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Rollup merge of #102287 - compiler-errors:unused-must-use-also-supertrait, r=fee1-dead

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Elaborate supertrait bounds when triggering `unused_must_use` on `impl Trait`

Given `impl Trait`, if one of its supertraits has a `#[must_use]`, then trigger the lint. This means that, for example, `-> impl ExactSizeIterator` also triggers the `must_use` on `trait Iterator`, which fixes #102183.

This might need `@rust-lang/lang` sign-off, since it changes the behavior of the lint, so cc'ing them.
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Dylan DPC 2022-10-21 17:29:57 +05:30 • committed by GitHub
commit 66d91d8276
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3 changed files with 32 additions and 2 deletions
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8
compiler/rustc_lint/src/unused.rs
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@ -7,6 +7,7 @@ use rustc_errors::{fluent, pluralize, Applicability, MultiSpan};
use rustc_hir as hir;
use rustc_hir::def::{DefKind, Res};
use rustc_hir::def_id::DefId;
use rustc_infer::traits::util::elaborate_predicates_with_span;
use rustc_middle::ty::adjustment;
use rustc_middle::ty::{self, Ty};
use rustc_span::symbol::Symbol;
@ -204,10 +205,13 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
ty::Adt(def, _) => check_must_use_def(cx, def.did(), span, descr_pre, descr_post),
ty::Opaque(def, _) => {
let mut has_emitted = false;
for &(predicate, _) in cx.tcx.explicit_item_bounds(def) {
for obligation in elaborate_predicates_with_span(
cx.tcx,
cx.tcx.explicit_item_bounds(def).iter().cloned(),
) {
// We only look at the `DefId`, so it is safe to skip the binder here.
if let ty::PredicateKind::Trait(ref poly_trait_predicate) =
predicate.kind().skip_binder()
obligation.predicate.kind().skip_binder()
{
let def_id = poly_trait_predicate.trait_ref.def_id;
let descr_pre =