) {
+    sandbox--;
+  }
+  textlock--;
+  clear_evalarg(&EVALARG_EVALUATE, NULL);
+
+  set_vim_var_string(VV_FNAME, NULL, 0);
+  current_sctx = saved_sctx;
+
+  return retlist;
+}
+
+/// Use 'findexpr' to find file 'findarg'.  The 'count' argument is used to find
+/// the n'th matching file.
+static char *findexpr_find_file(char *findarg, size_t findarg_len, int count)
+{
+  char *ret_fname = NULL;
+
+  const char cc = findarg[findarg_len];
+  findarg[findarg_len] = NUL;
+
+  list_T *fname_list = eval_findexpr(findarg, findarg_len);
+  int fname_count = tv_list_len(fname_list);
+
+  if (fname_count == 0) {
+    semsg(_(e_cant_find_file_str_in_path), findarg);
+  } else {
+    if (count > fname_count) {
+      semsg(_(e_no_more_file_str_found_in_path), findarg);
+    } else {
+      listitem_T *li = tv_list_find(fname_list, count - 1);
+      if (li != NULL && TV_LIST_ITEM_TV(li)->v_type == VAR_STRING) {
+        ret_fname = xstrdup(TV_LIST_ITEM_TV(li)->vval.v_string);
+      }
+    }
+  }
+
+  if (fname_list != NULL) {
+    tv_list_free(fname_list);
+  }
+
+  findarg[findarg_len] = cc;
+
+  return ret_fname;
+}
+
 /// :sview [+command] file       split window with new file, read-only
 /// :split [[+command] file]     split window with current or new file
 /// :vsplit [[+command] file]    split window vertically with current or new file
@@ -5196,13 +5280,17 @@ void ex_splitview(exarg_T *eap)
   }
 
   if (eap->cmdidx == CMD_sfind || eap->cmdidx == CMD_tabfind) {
-    char *file_to_find = NULL;
-    char *search_ctx = NULL;
-    fname = find_file_in_path(eap->arg, strlen(eap->arg),
-                              FNAME_MESS, true, curbuf->b_ffname,
-                              &file_to_find, &search_ctx);
-    xfree(file_to_find);
-    vim_findfile_cleanup(search_ctx);
+    if (*get_findexpr() != NUL) {
+      fname = findexpr_find_file(eap->arg, strlen(eap->arg),
+                                 eap->addr_count > 0 ? eap->line2 : 1);
+    } else {
+      char *file_to_find = NULL;
+      char *search_ctx = NULL;
+      fname = find_file_in_path(eap->arg, strlen(eap->arg), FNAME_MESS, true,
+                                curbuf->b_ffname, &file_to_find, &search_ctx);
+      xfree(file_to_find);
+      vim_findfile_cleanup(search_ctx);
+    }
     if (fname == NULL) {
       goto theend;
     }
@@ -5398,23 +5486,28 @@ static void ex_find(exarg_T *eap)
     return;
   }
 
-  char *file_to_find = NULL;
-  char *search_ctx = NULL;
-  char *fname = find_file_in_path(eap->arg, strlen(eap->arg),
-                                  FNAME_MESS, true, curbuf->b_ffname,
-                                  &file_to_find, &search_ctx);
-  if (eap->addr_count > 0) {
-    // Repeat finding the file "count" times.  This matters when it appears
-    // several times in the path.
-    linenr_T count = eap->line2;
-    while (fname != NULL && --count > 0) {
-      xfree(fname);
-      fname = find_file_in_path(NULL, 0, FNAME_MESS, false, curbuf->b_ffname,
-                                &file_to_find, &search_ctx);
+  char *fname = NULL;
+  if (*get_findexpr() != NUL) {
+    fname = findexpr_find_file(eap->arg, strlen(eap->arg),
+                               eap->addr_count > 0 ? eap->line2 : 1);
+  } else {
+    char *file_to_find = NULL;
+    char *search_ctx = NULL;
+    fname = find_file_in_path(eap->arg, strlen(eap->arg), FNAME_MESS, true,
+                              curbuf->b_ffname, &file_to_find, &search_ctx);
+    if (eap->addr_count > 0) {
+      // Repeat finding the file "count" times.  This matters when it appears
+      // several times in the path.
