Use weak tables in tree-sitter code (#17117)

feat(treesitter): use weak tables when possible

Also add the defaulttable function to create a table whose values are created when a key is missing.

runtime/lua/vim/shared.lua

@@ -715,5 +715,30 @@ function vim.is_callable(f)
   return type(m.__call) == 'function'
 end
 
+--- Creates a table whose members are automatically created when accessed, if they don't already
+--- exist.
+---
+--- They mimic defaultdict in python.
+---
+--- If @p create is @c nil, this will create a defaulttable whose constructor function is
+--- this function, effectively allowing to create nested tables on the fly:
+---
+--- <pre>
+--- local a = vim.defaulttable()
+--- a.b.c = 1
+--- </pre>
+---
+---@param create function|nil The function called to create a missing value.
+---@return table Empty table with metamethod
+function vim.defaulttable(create)
+  create = create or vim.defaulttable
+  return setmetatable({}, {
+    __index = function(tbl, key)
+      rawset(tbl, key, create())
+      return rawget(tbl, key)
+    end,
+  })
+end
+
 return vim
 -- vim:sw=2 ts=2 et

runtime/lua/vim/treesitter.lua

@@ -3,10 +3,7 @@ local query = require('vim.treesitter.query')
 local language = require('vim.treesitter.language')
 local LanguageTree = require('vim.treesitter.languagetree')
 
--- TODO(bfredl): currently we retain parsers for the lifetime of the buffer.
--- Consider use weak references to release parser if all plugins are done with
--- it.
-local parsers = {}
+local parsers = setmetatable({}, { __mode = 'v' })
 
 local M = vim.tbl_extend('error', query, language)
 

runtime/lua/vim/treesitter/query.lua

@@ -140,12 +140,9 @@ function M.get_query(lang, query_name)
   end
 end
 
-local query_cache = setmetatable({}, {
-  __index = function(tbl, key)
-    rawset(tbl, key, {})
-    return rawget(tbl, key)
-  end,
-})
+local query_cache = vim.defaulttable(function()
+  return setmetatable({}, { __mode = 'v' })
+end)
 
 --- Parse {query} as a string. (If the query is in a file, the caller
 --- should read the contents into a string before calling).