Summary

Generalize the #[macro_registrar] feature so it can register other kinds of compiler plugins.

Motivation

I want to implement loadable lints and use them for project-specific static analysis passes in Servo. Landing this first will allow more evolution of the plugin system without breaking source compatibility for existing users.

Detailed design

To register a procedural macro in current Rust:

use syntax::ast::Name;
use syntax::parse::token;
use syntax::ext::base::{SyntaxExtension, BasicMacroExpander, NormalTT};

#[macro_registrar]
pub fn macro_registrar(register: |Name, SyntaxExtension|) {
    register(token::intern("named_entities"),
        NormalTT(box BasicMacroExpander {
            expander: named_entities::expand,
            span: None
        },
        None));
}

I propose an interface like

use syntax::parse::token;
use syntax::ext::base::{BasicMacroExpander, NormalTT};

use rustc::plugin::Registry;

#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_macro(token::intern("named_entities"),
        NormalTT(box BasicMacroExpander {
            expander: named_entities::expand,
            span: None
        },
        None));
}

Then the struct Registry could provide additional methods such as register_lint as those features are implemented.

It could also provide convenience methods:

use rustc::plugin::Registry;

#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_simple_macro("named_entities", named_entities::expand);
}

phase(syntax) becomes phase(plugin), with the former as a deprecated synonym that warns. This is to avoid silent breakage of the very common #[phase(syntax)] extern crate log.

We only need one phase of loading plugin crates, even though the plugins we load may be used at different points (or not at all).

Drawbacks

Breaking change for existing procedural macros.

More moving parts.

Registry is provided by librustc, because it will have methods for registering lints and other librustc things. This means that syntax extensions must link librustc, when before they only needed libsyntax (but could link librustc anyway if desired). This was discussed on the RFC PR and the Rust PR and on IRC.

#![feature(macro_registrar)] becomes unknown, contradicting a comment in feature_gate.rs:

This list can never shrink, it may only be expanded (in order to prevent old programs from failing to compile)

Since when do we ensure that old programs will compile? ;) The #[macro_registrar] attribute wouldn’t work anyway.

Alternatives

We could add #[lint_registrar] etc. alongside #[macro_registrar]. This seems like it will produce more duplicated effort all around. It doesn’t provide convenience methods, and it won’t support API evolution as well.

We could support the old #[macro_registrar] by injecting an adapter shim. This is significant extra work to support a feature with no stability guarantee.

Unresolved questions

Naming bikeshed.