Rollup merge of #97609 - Elliot-Roberts:unused-trait-refactor, r=cjgillot

Iterate over `maybe_unused_trait_imports` when checking dead trait imports

Closes #96873
r? `@cjgillot`

Some questions, if you have time:

- Is there a way to shorten the `rustc_data_structures::fx::FxIndexSet` path in the query declaration? I wasn't sure where to put a `use`.
- Was returning by reference from the query the right choice here?
- How would I go about evaluating the importance of the `is_dummy()` call in `check_crate`? I don't see failing tests when I comment it out. Should I just try to determine whether dummy spans can ever be put into `maybe_unused_trait_imports`?
- Am I doing anything silly with the various ID types?
- Is that `let-else` with `unreachable!()` bad? (i.e is there a better idiom? Would `panic!("<explanation>")` be better?)
- If I want to evaluate the perf of using a `Vec` as mentioned in #96873, is the best way to use the CI or is it feasible locally?

Thanks :)

compiler/rustc_typeck/src/check_unused.rs

@@ -16,48 +16,32 @@ pub fn check_crate(tcx: TyCtxt<'_>) {
         used_trait_imports.extend(imports.iter());
     }
 
-    for id in tcx.hir().items() {
-        if matches!(tcx.def_kind(id.def_id), DefKind::Use) {
-            if tcx.visibility(id.def_id).is_public() {
-                continue;
-            }
-            let item = tcx.hir().item(id);
-            if item.span.is_dummy() {
-                continue;
-            }
-            if let hir::ItemKind::Use(path, _) = item.kind {
-                check_import(tcx, &mut used_trait_imports, item.item_id(), path.span);
-            }
+    for &id in tcx.maybe_unused_trait_imports(()) {
+        debug_assert_eq!(tcx.def_kind(id), DefKind::Use);
+        if tcx.visibility(id).is_public() {
+            continue;
         }
+        if used_trait_imports.contains(&id) {
+            continue;
+        }
+        let item = tcx.hir().expect_item(id);
+        if item.span.is_dummy() {
+            continue;
+        }
+        let hir::ItemKind::Use(path, _) = item.kind else { unreachable!() };
+        tcx.struct_span_lint_hir(lint::builtin::UNUSED_IMPORTS, item.hir_id(), path.span, |lint| {
+            let msg = if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(path.span) {
+                format!("unused import: `{}`", snippet)
+            } else {
+                "unused import".to_owned()
+            };
+            lint.build(&msg).emit();
+        });
     }
 
     unused_crates_lint(tcx);
 }
 
-fn check_import<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    used_trait_imports: &mut FxHashSet<LocalDefId>,
-    item_id: hir::ItemId,
-    span: Span,
-) {
-    if !tcx.maybe_unused_trait_import(item_id.def_id) {
-        return;
-    }
-
-    if used_trait_imports.contains(&item_id.def_id) {
-        return;
-    }
-
-    tcx.struct_span_lint_hir(lint::builtin::UNUSED_IMPORTS, item_id.hir_id(), span, |lint| {
-        let msg = if let Ok(snippet) = tcx.sess.source_map().span_to_snippet(span) {
-            format!("unused import: `{}`", snippet)
-        } else {
-            "unused import".to_owned()
-        };
-        lint.build(&msg).emit();
-    });
-}
-
 fn unused_crates_lint(tcx: TyCtxt<'_>) {
     let lint = lint::builtin::UNUSED_EXTERN_CRATES;