refactor(grid): make screen rendering more multibyte than ever before

Problem: buffer text with composing chars are converted from UTF-8
to an array of up to seven UTF-32 values and then converted back
to UTF-8 strings.

Solution: Convert buffer text directly to UTF-8 based schar_T values.

The limit of the text size is now in schar_T bytes, which is currently
31+1 but easily could be raised as it no longer multiplies the size
of the entire screen grid when not used, the full size is only required
for temporary scratch buffers.

Also does some general cleanup to win_line text handling, which was
unnecessarily complicated due to multibyte rendering being an "opt-in"
feature long ago. Nowadays, a char is just a char, regardless if it consists
of one ASCII byte or multiple bytes.

1 files changed, 8 insertions, 9 deletions

diff --git a/src/nvim/edit.c b/src/nvim/edit.c
index ce547b55fe..eb5ea2c873 100644
--- a/src/nvim/edit.c
+++ b/src/nvim/edit.c
@@ -1462,7 +1462,7 @@ void edit_putchar(int c, bool highlight)
       pc_status = PC_STATUS_SET;
     }
 
-    char buf[MB_MAXBYTES + 1];
+    char buf[MB_MAXCHAR + 1];
     grid_line_puts(pc_col, buf, utf_char2bytes(c, buf), attr);
     grid_line_flush();
   }
@@ -2176,7 +2176,7 @@ void insertchar(int c, int flags, int second_indent)
     int cc;
 
     if ((cc = utf_char2len(c)) > 1) {
-      char buf[MB_MAXBYTES + 1];
+      char buf[MB_MAXCHAR + 1];
 
       utf_char2bytes(c, buf);
       buf[cc] = NUL;
@@ -3681,7 +3681,6 @@ static bool ins_bs(int c, int mode, int *inserted_space_p)
   int cc;
   int temp = 0;                     // init for GCC
   bool did_backspace = false;
-  int cpc[MAX_MCO];                 // composing characters
   bool call_fix_indent = false;
 
   // can't delete anything in an empty file
@@ -3910,15 +3909,15 @@ static bool ins_bs(int c, int mode, int *inserted_space_p)
         if (State & REPLACE_FLAG) {
           replace_do_bs(-1);
         } else {
-          const int l_p_deco = p_deco;
-          if (l_p_deco) {
-            (void)utfc_ptr2char(get_cursor_pos_ptr(), cpc);
+          bool has_composing = false;
+          if (p_deco) {
+            char *p0 = get_cursor_pos_ptr();
+            has_composing = utf_composinglike(p0, p0 + utf_ptr2len(p0));
           }
           (void)del_char(false);
           // If there are combining characters and 'delcombine' is set
-          // move the cursor back.  Don't back up before the base
-          // character.
-          if (l_p_deco && cpc[0] != NUL) {
+          // move the cursor back.  Don't back up before the base character.
+          if (has_composing) {
             inc_cursor();
           }
           if (revins_chars) {