vim-patch:8.2.2658: :for cannot loop over a string

Problem:    :for cannot loop over a string.
Solution:   Accept a string argument and iterate over its characters.
https://github.com/vim/vim/commit/74e54fcb447e5db32f9c2df34c0554bbecdccca2

v8.2.2659 is already ported.

N/A patches for version.c:

vim-patch:8.2.2736: Vim9: for loop over string is a bit slow

Problem:    Vim9: for loop over string is a bit slow.
Solution:   Avoid using strlen().
https://github.com/vim/vim/commit/175a41c13f3e27e30c662f2f418c5a347dbc645d

3 files changed, 59 insertions, 7 deletions

diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index d6486073cf..aa6d994857 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -372,8 +372,8 @@ Changing the order of items in a list: >
 
 For loop ~
 
-The |:for| loop executes commands for each item in a |List| or |Blob|.
-A variable is set to each item in the sequence.  Example with a List: >
+The |:for| loop executes commands for each item in a |List|, |String| or |Blob|.
+A variable is set to each item in sequence.  Example with a List: >
 	:for item in mylist
 	:   call Doit(item)
 	:endfor
@@ -390,7 +390,7 @@ If all you want to do is modify each item in the list then the |map()|
 function will be a simpler method than a for loop.
 
 Just like the |:let| command, |:for| also accepts a list of variables.  This
-requires the argument to be a list of lists. >
+requires the argument to be a List of Lists. >
 	:for [lnum, col] in [[1, 3], [2, 8], [3, 0]]
 	:   call Doit(lnum, col)
 	:endfor
@@ -408,6 +408,12 @@ It is also possible to put remaining items in a List variable: >
 
 For a Blob one byte at a time is used.
 
+For a String one character, including any composing characters, is used as a
+String.  Example: >
+	for c in text
+	  echo 'This character is ' .. c
+	endfor
+
 
 List functions ~
 						*E714*
diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 5d9326eb24..aecb23f251 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -112,9 +112,11 @@ typedef struct {
   int fi_semicolon;             // TRUE if ending in '; var]'
   int fi_varcount;              // nr of variables in the list
   listwatch_T fi_lw;            // keep an eye on the item used.
-  list_T *fi_list;         // list being used
+  list_T *fi_list;              // list being used
   int fi_bi;                    // index of blob
   blob_T *fi_blob;              // blob being used
+  char_u *fi_string;            // copy of string being used
+  int fi_byte_idx;              // byte index in fi_string
 } forinfo_T;
 
 // values for vv_flags:
@@ -2641,6 +2643,13 @@ void *eval_for_line(const char_u *arg, bool *errp, char_u **nextcmdp, int skip)
           fi->fi_blob = btv.vval.v_blob;
         }
         tv_clear(&tv);
+      } else if (tv.v_type == VAR_STRING) {
+        fi->fi_byte_idx = 0;
+        fi->fi_string = tv.vval.v_string;
+        tv.vval.v_string = NULL;
+        if (fi->fi_string == NULL) {
+          fi->fi_string = vim_strsave((char_u *)"");
+        }
       } else {
         emsg(_(e_listblobreq));
         tv_clear(&tv);
@@ -2679,6 +2688,19 @@ bool next_for_item(void *fi_void, char_u *arg)
                        fi->fi_semicolon, fi->fi_varcount, false, NULL) == OK;
   }
 
+  if (fi->fi_string != NULL) {
+    const int len = utfc_ptr2len(fi->fi_string + fi->fi_byte_idx);
+    if (len == 0) {
+      return false;
+    }
+    typval_T tv;
+    tv.v_type = VAR_STRING;
+    tv.v_lock = VAR_FIXED;
+    tv.vval.v_string = vim_strnsave(fi->fi_string + fi->fi_byte_idx, len);
+    fi->fi_byte_idx += len;
+    return ex_let_vars(arg, &tv, true, fi->fi_semicolon, fi->fi_varcount, false, NULL) == OK;
+  }
+
   listitem_T *item = fi->fi_lw.lw_item;
   if (item == NULL) {
     return false;
@@ -2698,12 +2720,16 @@ void free_for_info(void *fi_void)
 {
   forinfo_T *fi = (forinfo_T *)fi_void;
 
-  if (fi != NULL && fi->fi_list != NULL) {
+  if (fi == NULL) {
+    return;
+  }
+  if (fi->fi_list != NULL) {
     tv_list_watch_remove(fi->fi_list, &fi->fi_lw);
     tv_list_unref(fi->fi_list);
-  }
-  if (fi != NULL && fi->fi_blob != NULL) {
+  } else if (fi->fi_blob != NULL) {
     tv_blob_unref(fi->fi_blob);
+  } else {
+    xfree(fi->fi_string);
   }
   xfree(fi);
 }
diff --git a/src/nvim/testdir/test_vimscript.vim b/src/nvim/testdir/test_vimscript.vim
index 34733f127f..c59cab5f36 100644
--- a/src/nvim/testdir/test_vimscript.vim
+++ b/src/nvim/testdir/test_vimscript.vim
@@ -1692,6 +1692,26 @@ func Test_function_defined_line()
     call delete('Xtest.vim')
 endfunc
 
+func Test_for_over_string()
+  let res = ''
+  for c in 'aéc̀d'
+    let res ..= c .. '-'
+  endfor
+  call assert_equal('a-é-c̀-d-', res)
+
+  let res = ''
+  for c in ''
+    let res ..= c .. '-'
+  endfor
+  call assert_equal('', res)
+
+  let res = ''
+  for c in v:_null_string
+    let res ..= c .. '-'
+  endfor
+  call assert_equal('', res)
+endfunc
+
 "-------------------------------------------------------------------------------
 " Modelines								    {{{1
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker