patch 9.1.1646: MS-Windows: completion cannot handle implicit drive letters

Problem:  MS-Windows: completion cannot handle implicit drive letters
Solution: Consider paths like \folder and /folder as absolute
          (Miguel Barro).

closes: #17829

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Miguel Barro <miguel.barro@live.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/version9.txt

@@ -1,4 +1,4 @@
-*version9.txt*  For Vim version 9.1.  Last change: 2025 Aug 16
+*version9.txt*  For Vim version 9.1.  Last change: 2025 Aug 17
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -41745,6 +41745,8 @@ Others: ~
   Unicode 16.
 - Two additional digraphs have been added: LEFT ANGLE BRACKET "<[" and RIGHT
   ANGLE BRACKET "]>".
+- MS-Winodws: Paths like "\Windows" and "/Windows" are now considered to be
+  absolute paths (to the current drive) and no longer relative.
 
 							*added-9.2*
 Added ~

src/buffer.c

@@ -5472,7 +5472,8 @@ fix_fname(char_u  *fname)
      * Also expand when there is ".." in the file name, try to remove it,
      * because "c:/src/../README" is equal to "c:/README".
      * Similarly "c:/src//file" is equal to "c:/src/file".
-     * For MS-Windows also expand names like "longna~1" to "longname".
+     * For MS-Windows also expand names like "longna~1" to "longname"
+     * and provide drive letter for all absolute paths.
      */
 #ifdef UNIX
     return FullName_save(fname, TRUE);
@@ -5485,6 +5486,8 @@ fix_fname(char_u  *fname)
 # endif
 # if defined(MSWIN)
 	    || vim_strchr(fname, '~') != NULL
+	    || fname[0] == '/'
+	    || fname[0] == '\\'
 # endif
 	    )
 	return FullName_save(fname, FALSE);

src/filepath.c

@@ -359,7 +359,11 @@ repeat:
 	}
 
 	// FullName_save() is slow, don't use it when not needed.
-	if (*p != NUL || !vim_isAbsName(*fnamep))
+	if (*p != NUL || !vim_isAbsName(*fnamep)
+#ifdef MSWIN	// enforce drive letter on windows paths
+		|| **fnamep == '/' || **fnamep == '\\'
+#endif
+	)
 	{
 	    *fnamep = FullName_save(*fnamep, *p != NUL);
 	    vim_free(*bufp);	// free any allocated file name
@@ -3110,6 +3114,42 @@ vim_fnamencmp(char_u *x, char_u *y, size_t len)
     int		cx = NUL;
     int		cy = NUL;
 
+#ifdef MSWIN
+    /*
+     * To allow proper comparisson of absolute paths:
+     *	 - one with explicit drive letter C:\xxx
+     *	 - another with implicit drive letter \xxx
+     * advance the pointer, of the explicit one, to skip the drive
+     */
+    for (int swap = 0, drive = NUL; swap < 2; ++swap)
+    {
+	// Handle absolute paths with implicit drive letter
+	cx = PTR2CHAR(px);
+	cy = PTR2CHAR(py);
+
+	if ((cx == '/' || cx == '\\') && ASCII_ISALPHA(cy))
+	{
+	    drive = MB_TOUPPER(cy) - 'A' + 1;
+
+	    // Check for the colon
+	    py += mb_ptr2len(py);
+	    cy = PTR2CHAR(py);
+	    if (cy == ':' && drive == _getdrive())
+	    { // skip the drive for comparisson
+		py += mb_ptr2len(py);
+		break;
+	    }
+	    else // ignore
+		py -= mb_ptr2len(py);
+	}
+
+	// swap pointers
+	char_u *tmp = px;
+	px = py;
+	py = tmp;
+    }
+#endif
+
     while (len > 0)
     {
 	cx = PTR2CHAR(px);

src/findfile.c

@@ -398,18 +398,6 @@ vim_findfile_init(
 	    search_ctx->ffsc_start_dir.length);
 	if (search_ctx->ffsc_start_dir.string == NULL)
 	    goto error_return;
-
-#ifdef BACKSLASH_IN_FILENAME
-	// A path that starts with "/dir" is relative to the drive, not to the
-	// directory (but not for "//machine/dir").  Only use the drive name.
-	if ((*path == '/' || *path == '\\')
-		&& path[1] != path[0]
-		&& search_ctx->ffsc_start_dir.string[1] == ':')
-	{
-	    search_ctx->ffsc_start_dir.string[2] = NUL;
-	    search_ctx->ffsc_start_dir.length = 2;
-	}
-#endif
     }
 
