fix(loader): remove cyclic dependency on vim.fs (when --luamod-dev)

Problem: Loading `vim.fs` via the `vim.loader` Lua package loader will
result in a stack overflow due to a cyclic dependency. This may happen
when the `vim.fs` module isn't byte-compiled, i.e. when `--luamod-dev`
is used (#27413).

Solution: `vim.loader` depends on `vim.fs`. Therefore `vim.fs` should
be loaded in advance.

2 files changed, 26 insertions, 3 deletions

diff --git a/runtime/lua/vim/loader.lua b/runtime/lua/vim/loader.lua
index b77251cf7e..ec99e417c2 100644
--- a/runtime/lua/vim/loader.lua
+++ b/runtime/lua/vim/loader.lua
@@ -1,3 +1,4 @@
+local fs = vim.fs -- "vim.fs" is a dependency, so must be loaded early.
 local uv = vim.uv
 local uri_encode = vim.uri_encode --- @type function
 
@@ -63,7 +64,7 @@ function Loader.get_hash(path)
 end
 
 local function normalize(path)
-  return vim.fs.normalize(path, { expand_env = false })
+  return fs.normalize(path, { expand_env = false })
 end
 
 --- Gets the rtp excluding after directories.
@@ -432,7 +433,7 @@ end
 function Loader.lsmod(path)
   if not Loader._indexed[path] then
     Loader._indexed[path] = {}
-    for name, t in vim.fs.dir(path .. '/lua') do
+    for name, t in fs.dir(path .. '/lua') do
       local modpath = path .. '/lua/' .. name
       -- HACK: type is not always returned due to a bug in luv
       t = t or Loader.get_hash(modpath).type
diff --git a/test/functional/lua/loader_spec.lua b/test/functional/lua/loader_spec.lua
index cdb561330a..4e42a18405 100644
--- a/test/functional/lua/loader_spec.lua
+++ b/test/functional/lua/loader_spec.lua
@@ -3,10 +3,32 @@ local helpers = require('test.functional.helpers')(after_each)
 
 local exec_lua = helpers.exec_lua
 local command = helpers.command
+local clear = helpers.clear
 local eq = helpers.eq
 
 describe('vim.loader', function()
-  before_each(helpers.clear)
+  before_each(clear)
+
+  it('can work in compatibility with --luamod-dev #27413', function()
+    clear({ args = { '--luamod-dev' } })
+    exec_lua [[
+      vim.loader.enable()
+
+      require("vim.fs")
+
+      -- try to load other vim submodules as well (Nvim Lua stdlib)
+      for key, _ in pairs(vim._submodules) do
+        local modname = 'vim.' .. key   -- e.g. "vim.fs"
+
+        local lhs = vim[key]
+        local rhs = require(modname)
+        assert(
+          lhs == rhs,
+          ('%s != require("%s"), %s != %s'):format(modname, modname, tostring(lhs), tostring(rhs))
+        )
+      end
+    ]]
+  end)
 
   it('handles changing files (#23027)', function()
     exec_lua [[