feat(statusline): support multibyte fillchar

This includes a partial port of Vim patch 8.2.2569 and some changes to
nvim_eval_statusline() to allow a multibyte fillchar. Literally every
line of C code touched by that patch has been refactored in Nvim, and
that patch contains some irrelevant foldcolumn tests I'm not sure how to
port (as Nvim's foldcolumn behavior has diverged from Vim's).

5 files changed, 46 insertions, 29 deletions

diff --git a/src/nvim/api/vim.c b/src/nvim/api/vim.c
index 59db12f2c0..7ef8faa67e 100644
--- a/src/nvim/api/vim.c
+++ b/src/nvim/api/vim.c
@@ -19,6 +19,7 @@
 #include "nvim/ascii.h"
 #include "nvim/buffer.h"
 #include "nvim/buffer_defs.h"
+#include "nvim/charset.h"
 #include "nvim/context.h"
 #include "nvim/decoration.h"
 #include "nvim/edit.h"
@@ -2234,7 +2235,7 @@ Dictionary nvim_eval_statusline(String str, Dict(eval_statusline) *opts, Error *
   Dictionary result = ARRAY_DICT_INIT;
 
   int maxwidth;
-  char fillchar = 0;
+  int fillchar = 0;
   Window window = 0;
   bool use_tabline = false;
   bool highlights = false;
@@ -2249,12 +2250,12 @@ Dictionary nvim_eval_statusline(String str, Dict(eval_statusline) *opts, Error *
   }
 
   if (HAS_KEY(opts->fillchar)) {
-    if (opts->fillchar.type != kObjectTypeString || opts->fillchar.data.string.size > 1) {
-      api_set_error(err, kErrorTypeValidation, "fillchar must be an ASCII character");
+    if (opts->fillchar.type != kObjectTypeString || opts->fillchar.data.string.size == 0
+        || char2cells(fillchar = utf_ptr2char((char_u *)opts->fillchar.data.string.data)) != 1
+        || (size_t)utf_char2len(fillchar) != opts->fillchar.data.string.size) {
+      api_set_error(err, kErrorTypeValidation, "fillchar must be a single-width character");
       return result;
     }
-
-    fillchar = opts->fillchar.data.string.data[0];
   }
 
   if (HAS_KEY(opts->highlights)) {
@@ -2285,7 +2286,7 @@ Dictionary nvim_eval_statusline(String str, Dict(eval_statusline) *opts, Error *
 
     if (fillchar == 0) {
       int attr;
-      fillchar = (char)fillchar_status(&attr, wp);
+      fillchar = fillchar_status(&attr, wp);
     }
   }
 
@@ -2313,7 +2314,7 @@ Dictionary nvim_eval_statusline(String str, Dict(eval_statusline) *opts, Error *
                                sizeof(buf),
                                (char_u *)str.data,
                                false,
-                               (char_u)fillchar,
+                               fillchar,
                                maxwidth,
                                hltab_ptr,
                                NULL);
diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index abd22fba26..9a02dff23b 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -3418,7 +3418,7 @@ typedef enum {
 ///
 /// @return The final width of the statusline
 int build_stl_str_hl(win_T *wp, char_u *out, size_t outlen, char_u *fmt, int use_sandbox,
-                     char_u fillchar, int maxwidth, stl_hlrec_t **hltab, StlClickRecord **tabtab)
+                     int fillchar, int maxwidth, stl_hlrec_t **hltab, StlClickRecord **tabtab)
 {
   static size_t stl_items_len = 20;  // Initial value, grows as needed.
   static stl_item_t *stl_items = NULL;
@@ -3461,9 +3461,6 @@ int build_stl_str_hl(win_T *wp, char_u *out, size_t outlen, char_u *fmt, int use
 
   if (fillchar == 0) {
     fillchar = ' ';
-  } else if (utf_char2len(fillchar) > 1) {
-    // Can't handle a multi-byte fill character yet.
-    fillchar = '-';
   }
 
   // The cursor in windows other than the current one isn't always
@@ -3661,7 +3658,7 @@ int build_stl_str_hl(win_T *wp, char_u *out, size_t outlen, char_u *fmt, int use
         out_p = out_p - n + 1;
         // Fill up space left over by half a double-wide char.
         while (++group_len < stl_items[stl_groupitems[groupdepth]].minwid) {
-          *out_p++ = fillchar;
+          MB_CHAR2BYTES(fillchar, out_p);
         }
         // }
 
@@ -3684,14 +3681,14 @@ int build_stl_str_hl(win_T *wp, char_u *out, size_t outlen, char_u *fmt, int use
         if (min_group_width < 0) {
           min_group_width = 0 - min_group_width;
           while (group_len++ < min_group_width && out_p < out_end_p) {
-            *out_p++ = fillchar;
+            MB_CHAR2BYTES(fillchar, out_p);
           }
           // If the group is right-aligned, shift everything to the right and
           // prepend with filler characters.
         } else {
           // { Move the group to the right
-          memmove(t + min_group_width - group_len, t, (size_t)(out_p - t));
-          group_len = min_group_width - group_len;
+          group_len = (min_group_width - group_len) * utf_char2len(fillchar);
+          memmove(t + group_len, t, (size_t)(out_p - t));
           if (out_p + group_len >= (out_end_p + 1)) {
             group_len = (long)(out_end_p - out_p);
           }
@@ -3705,7 +3702,7 @@ int build_stl_str_hl(win_T *wp, char_u *out, size_t outlen, char_u *fmt, int use
 
