feat(colorcolchar): make the colorcolumn more flexible

This PR creates the ability to optionally decorate the colorcolumn(s) in
other ways.

Specifically it adds the ability to set:

  * The highlight group to highlight a colorcolumn with
  * A character to draw in a colorcolumn
  * whether the colorcolumn should mix its highlighting with the
    character in the column.

The new syntax for colorcolumn is:

set colorcolumn=[+|-]<num>[/<char>[/<hl_group>[/<flags>]]]

By default the char is ' ', and the hl_group is 'ColorColumn'

This PR does not change the existing semantics, just adds to them.

4 files changed, 120 insertions, 28 deletions

diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index f01edd1ad2..dfcbb28c69 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -93,6 +93,21 @@ typedef uint64_t disptick_T;  // display tick type
 #include "nvim/syntax_defs.h"
 #include "nvim/terminal.h"
 
+typedef enum {
+  kColorcolBehind = 1
+} colorcol_flags_T;
+
+// Structure to define data associated with a colorcolumn.
+typedef struct {
+  int col;        // The column number to highlight.
+  int ch;         // The character to draw in the column.
+
+  char* syn_name; // The highlight group name. Must be free'd.
+  int syn_attr;   // The attribute. Will be set before a redraw.
+
+  int flags;      // Additional flags
+} colorcol_T;
+
 // The taggy struct is used to store the information about a :tag command.
 typedef struct taggy {
   char *tagname;                // tag name
@@ -1335,7 +1350,7 @@ struct window_S {
   uint32_t w_p_wbr_flags;           // flags for 'winbar'
   uint32_t w_p_fde_flags;           // flags for 'foldexpr'
   uint32_t w_p_fdt_flags;           // flags for 'foldtext'
-  int *w_p_cc_cols;                 // array of columns to highlight or NULL
+  colorcol_T *w_p_cc_cols;          // array of columns to highlight or NULL
   uint8_t w_p_culopt_flags;         // flags for cursorline highlighting
   long w_p_siso;                    // 'sidescrolloff' local value
   long w_p_so;                      // 'scrolloff' local value
diff --git a/src/nvim/drawline.c b/src/nvim/drawline.c
index 66e24e960f..ee6af01942 100644
--- a/src/nvim/drawline.c
+++ b/src/nvim/drawline.c
@@ -84,12 +84,12 @@ typedef struct {
 /// Advance **color_cols
 ///
 /// @return  true when there are columns to draw.
-static bool advance_color_col(int vcol, int **color_cols)
+static bool advance_color_col(int vcol, colorcol_T **color_cols)
 {
-  while (**color_cols >= 0 && vcol > **color_cols) {
+  while ((*color_cols)->col >= 0 && vcol > (*color_cols)->col) {
     (*color_cols)++;
   }
-  return **color_cols >= 0;
+  return (*color_cols)->col >= 0;
 }
 
 /// Used when 'cursorlineopt' contains "screenline": compute the margins between
@@ -660,7 +660,7 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, bool nochange,
   int save_did_emsg;
   int eol_hl_off = 0;                   // 1 if highlighted char after EOL
   bool draw_color_col = false;          // highlight colorcolumn
-  int *color_cols = NULL;               // pointer to according columns array
+  colorcol_T *color_cols = NULL;        // pointer to according columns array
   bool has_spell = false;               // this buffer has spell checking
 #define SPWORDLEN 150
   char nextline[SPWORDLEN * 2];         // text with start of the next line
@@ -2483,15 +2483,14 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, bool nochange,
 
         if (draw_color_col) {
           // determine rightmost colorcolumn to possibly draw
-          for (i = 0; color_cols[i] >= 0; i++) {
-            if (rightmost_vcol < color_cols[i]) {
-              rightmost_vcol = color_cols[i];
+          for (i = 0; color_cols[i].col >= 0; i++) {
+            if (rightmost_vcol < color_cols[i].col) {
+              rightmost_vcol = color_cols[i].col;
             }
           }
         }
 
         int cuc_attr = win_hl_attr(wp, HLF_CUC);
-        int mc_attr = win_hl_attr(wp, HLF_MC);
 
