Auto merge of #85312 - ehuss:macro_use-unused-attr, r=petrochenkov

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).

4 files changed, 70 insertions, 3 deletions

diff --git a/compiler/rustc_lint/src/late.rs b/compiler/rustc_lint/src/late.rs
index d325b5fe7f8..0a7ab4a2baf 100644
--- a/compiler/rustc_lint/src/late.rs
+++ b/compiler/rustc_lint/src/late.rs
@@ -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);
     })
diff --git a/compiler/rustc_target/src/asm/mod.rs b/compiler/rustc_target/src/asm/mod.rs
index c2f6bb76295..c17c2961434 100644
--- a/compiler/rustc_target/src/asm/mod.rs
+++ b/compiler/rustc_target/src/asm/mod.rs
@@ -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 {
         $(
@@ -51,7 +50,6 @@ macro_rules! def_reg_class {
     }
 }
 
-#[macro_use]
 macro_rules! def_regs {
     ($arch:ident $arch_reg:ident $arch_regclass:ident {
         $(
@@ -129,7 +127,6 @@ macro_rules! def_regs {
     }
 }
 
-#[macro_use]
 macro_rules! types {
     (
         $(_ : $($ty:expr),+;)?
diff --git a/src/test/ui/unused/unused-attr-macro-rules.rs b/src/test/ui/unused/unused-attr-macro-rules.rs
new file mode 100644
index 00000000000..396137a11d0
--- /dev/null
+++ b/src/test/ui/unused/unused-attr-macro-rules.rs
@@ -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() {}
diff --git a/src/test/ui/unused/unused-attr-macro-rules.stderr b/src/test/ui/unused/unused-attr-macro-rules.stderr
new file mode 100644
index 00000000000..4606be01ac0
--- /dev/null
+++ b/src/test/ui/unused/unused-attr-macro-rules.stderr
@@ -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
+