fix(vim.fs): joinpath() does not normalize slashes on Windows #31782

runtime/doc/lua.txt

@@ -3062,8 +3062,11 @@ vim.fs.find({names}, {opts})                                   *vim.fs.find()*
         items
 
 vim.fs.joinpath({...})                                     *vim.fs.joinpath()*
-    Concatenate directories and/or file paths into a single path with
-    normalization (e.g., `"foo/"` and `"bar"` get joined to `"foo/bar"`)
+    Concatenates partial paths into one path. Slashes are normalized
+    (redundant slashes are removed, and on Windows backslashes are replaced
+    with forward-slashes) (e.g., `"foo/"` and `"/bar"` get joined to
+    `"foo/bar"`) (windows: e.g `"a\foo\"` and `"\bar"` are joined to
+    `"a/foo/bar"`)
 
     Attributes: ~
         Since: 0.10.0

runtime/lua/vim/fs.lua

@@ -105,14 +105,19 @@ function M.basename(file)
   return file:match('/$') and '' or (file:match('[^/]*$'))
 end
 
---- Concatenate directories and/or file paths into a single path with normalization
---- (e.g., `"foo/"` and `"bar"` get joined to `"foo/bar"`)
+--- Concatenates partial paths into one path. Slashes are normalized (redundant slashes are removed, and on Windows backslashes are replaced with forward-slashes)
+--- (e.g., `"foo/"` and `"/bar"` get joined to `"foo/bar"`)
+--- (windows: e.g `"a\foo\"` and `"\bar"` are joined to `"a/foo/bar"`)
 ---
 ---@since 12
 ---@param ... string
 ---@return string
 function M.joinpath(...)
-  return (table.concat({ ... }, '/'):gsub('//+', '/'))
+  local path = table.concat({ ... }, '/')
+  if iswin then
+    path = path:gsub('\\', '/')
+  end
+  return (path:gsub('//+', '/'))
 end
 
 ---@alias Iterator fun(): string?, string?

test/functional/lua/fs_spec.lua

@@ -323,6 +323,20 @@ describe('vim.fs', function()
       eq('foo/bar/baz', vim.fs.joinpath('foo', 'bar', 'baz'))
       eq('foo/bar/baz', vim.fs.joinpath('foo', '/bar/', '/baz'))
     end)
+    it('rewrites backslashes on Windows', function()
+      if is_os('win') then
+        eq('foo/bar/baz/zub/', vim.fs.joinpath([[foo]], [[\\bar\\\\baz]], [[zub\]]))
+      else
+        eq([[foo/\\bar\\\\baz/zub\]], vim.fs.joinpath([[foo]], [[\\bar\\\\baz]], [[zub\]]))
+      end
+    end)
+    it('strips redundant slashes', function()
+      if is_os('win') then
+        eq('foo/bar/baz/zub/', vim.fs.joinpath([[foo//]], [[\\bar\\\\baz]], [[zub\]]))
+      else
+        eq('foo/bar/baz/zub/', vim.fs.joinpath([[foo]], [[//bar////baz]], [[zub/]]))
+      end
+    end)
   end)
 
   describe('normalize()', function()