     /*

src/os_mswin.c

@@ -319,6 +319,7 @@ mch_FullName(
 mch_isFullName(char_u *fname)
 {
     // A name like "d:/foo" and "//server/share" is absolute.  "d:foo" is not.
+    // /foo and \foo are absolute too because windows keeps a current drive.
     // Another way to check is to use mch_FullName() and see if the result is
     // the same as the name or mch_FullName() fails.  However, this has quite a
     // bit of overhead, so let's not do that.
@@ -326,7 +327,7 @@ mch_isFullName(char_u *fname)
 	return FALSE;
     return ((ASCII_ISALPHA(fname[0]) && fname[1] == ':'
 				      && (fname[2] == '/' || fname[2] == '\\'))
-	    || (fname[0] == fname[1] && (fname[0] == '/' || fname[0] == '\\')));
+	    || (fname[0] == '/' || fname[0] == '\\'));
 }
 
 /*

src/testdir/test_cd.vim

@@ -221,6 +221,39 @@ func Test_cd_completion()
     call assert_equal('"' .. cmd .. ' XComplDir1/ XComplDir2/ XComplDir3/', @:)
   endfor
   set cdpath&
+
+  if has('win32')
+    " Test windows absolute path completion
+    " Retrieve a suitable dir in the current drive
+    let dir = readdir('/', 'isdirectory("/" .. v:val) && len(v:val) > 2')[-1]
+    " Get partial path
+    let partial = dir[0:-2]
+    " Get the current drive letter
+    let old = chdir('/' . dir)
+    let full = getcwd()
+    let drive = full[0]
+    call chdir(old)
+
+    for cmd in ['cd', 'chdir', 'lcd', 'lchdir', 'tcd', 'tchdir']
+      for sep in [ '/', '\']
+
+        " Explicit drive letter
+        call feedkeys(':' .. cmd .. ' ' .. drive .. ':' .. sep ..
+                     \  partial .. "\<C-A>\<C-B>\"\<CR>", 'tx')
+        call assert_match(full, @:)
+
+        " Implicit drive letter
+        call feedkeys(':' .. cmd .. ' ' .. sep .. partial .. "\<C-A>\<C-B>\"\<CR>", 'tx')
+        call assert_match('/' .. dir .. '/', @:)
+
+        " UNC path
+        call feedkeys(':' .. cmd .. ' ' .. sep .. sep .. $COMPUTERNAME .. sep ..
+                     \ drive .. '$' .. sep .. partial .."\<C-A>\<C-B>\"\<CR>", 'tx')
+        call assert_match('//' .. $COMPUTERNAME .. '/' .. drive .. '$/' .. dir .. '/' , @:)
+
+      endfor
+    endfor
+  endif
 endfunc
 
 func Test_cd_unknown_dir()

src/testdir/test_functions.vim

@@ -4106,7 +4106,8 @@ func Test_isabsolutepath()
     call assert_true(isabsolutepath('A:\Foo'))
     call assert_true(isabsolutepath('A:/Foo'))
     call assert_false(isabsolutepath('A:Foo'))
-    call assert_false(isabsolutepath('\Windows'))
+    call assert_true(isabsolutepath('\Windows'))
+    call assert_true(isabsolutepath('/Windows'))
     call assert_true(isabsolutepath('\\Server2\Share\Test\Foo.txt'))
   else
     call assert_true(isabsolutepath('/'))

src/version.c

@@ -719,6 +719,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1646,
 /**/
     1645,
 /**/

src/vim9script.c

@@ -479,13 +479,7 @@ handle_import(
 	res = handle_import_fname(from_name, is_autoload, &sid);
 	vim_free(from_name);
     }
-    else if (mch_isFullName(tv.vval.v_string)
+    else if (mch_isFullName(tv.vval.v_string))
-#ifdef BACKSLASH_IN_FILENAME
-	    // On MS-Windows omitting the drive is still handled like an
-	    // absolute path, not using 'runtimepath'.
-	    || *tv.vval.v_string == '/' || *tv.vval.v_string == '\\'
-#endif
-	    )
     {
 	// Absolute path: "/tmp/name.vim"
 	res = handle_import_fname(tv.vval.v_string, is_autoload, &sid);