nvim/lua/plugins/treesitter.lua

-- since `vim.tbl_deep_extend`, can only merge tables and not lists, the code above
-- would overwrite `ensure_installed` with the new value.
-- If you'd rather extend the default config, use the code below instead:
return {
  {
    "nvim-treesitter/nvim-treesitter",
    opts = function(_, opts)
      vim.list_extend(opts.ensure_installed, {
        "rust",
      })
    end,
  },
  {
    "nvim-treesitter/playground",
    dependencies = { "nvim-treesitter/nvim-treesitter" },
    cmd = "TSPlaygroundToggle",
  },
}