fix(languagetree): apply `resolve_lang` to `metadata['injection.language']`

`resolve_lang` is applied to `@injection.language` when it's supplied as a
capture:

https://github.com/neovim/neovim/blob/f5953edbac14febce9d4f8a3c35bdec1eae26fbe/runtime/lua/vim/treesitter/languagetree.lua#L766-L768

If we want to support `metadata['injection.language']` (as per #22518 and
[tree-sitter upstream](https://tree-sitter.github.io/tree-sitter/syntax-highlighting#language-injection))
then the behavior should be consistent.

Fixes: nvim-treesitter/nvim-treesitter#4918

2 files changed, 84 insertions, 50 deletions

diff --git a/runtime/lua/vim/treesitter/languagetree.lua b/runtime/lua/vim/treesitter/languagetree.lua
index 79f36a27fd..4dd5a18396 100644
--- a/runtime/lua/vim/treesitter/languagetree.lua
+++ b/runtime/lua/vim/treesitter/languagetree.lua
@@ -734,7 +734,8 @@ local has_parser = function(lang)
     or #vim.api.nvim_get_runtime_file('parser/' .. lang .. '.*', false) > 0
 end
 
---- Return parser name for language (if exists) or filetype (if registered and exists)
+--- 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
@@ -743,10 +744,19 @@ 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
@@ -758,9 +768,10 @@ end
 function LanguageTree:_get_injection(match, metadata)
   local ranges = {} ---@type Range6[]
   local combined = metadata['injection.combined'] ~= nil
+  local injection_lang = metadata['injection.language'] --[[@as string?]]
   local lang = metadata['injection.self'] ~= nil and self:lang()
     or metadata['injection.parent'] ~= nil and self._parent_lang
-    or metadata['injection.language'] --[[@as string?]]
+    or (injection_lang and resolve_lang(injection_lang))
   local include_children = metadata['injection.include-children'] ~= nil
 
   for id, node in pairs(match) do
@@ -768,7 +779,7 @@ 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) or resolve_lang(text:lower())
+      lang = resolve_lang(text)
     elseif name == 'injection.content' then
       ranges = get_node_ranges(node, self._source, metadata[id], include_children)
     end
diff --git a/test/functional/treesitter/highlight_spec.lua b/test/functional/treesitter/highlight_spec.lua
index 0528370e2a..0aa0cdd6d6 100644
--- a/test/functional/treesitter/highlight_spec.lua
+++ b/test/functional/treesitter/highlight_spec.lua
@@ -85,6 +85,56 @@ void ui_refresh(void)
   }
 }]]
 
+local injection_text_c = [[
+int x = INT_MAX;
+#define READ_STRING(x, y) (char *)read_string((x), (size_t)(y))
+#define foo void main() { \
+              return 42;  \
+            }
+]]
+
+local injection_grid_c = [[
+  int x = INT_MAX;                                                 |
+  #define READ_STRING(x, y) (char *)read_string((x), (size_t)(y))  |
+  #define foo void main() { \                                      |
+                return 42;  \                                      |
+              }                                                    |
+  ^                                                                 |
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+                                                                   |
+]]
+
+local injection_grid_expected_c = [[
+  {3:int} x = {5:INT_MAX};                                                 |
+  #define {5:READ_STRING}(x, y) ({3:char} *)read_string((x), ({3:size_t})(y))  |
+  #define foo {3:void} main() { \                                      |
+                {4:return} {5:42};  \                                      |
+              }                                                    |
+  ^                                                                 |
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+  {1:~                                                                }|
+                                                                   |
+]]
+
 describe('treesitter highlighting (C)', function()
   local screen
 
@@ -411,34 +461,9 @@ describe('treesitter highlighting (C)', function()
   end)
 
   it("supports injected languages", function()
-    insert([[
-    int x = INT_MAX;
-    #define READ_STRING(x, y) (char *)read_string((x), (size_t)(y))
-    #define foo void main() { \
-                  return 42;  \
-                }
-    ]])
+    insert(injection_text_c)
 
-    screen:expect{grid=[[
-      int x = INT_MAX;                                                 |
-      #define READ_STRING(x, y) (char *)read_string((x), (size_t)(y))  |
-      #define foo void main() { \                                      |
-                    return 42;  \                                      |
-                  }                                                    |
-      ^                                                                 |
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-                                                                       |
-    ]]}
+    screen:expect{grid=injection_grid_c}
 
     exec_lua [[
       local parser = vim.treesitter.get_parser(0, "c", {
@@ -448,26 +473,24 @@ describe('treesitter highlighting (C)', function()
       test_hl = highlighter.new(parser, {queries = {c = hl_query}})
     ]]
 
-    screen:expect{grid=[[
-      {3:int} x = {5:INT_MAX};                                                 |
-      #define {5:READ_STRING}(x, y) ({3:char} *)read_string((x), ({3:size_t})(y))  |
-      #define foo {3:void} main() { \                                      |
-                    {4:return} {5:42};  \                                      |
-                  }                                                    |
-      ^                                                                 |
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-      {1:~                                                                }|
-                                                                       |
-    ]]}
+    screen:expect{grid=injection_grid_expected_c}
+  end)
+
+  it("supports injecting by ft name in metadata['injection.language']", function()
+    insert(injection_text_c)
+
+    screen:expect{grid=injection_grid_c}
+
+    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"))'}
+      })
+      local highlighter = vim.treesitter.highlighter
+      test_hl = highlighter.new(parser, {queries = {c = hl_query}})
+    ]]
+
+    screen:expect{grid=injection_grid_expected_c}
   end)
 
   it("supports overriding queries, like ", function()