[Backport release-0.7] fix(treesitter): create new parser if language…

… is not the same as cached parser (neovim#18220)

* fix(treesitter): create new parser if language is not the same as cached parser

Fixes neovim#18148

(cherry picked from commit 8e35894)

* test: create new parser in vim.treesitter.get_parser() when filetype changes

(cherry picked from commit 30e7b3f)

Co-authored-by: Chinmay Dalal <dalal.chinmay.0101@gmail.com>

runtime/lua/vim/treesitter.lua

@@ -90,7 +90,7 @@ function M.get_parser(bufnr, lang, opts)
     lang = a.nvim_buf_get_option(bufnr, "filetype")
   end
 
-  if parsers[bufnr] == nil then
+  if parsers[bufnr] == nil or parsers[bufnr]:lang() ~= lang then
     parsers[bufnr] = M._create_parser(bufnr, lang, opts)
   end
 

test/functional/treesitter/language_spec.lua

@@ -2,6 +2,7 @@ local helpers = require('test.functional.helpers')(after_each)
 
 local clear = helpers.clear
 local eq = helpers.eq
+local command = helpers.command
 local exec_lua = helpers.exec_lua
 local pcall_err = helpers.pcall_err
 local matches = helpers.matches
@@ -67,5 +68,15 @@ describe('treesitter API', function()
     end
     eq({true,true}, {has_named,has_anonymous})
   end)
+
+  it('checks if vim.treesitter.get_parser tries to create a new parser on filetype change', function ()
+    command("set filetype=c")
+    -- Should not throw an error when filetype is c
+    eq('c', exec_lua("return vim.treesitter.get_parser(0):lang()"))
+    command("set filetype=borklang")
+    -- Should throw an error when filetype changes to borklang
+    eq("Error executing lua: .../language.lua:0: no parser for 'borklang' language, see :help treesitter-parsers",
+       pcall_err(exec_lua, "new_parser = vim.treesitter.get_parser(0)"))
+  end)
 end)