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Rollup merge of #93977 - compiler-errors:sized-generic-metadata, r=wesleywiser

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Type params and assoc types have unit metadata if they are sized

Extend the logic in `Pointee` projection to ensure that we can satisfy `<T as Pointee>::Metadata = ()` if `T: Sized`.

cc: `@SimonSapin` and #93959
This commit is contained in:
Matthias Krüger 2022-03-14 17:24:57 +01:00 • committed by GitHub
commit 774655da5f
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5 changed files with 134 additions and 15 deletions
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35
compiler/rustc_trait_selection/src/traits/project.rs
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@ -1469,6 +1469,8 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
let tail = selcx.tcx().struct_tail_with_normalize(self_ty, |ty| {
// We throw away any obligations we get from this, since we normalize
// and confirm these obligations once again during confirmation
normalize_with_depth(
selcx,
obligation.param_env,
@ -1485,7 +1487,6 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Foreign(_)
| ty::Str
| ty::Array(..)
| ty::Slice(_)
@ -1498,6 +1499,8 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
| ty::Generator(..)
| ty::GeneratorWitness(..)
| ty::Never
// Extern types have unit metadata, according to RFC 2850
| ty::Foreign(_)
// If returned by `struct_tail_without_normalization` this is a unit struct
// without any fields, or not a struct, and therefore is Sized.
| ty::Adt(..)
@ -1506,9 +1509,18 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
// Integers and floats are always Sized, and so have unit type metadata.
| ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
ty::Projection(..)
// type parameters, opaques, and unnormalized projections have pointer
// metadata if they're known (e.g. by the param_env) to be sized
ty::Param(_) | ty::Projection(..) | ty::Opaque(..)
if tail.is_sized(selcx.tcx().at(obligation.cause.span), obligation.param_env) =>
{
true
}
// FIXME(compiler-errors): are Bound and Placeholder types ever known sized?
ty::Param(_)
| ty::Projection(..)
| ty::Opaque(..)
| ty::Param(..)
| ty::Bound(..)
| ty::Placeholder(..)
| ty::Infer(..)
@ -1517,7 +1529,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
candidate_set.mark_ambiguous();
}
false
},
}
}
}
super::ImplSource::Param(..) => {
@ -1727,7 +1739,7 @@ fn confirm_pointee_candidate<'cx, 'tcx>(
let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
let mut obligations = vec![];
let metadata_ty = self_ty.ptr_metadata_ty(tcx, |ty| {
let (metadata_ty, check_is_sized) = self_ty.ptr_metadata_ty(tcx, |ty| {
normalize_with_depth_to(
selcx,
obligation.param_env,
@ -1737,6 +1749,19 @@ fn confirm_pointee_candidate<'cx, 'tcx>(
&mut obligations,
)
});
if check_is_sized {
let sized_predicate = ty::Binder::dummy(ty::TraitRef::new(
tcx.require_lang_item(LangItem::Sized, None),
tcx.mk_substs_trait(self_ty, &[]),
))
.without_const()
.to_predicate(tcx);
obligations.push(Obligation::new(
obligation.cause.clone(),
obligation.param_env,
sized_predicate,
));
}
let substs = tcx.mk_substs([self_ty.into()].iter());
let metadata_def_id = tcx.require_lang_item(LangItem::Metadata, None);