feat(lua): add vim.func._memoize

Memoizes a function, using a custom function to hash the arguments.

Private for now until:

- There are other places in the codebase that could benefit from this
  (e.g. LSP), but might require other changes to accommodate.
- Invalidation of the cache needs to be controllable. Using weak tables
  is an acceptable invalidation policy, but it shouldn't be the only
  one.
- I don't think the story around `hash_fn` is completely thought out. We
  may be able to have a good default hash_fn by hashing each argument,
  so basically a better 'concat'.

1 files changed, 6 insertions, 4 deletions

diff --git a/runtime/lua/vim/treesitter/languagetree.lua b/runtime/lua/vim/treesitter/languagetree.lua
index f931291ed7..b2c4e9167d 100644
--- a/runtime/lua/vim/treesitter/languagetree.lua
+++ b/runtime/lua/vim/treesitter/languagetree.lua
@@ -738,12 +738,14 @@ local function add_injection(t, tree_index, pattern, lang, combined, ranges)
 end
 
 -- TODO(clason): replace by refactored `ts.has_parser` API (without registering)
----@param lang string parser name
----@return boolean # true if parser for {lang} exists on rtp
-local has_parser = function(lang)
+--- The result of this function is cached to prevent nvim_get_runtime_file from being
+--- called too often
+--- @param lang string parser name
+--- @return boolean # true if parser for {lang} exists on rtp
+local has_parser = vim.func._memoize(1, function(lang)
   return vim._ts_has_language(lang)
     or #vim.api.nvim_get_runtime_file('parser/' .. lang .. '.*', false) > 0
-end
+end)
 
 --- Return parser name for language (if exists) or filetype (if registered and exists).
 --- Also attempts with the input lower-cased.