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Rollup merge of #131857 - WaffleLapkin:dyn-drop-principal-3, r=compiler-errors

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Allow dropping dyn principal

Revival of #126660, which was a revival of #114679. Fixes #126313.

Allows dropping principal when coercing trait objects, e.g. `dyn Debug + Send` -> `dyn Send`.

cc `@compiler-errors` `@Jules-Bertholet`
r? `@lcnr`
This commit is contained in:
Matthias Krüger 2024-10-18 06:59:07 +02:00 • committed by GitHub
commit 13b398401f
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14 changed files with 167 additions and 35 deletions
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2
compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
View file

@ -1018,7 +1018,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
// #2 (region bounds).
let principal_def_id_a = a_data.principal_def_id();
let principal_def_id_b = b_data.principal_def_id();
if principal_def_id_a == principal_def_id_b {
if principal_def_id_a == principal_def_id_b || principal_def_id_b.is_none() {
// We may upcast to auto traits that are either explicitly listed in
// the object type's bounds, or implied by the principal trait ref's
// supertraits.

4
compiler/rustc_trait_selection/src/traits/select/confirmation.rs
View file

@ -1153,6 +1153,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
// We already checked the compatibility of auto traits within `assemble_candidates_for_unsizing`.
let iter = data_a
.principal()
.filter(|_| {
// optionally drop the principal, if we're unsizing to no principal
data_b.principal().is_some()
})
.map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
.into_iter()
.chain(