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_middle/src/query/mod.rs

@@ -1728,8 +1728,8 @@ rustc_queries! {
     query upvars_mentioned(def_id: DefId) -> Option<&'tcx FxIndexMap<hir::HirId, hir::Upvar>> {
         desc { |tcx| "collecting upvars mentioned in `{}`", tcx.def_path_str(def_id) }
     }
-    query maybe_unused_trait_import(def_id: LocalDefId) -> bool {
-        desc { |tcx| "maybe_unused_trait_import for `{}`", tcx.def_path_str(def_id.to_def_id()) }
+    query maybe_unused_trait_imports(_: ()) -> &'tcx FxIndexSet<LocalDefId> {
+        desc { "fetching potentially unused trait imports" }
     }
     query maybe_unused_extern_crates(_: ()) -> &'tcx [(LocalDefId, Span)] {
         desc { "looking up all possibly unused extern crates" }