bjoernager/rust
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Rollup merge of #95609 - compiler-errors:borrow-unsized-to-dyn, r=nagisa

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Suggest borrowing when trying to coerce unsized type into `dyn Trait`

A helpful error in response to #95598, since we can't coerce e.g. `&str` into `&dyn Display`, but we can coerce `&&str` into `&dyn Display` :)

Not sure if the suggestion message needs some help. Let me know, and I can refine this PR.
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Dylan DPC 2022-04-04 20:41:34 +02:00 • committed by GitHub
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6
compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
View file

@ -474,6 +474,12 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
err.span_label(span, explanation);
}
if let ObligationCauseCode::ObjectCastObligation(obj_ty) = obligation.cause.code().peel_derives() &&
let Some(self_ty) = trait_predicate.self_ty().no_bound_vars() &&
Some(trait_ref.def_id()) == self.tcx.lang_items().sized_trait() {
self.suggest_borrowing_for_object_cast(&mut err, &obligation, self_ty, *obj_ty);
}
if trait_predicate.is_const_if_const() && obligation.param_env.is_const() {
let non_const_predicate = trait_ref.without_const();
let non_const_obligation = Obligation {

37
compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
View file

@ -77,6 +77,14 @@ pub trait InferCtxtExt<'tcx> {
has_custom_message: bool,
) -> bool;
fn suggest_borrowing_for_object_cast(
&self,
err: &mut Diagnostic,
obligation: &PredicateObligation<'tcx>,
self_ty: Ty<'tcx>,
object_ty: Ty<'tcx>,
);
fn suggest_remove_reference(
&self,
obligation: &PredicateObligation<'tcx>,
@ -801,6 +809,35 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
}
}
// Suggest borrowing the type
fn suggest_borrowing_for_object_cast(
&self,
err: &mut Diagnostic,
obligation: &PredicateObligation<'tcx>,
self_ty: Ty<'tcx>,
object_ty: Ty<'tcx>,
) {
let ty::Dynamic(predicates, _) = object_ty.kind() else { return; };
let self_ref_ty = self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, self_ty);
for predicate in predicates.iter() {
if !self.predicate_must_hold_modulo_regions(
&obligation.with(predicate.with_self_ty(self.tcx, self_ref_ty)),
) {
return;
}
}
err.span_suggestion(
obligation.cause.span.shrink_to_lo(),
&format!(
"consider borrowing the value, since `&{self_ty}` can be coerced into `{object_ty}`"
),
"&".to_string(),
Applicability::MaybeIncorrect,
);
}
/// Whenever references are used by mistake, like `for (i, e) in &vec.iter().enumerate()`,
/// suggest removing these references until we reach a type that implements the trait.
fn suggest_remove_reference(