refactor(fs): replace vim.fn/vim.env in vim.fs (#20379)

Avoid using vim.env and vim.fn in vim.fs functions so that
they can be used in "fast" contexts.

runtime/lua/vim/fs.lua

@@ -1,5 +1,7 @@
 local M = {}
 
+local iswin = vim.loop.os_uname().sysname == 'Windows_NT'
+
 --- Iterate over all the parents of the given file or directory.
 ---
 --- Example:
@@ -40,7 +42,19 @@ function M.dirname(file)
   if file == nil then
     return nil
   end
-  return vim.fn.fnamemodify(file, ':h')
+  vim.validate({ file = { file, 's' } })
+  if iswin and file:match('^%w:[\\/]?$') then
+    return (file:gsub('\\', '/'))
+  elseif not file:match('[\\/]') then
+    return '.'
+  elseif file == '/' or file:match('^/[^/]+$') then
+    return '/'
+  end
+  local dir = file:match('[/\\]$') and file:sub(1, #file - 1) or file:match('^([/\\]?.+)[/\\]')
+  if iswin and dir:match('^%w:$') then
+    return dir .. '/'
+  end
+  return (dir:gsub('\\', '/'))
 end
 
 --- Return the basename of the given file or directory
@@ -48,7 +62,14 @@ end
 ---@param file (string) File or directory
 ---@return (string) Basename of {file}
 function M.basename(file)
-  return vim.fn.fnamemodify(file, ':t')
+  if file == nil then
+    return nil
+  end
+  vim.validate({ file = { file, 's' } })
+  if iswin and file:match('^%w:[\\/]?$') then
+    return ''
+  end
+  return file:match('[/\\]$') and '' or (file:match('[^\\/]*$'):gsub('\\', '/'))
 end
 
 --- Return an iterator over the files and directories located in {path}
@@ -229,7 +250,12 @@ end
 ---@return (string) Normalized path
 function M.normalize(path)
   vim.validate({ path = { path, 's' } })
-  return (path:gsub('^~/', vim.env.HOME .. '/'):gsub('%$([%w_]+)', vim.env):gsub('\\', '/'))
+  return (
+    path
+      :gsub('^~/', vim.loop.os_homedir() .. '/')
+      :gsub('%$([%w_]+)', vim.loop.os_getenv)
+      :gsub('\\', '/')
+  )
 end
 
 return M

test/functional/lua/fs_spec.lua

@@ -12,6 +12,39 @@ local is_os = helpers.is_os
 
 local nvim_prog_basename = is_os('win') and 'nvim.exe' or 'nvim'
 
+local test_basename_dirname_eq = {
+  '~/foo/',
+  '~/foo',
+  '~/foo/bar.lua',
+  'foo.lua',
+  ' ',
+  '',
+  '.',
+  '..',
+  '../',
+  '~',
+  '/usr/bin',
+  '/usr/bin/gcc',
+  '/',
+  '/usr/',
+  '/usr',
+  'c:/usr',
+  'c:/',
+  'c:',
+  'c:/users/foo',
+  'c:/users/foo/bar.lua',
+  'c:/users/foo/bar/../',
+}
+
+local tests_windows_paths = {
+  'c:\\usr',
+  'c:\\',
+  'c:',
+  'c:\\users\\foo',
+  'c:\\users\\foo\\bar.lua',
+  'c:\\users\\foo\\bar\\..\\',
+}
+
 before_each(clear)
 
 describe('vim.fs', function()
@@ -41,15 +74,58 @@ describe('vim.fs', function()
         local nvim_dir = ...
         return vim.fs.dirname(nvim_dir)
       ]], nvim_dir))
+
+      local function test_paths(paths)
+        for _, path in ipairs(paths) do
+          eq(
+            exec_lua([[
+              local path = ...
+              return vim.fn.fnamemodify(path,':h'):gsub('\\', '/')
+            ]], path),
+            exec_lua([[
+              local path = ...
+              return vim.fs.dirname(path)
+            ]], path),
+            path
+          )
+        end
+      end
+
+      test_paths(test_basename_dirname_eq)
+      if is_os('win') then
+        test_paths(tests_windows_paths)
+      end
     end)
   end)
 
   describe('basename()', function()
     it('works', function()
+
       eq(nvim_prog_basename, exec_lua([[
         local nvim_prog = ...
         return vim.fs.basename(nvim_prog)
       ]], nvim_prog))
+
+      local function test_paths(paths)
+        for _, path in ipairs(paths) do
+          eq(
+            exec_lua([[
+              local path = ...
+              return vim.fn.fnamemodify(path,':t'):gsub('\\', '/')
+            ]], path),
+            exec_lua([[
+              local path = ...
+              return vim.fs.basename(path)
+            ]], path),
+            path
+          )
+        end
+      end
+
+      test_paths(test_basename_dirname_eq)
+      if is_os('win') then
+        test_paths(tests_windows_paths)
+      end
     end)
   end)