Implement declarative (macro_rules!) attribute macros (RFC 3697)

[joshtriplett]

This implements RFC 3697, "Declarative (macro_rules!) attribute macros".

I would suggest reading this commit-by-commit. This first introduces the
feature gate, then adds parsing for attribute rules (doing nothing with them),
then adds the ability to look up and apply macro_rules! attributes by path,
then adds support for local attributes, then adds a test, and finally makes
various improvements to errors.

[rustbot]

r? @petrochenkov

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[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[theemathas]

Here's an edge case that probably needs a test.

Code and error

#![feature(macro_attr)]

#[foo]
macro_rules! foo {
    attr() { $($body:tt)* } => {
        $($body)*
    }
}
use foo;

warning: unused macro definition: `foo`
 --> src/lib.rs:4:14
  |
4 | macro_rules! foo {
  |              ^^^
  |
  = note: `#[warn(unused_macros)]` on by default

error: cannot determine resolution for the attribute macro `foo`
 --> src/lib.rs:3:3
  |
3 | #[foo]
  |   ^^^
  |
  = note: import resolution is stuck, try simplifying macro imports

warning: `foo` (lib) generated 1 warning
error: could not compile `foo` (lib) due to 1 previous error; 1 warning emitted

[joshtriplett]

@theemathas I can't reproduce that here. I get a different error instead:

error: cannot find attribute `foo` in this scope
  --> $DIR/macro-rules-attr-cycle.rs:4:3
   |
LL | #[foo]
   |   ^^^
LL | macro_rules! foo {
   |              --- `foo` exists, but has no `attr` rules

This isn't the right error message, but the error is getting caught.

(Suggestions welcome for how to catch and handle this case better.)

[joshtriplett]

@theemathas I looked into your test case more. I managed to find a way to reproduce the error you got; it only reproduces if there aren't other errors. So, I can reproduce it with the following test:

#![crate_type = "lib"]
#![feature(macro_attr)]

#[attr]
macro_rules! attr {
    attr() {} => {}
}

#[attr]
struct S;

This gives:

error: cannot find attribute `attr` in this scope
  --> /home/josh/src/rust/tests/ui/macros/macro-rules-attr-apply-to-self.rs:4:3
   |
LL | #[attr]
   |   ^^^^
LL | macro_rules! attr {
   |              ---- `attr` exists, but has no `attr` rules

error: cannot determine resolution for the attribute macro `attr`
  --> /home/josh/src/rust/tests/ui/macros/macro-rules-attr-apply-to-self.rs:9:3
   |
LL | #[attr]
   |   ^^^^
   |
   = note: import resolution is stuck, try simplifying macro imports

The second error seems reasonable for attempting to invoke an attribute on itself.

The first error also largely seems fine, except that it shouldn't say has no `attr` rules when the real problem is a cycle.

Note that "cannot find" is the same error that you get if you try this:

mac! {
    macro_rules! mac {
        {} => {}
    }
}

mac! {
    struct S;
}

This gives:

error: cannot find macro `mac` in this scope
 --> src/main.rs:1:1
  |
1 | mac! {
  | ^^^

error: cannot find macro `mac` in this scope
 --> src/main.rs:7:1
  |
7 | mac! {
  | ^^^

So, I think "cannot find ... in this scope" is an acceptable error, albeit one we might be able to improve. I think the main problem here is the spurious `attr` exists, but has no `attr` rules.

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] rustc_public test:false 5.657
error: using `.clone()` on a ref-counted pointer
   --> compiler/rustc_resolve/src/macros.rs:838:85
    |
838 |                 Some(MacroKind::Attr) if let Some(ref ext) = macro_data.attr_ext => ext.clone(),
    |                                                                                     ^^^^^^^^^^^ help: try: `Arc::<rustc_expand::base::SyntaxExtension>::clone(&ext)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_ref_ptr
    = note: requested on the command line with `-D clippy::clone-on-ref-ptr`

error: using `.clone()` on a ref-counted pointer
   --> compiler/rustc_resolve/src/macros.rs:839:22
    |
839 |                 _ => macro_data.ext.clone(),
    |                      ^^^^^^^^^^^^^^^^^^^^^^ help: try: `Arc::<rustc_expand::base::SyntaxExtension>::clone(&macro_data.ext)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_ref_ptr

[RUSTC-TIMING] rustc_resolve test:false 8.357
error: could not compile `rustc_resolve` (lib) due to 2 previous errors

[joshtriplett]

I think this is ready for review. The error messages could always be better, and there are additional cases that need handling (notably glob imports), but I think it would help to have some review before tackling those.

[petrochenkov]

(I started looking today, but will continue only on ~Monday.)