+      linenr_T count = eap->line2;
+      while (fname != NULL && --count > 0) {
+        xfree(fname);
+        fname = find_file_in_path(NULL, 0, FNAME_MESS, false,
+                                  curbuf->b_ffname, &file_to_find, &search_ctx);
+      }
     }
+    xfree(file_to_find);
+    vim_findfile_cleanup(search_ctx);
   }
-  xfree(file_to_find);
-  vim_findfile_cleanup(search_ctx);
 
   if (fname == NULL) {
     return;
diff --git a/src/nvim/file_search.c b/src/nvim/file_search.c
index cdfd281718..aeaf448a05 100644
--- a/src/nvim/file_search.c
+++ b/src/nvim/file_search.c
@@ -1489,15 +1489,15 @@ char *find_file_in_path_option(char *ptr, size_t len, int options, int first, ch
   if (file_name == NULL && (options & FNAME_MESS)) {
     if (first == true) {
       if (find_what == FINDFILE_DIR) {
-        semsg(_("E344: Can't find directory \"%s\" in cdpath"), *file_to_find);
+        semsg(_(e_cant_find_directory_str_in_cdpath), *file_to_find);
       } else {
-        semsg(_("E345: Can't find file \"%s\" in path"), *file_to_find);
+        semsg(_(e_cant_find_file_str_in_path), *file_to_find);
       }
     } else {
       if (find_what == FINDFILE_DIR) {
-        semsg(_("E346: No more directory \"%s\" found in cdpath"), *file_to_find);
+        semsg(_(e_no_more_directory_str_found_in_cdpath), *file_to_find);
       } else {
-        semsg(_("E347: No more file \"%s\" found in path"), *file_to_find);
+        semsg(_(e_no_more_file_str_found_in_path), *file_to_find);
       }
     }
   }
diff --git a/src/nvim/option.c b/src/nvim/option.c
index 7c32c99d0e..65f03ca77f 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -4530,6 +4530,8 @@ void *get_varp_scope_from(vimoption_T *p, int scope, buf_T *buf, win_T *win)
     switch ((int)p->indir) {
     case PV_FP:
       return &(buf->b_p_fp);
+    case PV_FEXPR:
+      return &(buf->b_p_fexpr);
     case PV_EFM:
       return &(buf->b_p_efm);
     case PV_GP:
@@ -4651,6 +4653,8 @@ void *get_varp_from(vimoption_T *p, buf_T *buf, win_T *win)
     return *buf->b_p_tsrfu != NUL ? &(buf->b_p_tsrfu) : p->var;
   case PV_FP:
     return *buf->b_p_fp != NUL ? &(buf->b_p_fp) : p->var;
+  case PV_FEXPR:
+    return *buf->b_p_fexpr != NUL ? &(buf->b_p_fexpr) : p->var;
   case PV_EFM:
     return *buf->b_p_efm != NUL ? &(buf->b_p_efm) : p->var;
   case PV_GP:
@@ -4922,6 +4926,15 @@ char *get_equalprg(void)
   return curbuf->b_p_ep;
 }
 
+/// Get the value of 'findexpr', either the buffer-local one or the global one.
+char *get_findexpr(void)
+{
+  if (*curbuf->b_p_fexpr == NUL) {
+    return p_fexpr;
+  }
+  return curbuf->b_p_fexpr;
+}
+
 /// Copy options from one window to another.
 /// Used when splitting a window.
