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 :)

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 73c8a9d28bd..49c15d2c9ef 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -28,7 +28,7 @@ use rustc_ast::node_id::NodeMap;
 use rustc_ast::{self as ast, NodeId, CRATE_NODE_ID};
 use rustc_ast::{AngleBracketedArg, Crate, Expr, ExprKind, GenericArg, GenericArgs, LitKind, Path};
 use rustc_ast_lowering::{LifetimeRes, ResolverAstLowering};
-use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
+use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_data_structures::intern::Interned;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, DiagnosticBuilder, ErrorGuaranteed};
@@ -941,7 +941,7 @@ pub struct Resolver<'a> {
     visibilities: FxHashMap<LocalDefId, ty::Visibility>,
     has_pub_restricted: bool,
     used_imports: FxHashSet<NodeId>,
-    maybe_unused_trait_imports: FxHashSet<LocalDefId>,
+    maybe_unused_trait_imports: FxIndexSet<LocalDefId>,
     maybe_unused_extern_crates: Vec<(LocalDefId, Span)>,
 
     /// Privacy errors are delayed until the end in order to deduplicate them.