Auto merge of #97903 - est31:unused_macro_rules_compile_error, r=petrochenkov

Never regard macro rules with compile_error! invocations as unused

The very point of compile_error! is to never be reached, and one of
the use cases of the macro, currently also listed as examples in the
documentation of compile_error, is to create nicer errors for wrong
macro invocations. Thus, we should never warn about unused macro arms
that contain invocations of compile_error.

See also https://github.com/rust-lang/rust/pull/96150#issuecomment-1126599107 and the discussion after that.

Furthermore, the PR also contains two commits to silence `unused_macro_rules` when a macro has an invalid rule, and to add a test that `unused_macros` does not behave badly in the same situation.

r? `@petrochenkov` as I've talked to them about this

2 files changed, 4 insertions, 4 deletions

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 20d9123e411..f8fa7a0941d 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -1220,12 +1220,12 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
         ident: Ident,
         def_id: LocalDefId,
         node_id: NodeId,
-        rule_spans: &[Span],
+        rule_spans: &[(usize, Span)],
     ) {
         if !ident.as_str().starts_with('_') {
             self.r.unused_macros.insert(def_id, (node_id, ident));
-            for (rule_i, rule_span) in rule_spans.iter().enumerate() {
-                self.r.unused_macro_rules.insert((def_id, rule_i), (ident, *rule_span));
+            for (rule_i, rule_span) in rule_spans.iter() {
+                self.r.unused_macro_rules.insert((def_id, *rule_i), (ident, *rule_span));
             }
         }
     }
diff --git a/compiler/rustc_resolve/src/macros.rs b/compiler/rustc_resolve/src/macros.rs
index 2e2d3674560..3fb34cdcd9b 100644
--- a/compiler/rustc_resolve/src/macros.rs
+++ b/compiler/rustc_resolve/src/macros.rs
@@ -870,7 +870,7 @@ impl<'a> Resolver<'a> {
         &mut self,
         item: &ast::Item,
         edition: Edition,
-    ) -> (SyntaxExtension, Vec<Span>) {
+    ) -> (SyntaxExtension, Vec<(usize, Span)>) {
         let (mut result, mut rule_spans) = compile_declarative_macro(
             &self.session,
             self.session.features_untracked(),