The tracking issue for this feature is: #29597

This feature is part of "compiler plugins." It will often be used with the plugin_registrar and rustc_private features.

rustc can load compiler plugins, which are user-provided libraries that extend the compiler's behavior with new syntax extensions, lint checks, etc.

A plugin is a dynamic library crate with a designated registrar function that registers extensions with rustc. Other crates can load these extensions using the crate attribute #![plugin(...)]. See the rustc_plugin documentation for more about the mechanics of defining and loading a plugin.

If present, arguments passed as #![plugin(foo(... args ...))] are not interpreted by rustc itself. They are provided to the plugin through the Registry's args method.

In the vast majority of cases, a plugin should only be used through #![plugin] and not through an extern crate item. Linking a plugin would pull in all of libsyntax and librustc as dependencies of your crate. This is generally unwanted unless you are building another plugin. The plugin_as_library lint checks these guidelines.

The usual practice is to put compiler plugins in their own crate, separate from any macro_rules! macros or ordinary Rust code meant to be used by consumers of a library.

Syntax extensions

Plugins can extend Rust's syntax in various ways. One kind of syntax extension is the procedural macro. These are invoked the same way as ordinary macros, but the expansion is performed by arbitrary Rust code that manipulates syntax trees at compile time.

Let's write a plugin that implements Roman numeral integer literals.

#![feature(plugin_registrar, rustc_private)]

extern crate syntax;
extern crate rustc;
extern crate rustc_plugin;

use syntax::parse::token;
use syntax::tokenstream::TokenTree;
use syntax::ext::base::{ExtCtxt, MacResult, DummyResult, MacEager};
use syntax::ext::build::AstBuilder;  // A trait for expr_usize.
use syntax::ext::quote::rt::Span;
use rustc_plugin::Registry;

fn expand_rn(cx: &mut ExtCtxt, sp: Span, args: &[TokenTree])
        -> Box<MacResult + 'static> {

    static NUMERALS: &'static [(&'static str, usize)] = &[
        ("M", 1000), ("CM", 900), ("D", 500), ("CD", 400),
        ("C",  100), ("XC",  90), ("L",  50), ("XL",  40),
        ("X",   10), ("IX",   9), ("V",   5), ("IV",   4),
        ("I",    1)];

    if args.len() != 1 {
            &format!("argument should be a single identifier, but got {} arguments", args.len()));
        return DummyResult::any(sp);

    let text = match args[0] {
        TokenTree::Token(_, token::Ident(s)) => s.to_string(),
        _ => {
            cx.span_err(sp, "argument should be a single identifier");
            return DummyResult::any(sp);

    let mut text = &*text;
    let mut total = 0;
    while !text.is_empty() {
        match NUMERALS.iter().find(|&&(rn, _)| text.starts_with(rn)) {
            Some(&(rn, val)) => {
                total += val;
                text = &text[rn.len()..];
            None => {
                cx.span_err(sp, "invalid Roman numeral");
                return DummyResult::any(sp);

    MacEager::expr(cx.expr_usize(sp, total))

pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_macro("rn", expand_rn);

Then we can use rn!() like any other macro:


fn main() {
    assert_eq!(rn!(MMXV), 2015);

The advantages over a simple fn(&str) -> u32 are:

  • The (arbitrarily complex) conversion is done at compile time.
  • Input validation is also performed at compile time.
  • It can be extended to allow use in patterns, which effectively gives a way to define new literal syntax for any data type.

In addition to procedural macros, you can define new derive-like attributes and other kinds of extensions. See Registry::register_syntax_extension and the SyntaxExtension enum. For a more involved macro example, see regex_macros.

Tips and tricks

Some of the macro debugging tips are applicable.

You can use syntax::parse to turn token trees into higher-level syntax elements like expressions:

fn expand_foo(cx: &mut ExtCtxt, sp: Span, args: &[TokenTree])
        -> Box<MacResult+'static> {

    let mut parser = cx.new_parser_from_tts(args);

    let expr: P<Expr> = parser.parse_expr();

Looking through libsyntax parser code will give you a feel for how the parsing infrastructure works.

Keep the Spans of everything you parse, for better error reporting. You can wrap Spanned around your custom data structures.

Calling ExtCtxt::span_fatal will immediately abort compilation. It's better to instead call ExtCtxt::span_err and return DummyResult so that the compiler can continue and find further errors.

To print syntax fragments for debugging, you can use span_note together with syntax::print::pprust::*_to_string.

The example above produced an integer literal using AstBuilder::expr_usize. As an alternative to the AstBuilder trait, libsyntax provides a set of quasiquote macros. They are undocumented and very rough around the edges. However, the implementation may be a good starting point for an improved quasiquote as an ordinary plugin library.

Lint plugins

Plugins can extend Rust's lint infrastructure with additional checks for code style, safety, etc. Now let's write a plugin that warns about any item named lintme.

#![feature(box_syntax, rustc_private)]

extern crate syntax;

// Load rustc as a plugin to get macros
extern crate rustc;
extern crate rustc_plugin;

use rustc::lint::{EarlyContext, LintContext, LintPass, EarlyLintPass,
                  EarlyLintPassObject, LintArray};
use rustc_plugin::Registry;
use syntax::ast;

declare_lint!(TEST_LINT, Warn, "Warn about items named 'lintme'");

struct Pass;

impl LintPass for Pass {
    fn get_lints(&self) -> LintArray {

impl EarlyLintPass for Pass {
    fn check_item(&mut self, cx: &EarlyContext, it: &ast::Item) {
        if == "lintme" {
            cx.span_lint(TEST_LINT, it.span, "item is named 'lintme'");

pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_early_lint_pass(box Pass as EarlyLintPassObject);

Then code like


fn lintme() { }

will produce a compiler warning: 4:16 warning: item is named 'lintme', #[warn(test_lint)] on by default fn lintme() { }

The components of a lint plugin are:

  • one or more declare_lint! invocations, which define static Lint structs;

  • a struct holding any state needed by the lint pass (here, none);

  • a LintPass implementation defining how to check each syntax element. A single LintPass may call span_lint for several different Lints, but should register them all through the get_lints method.

Lint passes are syntax traversals, but they run at a late stage of compilation where type information is available. rustc's built-in lints mostly use the same infrastructure as lint plugins, and provide examples of how to access type information.

Lints defined by plugins are controlled by the usual attributes and compiler flags, e.g. #[allow(test_lint)] or -A test-lint. These identifiers are derived from the first argument to declare_lint!, with appropriate case and punctuation conversion.

You can run rustc -W help to see a list of lints known to rustc, including those provided by plugins loaded by