           // Prepend the fill characters
           for (; group_len > 0; group_len--) {
-            *t++ = fillchar;
+            MB_CHAR2BYTES(fillchar, t);
           }
         }
       }
@@ -4237,7 +4234,7 @@ int build_stl_str_hl(win_T *wp, char_u *out, size_t outlen, char_u *fmt, int use
           if (l + 1 == minwid && fillchar == '-' && ascii_isdigit(*t)) {
             *out_p++ = ' ';
           } else {
-            *out_p++ = fillchar;
+            MB_CHAR2BYTES(fillchar, out_p);
           }
         }
         minwid = 0;
@@ -4248,20 +4245,21 @@ int build_stl_str_hl(win_T *wp, char_u *out, size_t outlen, char_u *fmt, int use
       }
 
       // { Copy the string text into the output buffer
-      while (*t && out_p < out_end_p) {
-        *out_p++ = *t++;
+      for (; *t && out_p < out_end_p; t++) {
         // Change a space by fillchar, unless fillchar is '-' and a
         // digit follows.
-        if (fillable && out_p[-1] == ' '
-            && (!ascii_isdigit(*t) || fillchar != '-')) {
-          out_p[-1] = fillchar;
+        if (fillable && *t == ' '
+            && (!ascii_isdigit(*(t + 1)) || fillchar != '-')) {
+          MB_CHAR2BYTES(fillchar, out_p);
+        } else {
+          *out_p++ = *t;
         }
       }
       // }
 
       // For left-aligned items, fill any remaining space with the fillchar
       for (; l < minwid && out_p < out_end_p; l++) {
-        *out_p++ = fillchar;
+        MB_CHAR2BYTES(fillchar, out_p);
       }
 
       // Otherwise if the item is a number, copy that to the output buffer.
@@ -4454,7 +4452,7 @@ int build_stl_str_hl(win_T *wp, char_u *out, size_t outlen, char_u *fmt, int use
 
       // Fill up for half a double-wide character.
       while (++width < maxwidth) {
-        *trunc_p++ = fillchar;
+        MB_CHAR2BYTES(fillchar, trunc_p);
         *trunc_p = NUL;
       }
       // }
@@ -4505,13 +4503,13 @@ int build_stl_str_hl(win_T *wp, char_u *out, size_t outlen, char_u *fmt, int use
                          standard_spaces * (num_separators - 1);
 
       for (int i = 0; i < num_separators; i++) {
-        int dislocation = (i == (num_separators - 1))
-                          ? final_spaces : standard_spaces;
+        int dislocation = (i == (num_separators - 1)) ? final_spaces : standard_spaces;
+        dislocation *= utf_char2len(fillchar);
         char_u *start = stl_items[stl_separator_locations[i]].start;
         char_u *seploc = start + dislocation;
         STRMOVE(seploc, start);
-        for (char_u *s = start; s < seploc; s++) {
-          *s = fillchar;
+        for (char_u *s = start; s < seploc; ) {
+          MB_CHAR2BYTES(fillchar, s);
         }
 
         for (int item_idx = stl_separator_locations[i] + 1;
diff --git a/src/nvim/macros.h b/src/nvim/macros.h
index c2b2c89abf..d5611f587b 100644
--- a/src/nvim/macros.h
+++ b/src/nvim/macros.h
@@ -105,6 +105,9 @@
 #define MB_PTR_BACK(s, p) \
   (p -= utf_head_off((char_u *)s, (char_u *)p - 1) + 1)
 
+// MB_CHAR2BYTES(): convert character to bytes and advance pointer to bytes
+#define MB_CHAR2BYTES(c, b) ((b) += utf_char2bytes((c), (b)))
+
 #define RESET_BINDING(wp) \
   do { \
     (wp)->w_p_scb = false; \
diff --git a/src/nvim/screen.c b/src/nvim/screen.c
index 538604cf79..0cc25552fe 100644
--- a/src/nvim/screen.c
+++ b/src/nvim/screen.c
@@ -1960,7 +1960,7 @@ static size_t fill_foldcolumn(char_u *p, win_T *wp, foldinfo_T foldinfo, linenr_
   level = foldinfo.fi_level;
 
   // If the column is too narrow, we start at the lowest level that
-  // fits and use numbers to indicated the depth.
+  // fits and use numbers to indicate the depth.
   first_level = level - fdc - closed + 1;
   if (first_level < 1) {
     first_level = 1;
diff --git a/src/nvim/testdir/test_statusline.vim b/src/nvim/testdir/test_statusline.vim
index a3e4dcdd25..9c9853674b 100644
--- a/src/nvim/testdir/test_statusline.vim
+++ b/src/nvim/testdir/test_statusline.vim
@@ -498,5 +498,20 @@ func Test_statusline_after_split_vsplit()
   set ls& stl&
 endfunc
 
+" Test using a multibyte character for 'stl' and 'stlnc' items in 'fillchars'
+" with a custom 'statusline'
+func Test_statusline_mbyte_fillchar()
+  only
+  set laststatus=2
+  set fillchars=vert:\|,fold:-,stl:━,stlnc:═
+  set statusline=a%=b
+  call assert_match('^a\+━\+b$', s:get_statusline())
+  vnew
+  call assert_match('^a\+━\+b━a\+═\+b$', s:get_statusline())
+  wincmd w
+  call assert_match('^a\+═\+b═a\+━\+b$', s:get_statusline())
+  set statusline& fillchars& laststatus&
+  %bw!
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab