fix(completion): complete drive-letter filepath on Windows #36353

Problem:
On MSWIN, file completion (CTRL-X CTRL-F) only works for the current
drive (so not for actual absolute paths), since drive letters are never
included in the completion pattern.

e.g. when completing "F:\Hello" Nvim currently completes "\Hello"
which is relative to the current drive/volume.

vim solves this by adding ':' to the default 'isfname' value on mswin,
but that causes issues as ':' is not a valid windows path char anywhere
_except_ after the drive letter.

Solution:
detect drive letters in front of the path when creating the completion
pattern.

src/nvim/file_search.c

@@ -1679,7 +1679,7 @@ char *file_name_in_line(char *line, int col, int options, int count, char *rel_f
 
   // Search forward for the last char of the file name.
   // Also allow ":/" when ':' is not in 'isfname'.
-  len = path_has_drive_letter(ptr) ? 2 : 0;
+  len = path_has_drive_letter(ptr, strlen(ptr)) ? 2 : 0;
   while (vim_isfilec((uint8_t)ptr[len]) || (ptr[len] == '\\' && ptr[len + 1] == ' ')
          || ((options & FNAME_HYP) && path_is_url(ptr + len))
          || (is_url && vim_strchr(":?&=", (uint8_t)ptr[len]) != NULL)) {

src/nvim/insexpand.c

@@ -5786,7 +5786,17 @@ static int get_filename_compl_info(char *line, int startcol, colnr_T curs_col)
     while (p > line && vim_isfilec(utf_ptr2char(p))) {
       MB_PTR_BACK(line, p);
     }
-    if (p == line && vim_isfilec(utf_ptr2char(p))) {
+    bool p_is_filec = false;
+#ifdef MSWIN
+    // check for drive letters on mswin
+    if (p > line && path_has_drive_letter(p - 1, line + startcol - (p - 1))) {
+      p -= p == line + 1 ? 1 : 2;
+      p_is_filec = true;
+    }
+#endif
+    p_is_filec = p_is_filec || vim_isfilec(utf_ptr2char(p));
+
+    if (p == line && p_is_filec) {
       startcol = 0;
     } else {
       startcol = (int)(p - line) + 1;

src/nvim/path.c

@@ -1715,13 +1715,13 @@ size_t simplify_filename(char *filename)
 /// Checks for a Windows drive letter ("C:/") at the start of the path.
 ///
 /// @see https://url.spec.whatwg.org/#start-with-a-windows-drive-letter
-bool path_has_drive_letter(const char *p)
+bool path_has_drive_letter(const char *p, size_t path_len)
   FUNC_ATTR_NONNULL_ALL
 {
-  return strlen(p) >= 2
+  return path_len >= 2
          && ASCII_ISALPHA(p[0])
          && (p[1] == ':' || p[1] == '|')
-         && (strlen(p) == 2 || ((p[2] == '/') | (p[2] == '\\') | (p[2] == '?') | (p[2] == '#')));
+         && (path_len == 2 || ((p[2] == '/') | (p[2] == '\\') | (p[2] == '?') | (p[2] == '#')));
 }
 
 // Check if the ":/" of a URL is at the pointer, return URL_SLASH.
@@ -1758,7 +1758,7 @@ int path_with_url(const char *fname)
     return 0;
   }
 
-  if (path_has_drive_letter(fname)) {
+  if (path_has_drive_letter(fname, strlen(fname))) {
     return 0;
   }
 

test/functional/editor/completion_spec.lua

@@ -30,6 +30,16 @@ describe('completion', function()
     }
   end)
 
+  it('ctrl-x_ctrl-f completes Windows drive letter', function()
+    if not t.is_os('win') then
+      return
+    end
+    feed('iblablaC:/W<C-x><C-f>')
+    screen:expect {
+      any = [[C:\Windows\]],
+    }
+  end)
+
   describe('v:completed_item', function()
     it('is empty dict until completion', function()
       eq({}, eval('v:completed_item'))