 void win_copy_options(win_T *wp_from, win_T *wp_to)
@@ -5320,6 +5333,8 @@ void buf_copy_options(buf_T *buf, int flags)
       buf->b_p_mp = empty_string_option;
       buf->b_p_efm = empty_string_option;
       buf->b_p_ep = empty_string_option;
+      buf->b_p_fexpr = xstrdup(p_fexpr);
+      COPY_OPT_SCTX(buf, BV_FEXPR);
       buf->b_p_kp = empty_string_option;
       buf->b_p_path = empty_string_option;
       buf->b_p_tags = empty_string_option;
diff --git a/src/nvim/option_vars.h b/src/nvim/option_vars.h
index 5b630117ab..a88b51dae7 100644
--- a/src/nvim/option_vars.h
+++ b/src/nvim/option_vars.h
@@ -451,6 +451,7 @@ EXTERN char *p_ffs;             ///< 'fileformats'
 EXTERN int p_fic;               ///< 'fileignorecase'
 EXTERN char *p_ft;              ///< 'filetype'
 EXTERN char *p_fcs;             ///< 'fillchar'
+EXTERN char *p_fexpr;           ///< 'findexpr'
 EXTERN int p_fixeol;            ///< 'fixendofline'
 EXTERN char *p_fcl;             ///< 'foldclose'
 EXTERN OptInt p_fdls;           ///< 'foldlevelstart'
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index a891d18364..65401fb3a9 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -2906,6 +2906,59 @@ return {
       varname = 'p_fcs',
     },
     {
+      abbreviation = 'fexpr',
+      cb = 'did_set_optexpr',
+      defaults = { if_true = '' },
+      desc = [=[
+        Expression that is evaluated to obtain the filename(s) for the |:find|
+        command.  When this option is empty, the internal |file-searching|
+        mechanism is used.
+
+        While evaluating the expression, the |v:fname| variable is set to the
+        argument of the |:find| command.
+
+        The expression is evaluated only once per |:find| command invocation.
+        The expression can process all the directories specified in 'path'.
+
+        If a match is found, the expression should return a |List| containing
+        one or more file names.  If a match is not found, the expression
+        should return an empty List.
+
+        If any errors are encountered during the expression evaluation, an
+        empty List is used as the return value.
+
+        Using a function call without arguments is faster |expr-option-function|
+
+        It is not allowed to change text or jump to another window while
+        evaluating 'findexpr' |textlock|.
+
+        This option cannot be set from a |modeline| or in the |sandbox|, for
+        security reasons.
+
+        Examples:
+        >vim
+            " Use glob()
+            func FindExprGlob()
+        	return glob(v:fname, v:false, v:true)
+            endfunc
+            set findexpr=FindExprGlob()
+
+            " Use the 'git ls-files' output
+            func FindGitFiles()
+        	let fnames = systemlist('git ls-files')
+        	return fnames->filter('v:val =~? v:fname')
+            endfunc
+            set findexpr=FindGitFiles()
+        <
+      ]=],
+      full_name = 'findexpr',
+      scope = { 'global', 'buffer' },
+      secure = true,
+      short_desc = N_('expression used for :find'),
+      type = 'string',
+      varname = 'p_fexpr',
+    },
+    {
       abbreviation = 'fixeol',
       cb = 'did_set_eof_eol_fixeol_bomb',
       defaults = { if_true = true },
diff --git a/src/nvim/optionstr.c b/src/nvim/optionstr.c
index e07fcf2f0e..c66849800c 100644
--- a/src/nvim/optionstr.c
+++ b/src/nvim/optionstr.c
@@ -233,6 +233,7 @@ void check_buf_options(buf_T *buf)
   check_string_option(&buf->b_p_mp);
   check_string_option(&buf->b_p_efm);
   check_string_option(&buf->b_p_ep);
+  check_string_option(&buf->b_p_fexpr);
   check_string_option(&buf->b_p_path);
   check_string_option(&buf->b_p_tags);
   check_string_option(&buf->b_p_tfu);
@@ -1885,8 +1886,9 @@ int expand_set_nrformats(optexpand_T *args, int *numMatches, char ***matches)
                                matches);
 }
 
-/// One of the '*expr' options is changed:, 'diffexpr', 'foldexpr', 'foldtext',
-/// 'formatexpr', 'includeexpr', 'indentexpr', 'patchexpr' and 'charconvert'.
+/// One of the '*expr' options is changed:, 'diffexpr', 'findexpr',
+/// 'foldexpr', 'foldtext', 'formatexpr', 'includeexpr', 'indentexpr',
+/// 'patchexpr' and 'charconvert'.
 const char *did_set_optexpr(optset_T *args)
 {
   char **varp = (char **)args->os_varp;
diff --git a/src/nvim/vvars.lua b/src/nvim/vvars.lua
index ad139bbbfe..5c9f0f9e27 100644
--- a/src/nvim/vvars.lua
+++ b/src/nvim/vvars.lua
@@ -284,7 +284,8 @@ M.vars = {
     type = 'string',
     desc = [=[
       When evaluating 'includeexpr': the file name that was
-      detected.  Empty otherwise.