         int diff_attr = 0;
         if (diff_hlf == HLF_TXD) {
@@ -2519,9 +2518,9 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, bool nochange,
 
           if (wp->w_p_cuc && VCOL_HLC == (long)wp->w_virtcol) {
             col_attr = cuc_attr;
-          } else if (draw_color_col && VCOL_HLC == *color_cols) {
-            col_attr = mc_attr;
-            c = ' ';
+          } else if (draw_color_col && VCOL_HLC == color_cols->col) {
+            col_attr = color_cols->syn_attr;
+            c = color_cols->ch;
             schar_from_char(linebuf_char[off], c);
           }
 
@@ -2609,9 +2608,11 @@ int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, bool nochange,
           && lnum != wp->w_cursor.lnum) {
         vcol_save_attr = char_attr;
         char_attr = hl_combine_attr(win_hl_attr(wp, HLF_CUC), char_attr);
-      } else if (draw_color_col && VCOL_HLC == *color_cols) {
+      } else if (draw_color_col && VCOL_HLC == color_cols->col) {
         vcol_save_attr = char_attr;
-        char_attr = hl_combine_attr(win_hl_attr(wp, HLF_MC), char_attr);
+        if (!(color_cols->flags & kColorcolBehind)) {
+          char_attr = hl_combine_attr(color_cols->syn_attr, char_attr);
+        }
       }
     }
 
diff --git a/src/nvim/drawscreen.c b/src/nvim/drawscreen.c
index 04c342e068..f5f72b711f 100644
--- a/src/nvim/drawscreen.c
+++ b/src/nvim/drawscreen.c
@@ -1016,6 +1016,19 @@ static void win_update(win_T *wp, DecorProviders *providers)
     return;
   }
 
+  // Link colorcolumn syn_attrs to syn_names. Needs to be done at a redraw
+  // as the syn names are volitile and can change.
+  if (wp->w_p_cc_cols) {
+    for (int i = 0; wp->w_p_cc_cols[i].col >= 0; ++ i) {
+      const char* syn_name = wp->w_p_cc_cols[i].syn_name;
+      if (syn_name == NULL) {
+        wp->w_p_cc_cols[i].syn_attr = win_hl_attr(wp, HLF_MC);
+      } else {
+        wp->w_p_cc_cols[i].syn_attr = syn_name2attr(syn_name);
+      }
+    }
+  }
+
   buf_T *buf = wp->w_buffer;
 
   // reset got_int, otherwise regexp won't work
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 05f84b5a91..cf5d0013a3 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -5061,6 +5061,17 @@ void free_wininfo(wininfo_T *wip, buf_T *bp)
   xfree(wip);
 }
 
+// Free colorcolumns. Has syntax names.
+static void free_wp_cc_cols(colorcol_T* cc)
+{
+  if (cc) {
+    for (int i = 0; cc[i].col >= 0; i ++) {
+      xfree(cc[i].syn_name);
+    }
+  }
+  xfree(cc);
+}
+
 /// Remove window 'wp' from the window list and free the structure.
 ///
 /// @param tp  tab page "win" is in, NULL for current
@@ -5154,7 +5165,7 @@ static void win_free(win_T *wp, tabpage_T *tp)
 
   qf_free_all(wp);
 
-  xfree(wp->w_p_cc_cols);
+  free_wp_cc_cols(wp->w_p_cc_cols);
 
   win_free_grid(wp, false);
 
@@ -7395,10 +7406,11 @@ static bool frame_check_width(const frame_T *topfrp, int width)
   return true;
 }
 
-/// Simple int comparison function for use with qsort()
-static int int_cmp(const void *a, const void *b)
+/// Simple colorcol_T comparison function for use with qsort().
+/// Compares the column numbers
+static int colorcol_cmp(const void *a, const void *b)
 {
-  return *(const int *)a - *(const int *)b;
+  return ((const colorcol_T *)a)->col - ((const colorcol_T *)b)->col;
 }
 
