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).

2 files changed, 66 insertions, 0 deletions

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
+