+      detected.  When evaluating 'findexpr': the argument passed to
+      the |:find| command.  Empty otherwise.
     ]=],
   },
   fname_diff = {
diff --git a/test/old/testdir/test_findfile.vim b/test/old/testdir/test_findfile.vim
index 06d781ed69..baf33898fe 100644
--- a/test/old/testdir/test_findfile.vim
+++ b/test/old/testdir/test_findfile.vim
@@ -1,5 +1,7 @@
 " Test findfile() and finddir()
 
+source check.vim
+
 let s:files = [ 'Xfinddir1/foo',
       \         'Xfinddir1/bar',
       \         'Xfinddir1/Xdir2/foo',
@@ -286,4 +288,170 @@ func Test_find_non_existing_path()
   let &path = save_path
 endfunc
 
+" Test for 'findexpr'
+func Test_findexpr()
+  CheckUnix
+  call assert_equal('', &findexpr)
+  call writefile(['aFile'], 'Xfindexpr1.c', 'D')
+  call writefile(['bFile'], 'Xfindexpr2.c', 'D')
+  call writefile(['cFile'], 'Xfindexpr3.c', 'D')
+
+  " basic tests
+  func FindExpr1()
+    let fnames = ['Xfindexpr1.c', 'Xfindexpr2.c', 'Xfindexpr3.c']
+    "return fnames->copy()->filter('v:val =~? v:fname')->join("\n")
+    return fnames->copy()->filter('v:val =~? v:fname')
+  endfunc
+
+  set findexpr=FindExpr1()
+  find Xfindexpr3
+  call assert_match('Xfindexpr3.c', @%)
+  bw!
+  2find Xfind
+  call assert_match('Xfindexpr2.c', @%)
+  bw!
+  call assert_fails('4find Xfind', 'E347: No more file "Xfind" found in path')
+  call assert_fails('find foobar', 'E345: Can''t find file "foobar" in path')
+
+  sfind Xfindexpr2.c
+  call assert_match('Xfindexpr2.c', @%)
+  call assert_equal(2, winnr('$'))
+  %bw!
+  call assert_fails('sfind foobar', 'E345: Can''t find file "foobar" in path')
+
+  tabfind Xfindexpr3.c
+  call assert_match('Xfindexpr3.c', @%)
+  call assert_equal(2, tabpagenr())
+  %bw!
+  call assert_fails('tabfind foobar', 'E345: Can''t find file "foobar" in path')
+
+  " Buffer-local option
+  set findexpr=['abc']
+  new
+  setlocal findexpr=['def']
+  find xxxx
+  call assert_equal('def', @%)
+  wincmd w
+  find xxxx
+  call assert_equal('abc', @%)
+  aboveleft new
+  call assert_equal("['abc']", &findexpr)
+  wincmd k
+  aboveleft new
+  call assert_equal("['abc']", &findexpr)
+  %bw!
+
+  " Empty list
+  set findexpr=[]
+  call assert_fails('find xxxx', 'E345: Can''t find file "xxxx" in path')
+
+  " Error cases
+
+  " Syntax error in the expression
+  set findexpr=FindExpr1{}
+  call assert_fails('find Xfindexpr1.c', 'E15: Invalid expression')
+
+  " Find expression throws an error
+  func FindExpr2()
+    throw 'find error'
+  endfunc
+  set findexpr=FindExpr2()
+  call assert_fails('find Xfindexpr1.c', 'find error')
+
+  " Try using a null string as the expression
+  set findexpr=v:_null_string
+  call assert_fails('find Xfindexpr1.c', 'E345: Can''t find file "Xfindexpr1.c" in path')
+
+  " Try to create a new window from the find expression
+  func FindExpr3()
+    new
+    return ["foo"]
+  endfunc
+  set findexpr=FindExpr3()
+  call assert_fails('find Xfindexpr1.c', 'E565: Not allowed to change text or change window')
+
+  " Try to modify the current buffer from the find expression
+  func FindExpr4()
+    call setline(1, ['abc'])
+    return ["foo"]
+  endfunc
+  set findexpr=FindExpr4()
+  call assert_fails('find Xfindexpr1.c', 'E565: Not allowed to change text or change window')
+
+  set findexpr&
+  delfunc! FindExpr1
+  delfunc! FindExpr2
+  delfunc! FindExpr3
+  delfunc! FindExpr4
+endfunc
+
+" Test for using a script-local function for 'findexpr'
+func Test_findexpr_scriptlocal_func()
+  func! s:FindExprScript()
+    let g:FindExprArg = v:fname
+    return ['xxx']
+  endfunc
+
+  set findexpr=s:FindExprScript()
+  call assert_equal(expand('<SID>') .. 'FindExprScript()', &findexpr)
+  call assert_equal(expand('<SID>') .. 'FindExprScript()', &g:findexpr)
+  new | only
+  let g:FindExprArg = ''
+  find abc
+  call assert_equal('abc', g:FindExprArg)
+  bw!
