- Start Date: 2012-03-20
- RFC PR: [rust-lang/rfcs#3](https://github.com/rust-lang/rfcs/pull/3)
- Rust Issue: [rust-lang/rust#14373](https://github.com/rust-lang/rust/issues/14373)

## Summary

Rust currently has an attribute usage lint but it does not work particularly
well. This RFC proposes a new implementation strategy that should make it
significantly more useful.

## Motivation

The current implementation has two major issues:

+ There are very limited warnings for valid attributes that end up in the
wrong place. Something like this will be silently ignored:
```rust
#[deriving(Clone)]; // Shouldn't have put a ; here
struct Foo;

#[ignore(attribute-usage)] // Should have used #[allow(attribute-usage)] instead!
mod bar {
    //...
}
```
+ `ItemDecorators` can now be defined outside of the compiler, and there's no
way to tag them and associated attributes as valid. Something like this
requires an `#[allow(attribute-usage)]`:
```rust
#[feature(phase)];
#[phase(syntax, link)]
extern crate some_orm;

#[ormify]
pub struct Foo {
    #[column(foo_)]
    #[primary_key]
    foo: int
}
```

## Detailed design

The current implementation is implemented as a simple fold over the AST,
comparing attributes against a whitelist. Crate-level attributes use a separate
whitelist, but no other distinctions are made.

This RFC would change the implementation to actually track which attributes are
used during the compilation process. `syntax::ast::Attribute_` would be
modified to add an ID field:
```rust
pub struct AttrId(uint);

pub struct Attribute_ {
    id: AttrId,
    style: AttrStyle,
    value: @MetaItem,
    is_sugared_doc: bool,
}
```

`syntax::ast::parse::ParseSess` will generate new `AttrId`s on demand. I
believe that attributes will only be created during parsing and expansion, and
the `ParseSess` is accessible in both.

The `AttrId`s will be used to create a side table of used attributes. This will
most likely be a thread local to make it easily accessible during all stages of
compilation by calling a function in `syntax::attr`:
```rust
fn mark_used(attr: &Attribute) { }
```

The `attribute-usage` lint would run at the end of compilation and warn on all
attributes whose ID does not appear in the side table.

One interesting edge case is attributes like `doc` that are used, but not in
the normal compilation process. There could either be a separate fold pass to
mark all `doc` attributes as used or `doc` could simply be whitelisted in the
`attribute-usage` lint.

Attributes in code that has been eliminated with `#[cfg()]` will not be linted,
but I feel that this is consistent with the way `#[cfg()]` works in general
(e.g. the code won't be type-checked either).

## Alternatives

An alternative would be to rewrite `rustc::middle::lint` to robustly check
that attributes are used where they're supposed to be. This will be fairly
complex and be prone to failure if/when more nodes are added to the AST. This
also doesn't solve motivation #2, which would require externally loaded lint
support.

## Unresolved questions

+ This implementation doesn't allow for a distinction between "unused" and
"unknown" attributes. The `#[phase(syntax)]` crate loading infrastructure could
be extended to pull a list of attributes from crates to use in the lint pass,
but I'm not sure if the extra complexity is worth it.
+ The side table could be threaded through all of the compilation stages that
need to use it instead of being a thread local. This would probably require
significantly more work than the thread local approach, however. The thread
local approach should not negatively impact any future parallelization work as
each thread can keep its own side table, which can be merged into one for the
lint pass.