 /// Handle setting 'colorcolumn' or 'textwidth' in window "wp".
@@ -7411,9 +7423,16 @@ char *check_colorcolumn(win_T *wp)
   }
 
   unsigned int count = 0;
-  int color_cols[256];
+  colorcol_T color_cols[256];
+  bool do_skip = false;
+
   for (char *s = wp->w_p_cc; *s != NUL && count < 255;) {
     int col;
+    int ch = ' ';
+    char syn_name[256];
+    int flags = 0;
+    syn_name[0] = 0;
+
     if (*s == '-' || *s == '+') {
       // -N and +N: add to 'textwidth'
       col = (*s == '-') ? -1 : 1;
@@ -7423,7 +7442,7 @@ char *check_colorcolumn(win_T *wp)
       }
       col = col * getdigits_int(&s, true, 0);
       if (wp->w_buffer->b_p_tw == 0) {
-        goto skip;          // 'textwidth' not set, skip this item
+        do_skip = true;          // 'textwidth' not set, skip this item
       }
       assert((col >= 0
               && wp->w_buffer->b_p_tw <= INT_MAX - col
@@ -7433,15 +7452,58 @@ char *check_colorcolumn(win_T *wp)
                  && wp->w_buffer->b_p_tw + col <= INT_MAX));
       col += (int)wp->w_buffer->b_p_tw;
       if (col < 0) {
-        goto skip;
+        do_skip = true;
       }
     } else if (ascii_isdigit(*s)) {
       col = getdigits_int(&s, true, 0);
     } else {
       return e_invarg;
     }
-    color_cols[count++] = col - 1;      // 1-based to 0-based
-skip:
+
+    // Parse the character.
+    if (*s == '/') {
+      s ++;
+      ch = mb_ptr2char_adv((const char**) &s);
+      if (!ch) {
+        return e_invarg;
+      }
+    }
+
+    // Parse the highlight group.
+    if (*s == '/') {
+      s ++;
+      size_t i = 0;
+      while(i < sizeof(syn_name) && *s && *s != '/' && *s != ',') {
+        syn_name[i ++] = *(s++);
+      }
+      syn_name[i] = 0;
+    }
+
+    // Parse extra flags
+    if (*s == '/') {
+      s ++;
+      while (*s != ',' && *s) {
+        switch (*(s ++)) {
+          case 'b':
+            flags |= kColorcolBehind;
+            break;
+
+          default:
+            return e_invarg;
+        }
+      }
+    }
+
+    if (!do_skip) {
+      color_cols[count++] = (colorcol_T) {
+        .col = col - 1,      // 1-based to 0-based
+        .syn_attr = 0,
+        .ch = ch,
+        .syn_name = syn_name[0] == 0 ? NULL : xstrdup(syn_name),
+        .flags = flags,
+      };
+    }
+
     if (*s == NUL) {
       break;
     }
@@ -7453,23 +7515,24 @@ skip:
     }
   }
 
-  xfree(wp->w_p_cc_cols);
+  free_wp_cc_cols(wp->w_p_cc_cols);
   if (count == 0) {
     wp->w_p_cc_cols = NULL;
   } else {
-    wp->w_p_cc_cols = xmalloc(sizeof(int) * (count + 1));
+    wp->w_p_cc_cols = xmalloc(sizeof(colorcol_T) * (count + 1));
     // sort the columns for faster usage on screen redraw inside
     // win_line()
-    qsort(color_cols, count, sizeof(int), int_cmp);
+    qsort(color_cols, count, sizeof(colorcol_T), colorcol_cmp);
 
     int j = 0;
     for (unsigned int i = 0; i < count; i++) {
       // skip duplicates
-      if (j == 0 || wp->w_p_cc_cols[j - 1] != color_cols[i]) {
+      if (j == 0 || wp->w_p_cc_cols[j - 1].col != color_cols[i].col) {
         wp->w_p_cc_cols[j++] = color_cols[i];
       }
     }
-    wp->w_p_cc_cols[j] = -1;        // end marker
+    memset(&wp->w_p_cc_cols[j], 0, sizeof(wp->w_p_cc_cols[j]));
+    wp->w_p_cc_cols[j].col = -1;        // end marker
   }
 
   return NULL;    // no error