+
+  set findexpr=<SID>FindExprScript()
+  call assert_equal(expand('<SID>') .. 'FindExprScript()', &findexpr)
+  call assert_equal(expand('<SID>') .. 'FindExprScript()', &g:findexpr)
+  new | only
+  let g:FindExprArg = ''
+  find abc
+  call assert_equal('abc', g:FindExprArg)
+  bw!
+
+  let &findexpr = 's:FindExprScript()'
+  call assert_equal(expand('<SID>') .. 'FindExprScript()', &g:findexpr)
+  new | only
+  let g:FindExprArg = ''
+  find abc
+  call assert_equal('abc', g:FindExprArg)
+  bw!
+
+  let &findexpr = '<SID>FindExprScript()'
+  call assert_equal(expand('<SID>') .. 'FindExprScript()', &g:findexpr)
+  new | only
+  let g:FindExprArg = ''
+  find abc
+  call assert_equal('abc', g:FindExprArg)
+  bw!
+
+  set findexpr=
+  setglobal findexpr=s:FindExprScript()
+  setlocal findexpr=
+  call assert_equal(expand('<SID>') .. 'FindExprScript()', &findexpr)
+  call assert_equal(expand('<SID>') .. 'FindExprScript()', &g:findexpr)
+  call assert_equal('', &l:findexpr)
+  new | only
+  let g:FindExprArg = ''
+  find abc
+  call assert_equal('abc', g:FindExprArg)
+  bw!
+
+  new | only
+  set findexpr=
+  setglobal findexpr=
+  setlocal findexpr=s:FindExprScript()
+  call assert_equal(expand('<SID>') .. 'FindExprScript()', &findexpr)
+  call assert_equal(expand('<SID>') .. 'FindExprScript()', &l:findexpr)
+  call assert_equal('', &g:findexpr)
+  let g:FindExprArg = ''
+  find abc
+  call assert_equal('abc', g:FindExprArg)
+  bw!
+
+  set findexpr=
+  delfunc s:FindExprScript
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/test/old/testdir/test_modeline.vim b/test/old/testdir/test_modeline.vim
index 487a89e038..7b7163d372 100644
--- a/test/old/testdir/test_modeline.vim
+++ b/test/old/testdir/test_modeline.vim
@@ -217,6 +217,7 @@ func Test_modeline_fails_always()
   call s:modeline_fails('equalprg', 'equalprg=Something()', 'E520:')
   call s:modeline_fails('errorfile', 'errorfile=Something()', 'E520:')
   call s:modeline_fails('exrc', 'exrc=Something()', 'E520:')
+  call s:modeline_fails('findexpr', 'findexpr=Something()', 'E520:')
   call s:modeline_fails('formatprg', 'formatprg=Something()', 'E520:')
   call s:modeline_fails('fsync', 'fsync=Something()', 'E520:')
   call s:modeline_fails('grepprg', 'grepprg=Something()', 'E520:')
diff --git a/test/old/testdir/test_options.vim b/test/old/testdir/test_options.vim
index ba93778404..6902560518 100644
--- a/test/old/testdir/test_options.vim
+++ b/test/old/testdir/test_options.vim
@@ -1559,7 +1559,7 @@ endfunc
 
 " Test for changing options in a sandbox
 func Test_opt_sandbox()
-  for opt in ['backupdir', 'cdpath', 'exrc']
+  for opt in ['backupdir', 'cdpath', 'exrc', 'findexpr']
     call assert_fails('sandbox set ' .. opt .. '?', 'E48:')
     call assert_fails('sandbox let &' .. opt .. ' = 1', 'E48:')
   endfor