fix(treesitter): enforce lowercase language names (#28546)

* fix(treesitter): enforce lowercase language names

Problem: On case-insensitive file systems (e.g., macOS), `has_parser`
will return `true` for uppercase aliases, which will then try to inject
the uppercase language unsuccessfully.

Solution: Enforce and assume parser names to be lowercase when
resolving language names.

4 files changed, 8 insertions, 12 deletions

diff --git a/runtime/doc/treesitter.txt b/runtime/doc/treesitter.txt
index 67cbeda2a8..2b9eba1520 100644
--- a/runtime/doc/treesitter.txt
+++ b/runtime/doc/treesitter.txt
@@ -30,6 +30,9 @@ A parser can also be loaded manually using a full path: >lua
 
     vim.treesitter.language.add('python', { path = "/path/to/python.so" })
 <
+Parser names are assumed to be lower case if the file system is
+case-sensitive.
+
 To associate certain |filetypes| with a treesitter language (name of parser),
 use |vim.treesitter.language.register()|. For example, to use the `xml`
 treesitter parser for buffers with filetype `svg` or `xslt`, use: >lua
diff --git a/runtime/lua/vim/treesitter/language.lua b/runtime/lua/vim/treesitter/language.lua
index 47abf65332..d0a74daa6c 100644
--- a/runtime/lua/vim/treesitter/language.lua
+++ b/runtime/lua/vim/treesitter/language.lua
@@ -88,6 +88,9 @@ function M.add(lang, opts)
     filetype = { filetype, { 'string', 'table' }, true },
   })
 
+  -- parser names are assumed to be lowercase (consistent behavior on case-insensitive file systems)
+  lang = lang:lower()
+
   if vim._ts_has_language(lang) then
     M.register(lang, filetype)
     return
diff --git a/runtime/lua/vim/treesitter/languagetree.lua b/runtime/lua/vim/treesitter/languagetree.lua
index 990debc77b..e618f29f8f 100644
--- a/runtime/lua/vim/treesitter/languagetree.lua
+++ b/runtime/lua/vim/treesitter/languagetree.lua
@@ -758,7 +758,6 @@ local has_parser = vim.func._memoize(1, function(lang)
 end)
 
 --- Return parser name for language (if exists) or filetype (if registered and exists).
---- Also attempts with the input lower-cased.
 ---
 ---@param alias string language or filetype name
 ---@return string? # resolved parser name
@@ -772,19 +771,10 @@ local function resolve_lang(alias)
     return alias
   end
 
-  if has_parser(alias:lower()) then
-    return alias:lower()
-  end
-
   local lang = vim.treesitter.language.get_lang(alias)
   if lang and has_parser(lang) then
     return lang
   end
-
-  lang = vim.treesitter.language.get_lang(alias:lower())
-  if lang and has_parser(lang) then
-    return lang
-  end
 end
 
 ---@private
@@ -808,7 +798,7 @@ function LanguageTree:_get_injection(match, metadata)
       -- Lang should override any other language tag
       if name == 'injection.language' then
         local text = vim.treesitter.get_node_text(node, self._source, { metadata = metadata[id] })
-        lang = resolve_lang(text)
+        lang = resolve_lang(text:lower()) -- language names are always lower case
       elseif name == 'injection.filename' then
         local text = vim.treesitter.get_node_text(node, self._source, { metadata = metadata[id] })
         local ft = vim.filetype.match({ filename = text })
diff --git a/test/functional/treesitter/highlight_spec.lua b/test/functional/treesitter/highlight_spec.lua
index 1d7ee00818..69984b3233 100644
--- a/test/functional/treesitter/highlight_spec.lua
+++ b/test/functional/treesitter/highlight_spec.lua
@@ -484,7 +484,7 @@ describe('treesitter highlighting (C)', function()
     exec_lua [[
       vim.treesitter.language.register("c", "foo")
       local parser = vim.treesitter.get_parser(0, "c", {
-        injections = {c = '(preproc_def (preproc_arg) @injection.content (#set! injection.language "fOO")) (preproc_function_def value: (preproc_arg) @injection.content (#set! injection.language "fOO"))'}
+        injections = {c = '(preproc_def (preproc_arg) @injection.content (#set! injection.language "foo")) (preproc_function_def value: (preproc_arg) @injection.content (#set! injection.language "foo"))'}
       })
       local highlighter = vim.treesitter.highlighter
       test_hl = highlighter.new(parser, {queries = {c = hl_query}})