# Compiler Plugins

# Introduction

`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 `roman_numerals.rs` that implements Roman numeral integer literals.

    #![crate_type="dylib"]
    #![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 {
            cx.span_err(
                sp,
                &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))
    }

    #[plugin_registrar]
    pub fn plugin_registrar(reg: &mut Registry) {
        reg.register_macro("rn", expand_rn);
    }Run

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

    #![feature(plugin)]
    #![plugin(roman_numerals)]

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

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();Run

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

Keep the `Span`s 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 `lint_plugin_test.rs` that warns about any item named `lintme`.

    #![feature(plugin_registrar)]
    #![feature(box_syntax, rustc_private)]

    extern crate syntax;

    // Load rustc as a plugin to get macros
    #[macro_use]
    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 {
            lint_array!(TEST_LINT)
        }
    }

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

    #[plugin_registrar]
    pub fn plugin_registrar(reg: &mut Registry) {
        reg.register_early_lint_pass(box Pass as EarlyLintPassObject);
    }Run

Then code like

    #![plugin(lint_plugin_test)]

    fn lintme() { }Run

will produce a compiler warning:

    foo.rs:4:1: 4:16 warning: item is named 'lintme', #[warn(test_lint)] on by default
    foo.rs:4 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 `Lint`s, 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 foo.rs` to see a list of lints known to `rustc`, including those provided by plugins loaded by `foo.rs`.