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Auto merge of #85312 - ehuss:macro_use-unused-attr, r=petrochenkov
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Fix unused attributes on macro_rules.

The `unused_attributes` lint wasn't firing on attributes of `macro_rules` definitions. The consequence is that many attributes are silently ignored on `macro_rules`. The reason is that `unused_attributes` is a late-lint pass, and only has access to the HIR, which does not have macro_rules definitions.

My solution here is to change `non_exported_macro_attrs` to be `macro_attrs` (a list of all attributes used for `macro_rules`, instead of just those for `macro_export`), and then to check this list in the `unused_attributes` lint. There are a number of alternate approaches, but this seemed the most reliable and least invasive. I am open to completely different approaches, though.

One concern is that I don't fully understand the implications of extending `non_exported_macro_attrs` to include non-exported macros. That list was originally added in #62042 to handle stability attributes, so I suspect it was just an optimization since that was all that was needed. It was later extended to be included in SVH in #83901. #80641 also added a use to check for `invalid` attributes, which seems a little odd to me (it didn't validate non-exported macros, and seems highly specific).

Overall, there doesn't seem to be a clear story of when `unused_attributes` should be used versus an error like E0518. I considered alternatively using an "allow list" of built-in attributes that can be used on macro_rules (allow, warn, deny, forbid, cfg, cfg_attr, macro_export, deprecated, doc), but I feel like that could be a pain to maintain.

Some built-in attributes already present hard-errors when used with macro_rules. These are each hard-coded in various places:
- `derive`
- `test` and `bench`
- `proc_macro` and `proc_macro_derive`
- `inline`
- `global_allocator`

The primary motivation is that I sometimes see people use `#[macro_use]` in front of `macro_rules`, which indicates there is some confusion out there (evident that there was even a case of it in rustc).
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bors committed May 16, 2021
2 parents 7dc9ff5 + 5bbc240 commit fe72845
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Showing 4 changed files with 70 additions and 3 deletions.
4 changes: 4 additions & 0 deletions compiler/rustc_lint/src/late.rs
Original file line number Diff line number Diff line change
Expand Up @@ -448,6 +448,10 @@ fn late_lint_pass_crate<'tcx, T: LateLintPass<'tcx>>(tcx: TyCtxt<'tcx>, pass: T)
lint_callback!(cx, check_crate, krate);

hir_visit::walk_crate(cx, krate);
for attr in krate.non_exported_macro_attrs {
// This HIR ID is a lie, since the macro ID isn't available.
cx.visit_attribute(hir::CRATE_HIR_ID, attr);
}

lint_callback!(cx, check_crate_post, krate);
})
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3 changes: 0 additions & 3 deletions compiler/rustc_target/src/asm/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,6 @@ use rustc_span::Symbol;
use std::fmt;
use std::str::FromStr;

#[macro_use]
macro_rules! def_reg_class {
($arch:ident $arch_regclass:ident {
$(
Expand Down Expand Up @@ -51,7 +50,6 @@ macro_rules! def_reg_class {
}
}

#[macro_use]
macro_rules! def_regs {
($arch:ident $arch_reg:ident $arch_regclass:ident {
$(
Expand Down Expand Up @@ -129,7 +127,6 @@ macro_rules! def_regs {
}
}

#[macro_use]
macro_rules! types {
(
$(_ : $($ty:expr),+;)?
Expand Down
34 changes: 34 additions & 0 deletions src/test/ui/unused/unused-attr-macro-rules.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,34 @@
#![deny(unused_attributes)]
// Unused attributes on macro_rules requires special handling since the
// macro_rules definition does not survive towards HIR.

// A sample of various built-in attributes.
#[macro_export]
#[macro_use] //~ ERROR unused attribute
#[path="foo"] //~ ERROR unused attribute
#[recursion_limit="1"] //~ ERROR unused attribute
//~| ERROR crate-level attribute should be an inner attribute
macro_rules! foo {
() => {};
}

// The following should not warn about unused attributes.
#[allow(unused)]
macro_rules! foo2 {
() => {};
}

#[cfg(FALSE)]
macro_rules! foo {
() => {};
}

/// Some docs
#[deprecated]
#[doc = "more docs"]
#[macro_export]
macro_rules! bar {
() => {};
}

fn main() {}
32 changes: 32 additions & 0 deletions src/test/ui/unused/unused-attr-macro-rules.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,32 @@
error: unused attribute
--> $DIR/unused-attr-macro-rules.rs:7:1
|
LL | #[macro_use]
| ^^^^^^^^^^^^
|
note: the lint level is defined here
--> $DIR/unused-attr-macro-rules.rs:1:9
|
LL | #![deny(unused_attributes)]
| ^^^^^^^^^^^^^^^^^

error: unused attribute
--> $DIR/unused-attr-macro-rules.rs:8:1
|
LL | #[path="foo"]
| ^^^^^^^^^^^^^

error: unused attribute
--> $DIR/unused-attr-macro-rules.rs:9:1
|
LL | #[recursion_limit="1"]
| ^^^^^^^^^^^^^^^^^^^^^^

error: crate-level attribute should be an inner attribute: add an exclamation mark: `#![foo]`
--> $DIR/unused-attr-macro-rules.rs:9:1
|
LL | #[recursion_limit="1"]
| ^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 4 previous errors

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