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Auto merge of #117610 - compiler-errors:object-hmm, r=aliemjay
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Only instantiate binder during dyn's built-in trait candidate probe once

See UI test for demonstration of the issue.

This was "caused" by #117131, but only because we're using the `normalize_param_env` (which has been augmented with a projection clause used to normalize GATs) which features non-lifetime bound vars in it.

Fixes #117602 technically, though that's also fixed by #117542.

r? types
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bors committed Nov 7, 2023
2 parents 504f63e + 171d558 commit 114f1f6
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Showing 5 changed files with 64 additions and 14 deletions.
Original file line number Diff line number Diff line change
Expand Up @@ -628,15 +628,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}

self.infcx.probe(|_snapshot| {
if obligation.has_non_region_late_bound() {
return;
}
let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
let placeholder_trait_predicate =
self.infcx.instantiate_binder_with_placeholders(poly_trait_predicate);

// The code below doesn't care about regions, and the
// self-ty here doesn't escape this probe, so just erase
// any LBR.
let self_ty = self.tcx().erase_late_bound_regions(obligation.self_ty());
let poly_trait_ref = match self_ty.kind() {
let self_ty = placeholder_trait_predicate.self_ty();
let principal_trait_ref = match self_ty.kind() {
ty::Dynamic(ref data, ..) => {
if data.auto_traits().any(|did| did == obligation.predicate.def_id()) {
debug!(
Expand Down Expand Up @@ -668,18 +665,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
_ => return,
};

debug!(?poly_trait_ref, "assemble_candidates_from_object_ty");

let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
let placeholder_trait_predicate =
self.infcx.instantiate_binder_with_placeholders(poly_trait_predicate);
debug!(?principal_trait_ref, "assemble_candidates_from_object_ty");

// Count only those upcast versions that match the trait-ref
// we are looking for. Specifically, do not only check for the
// correct trait, but also the correct type parameters.
// For example, we may be trying to upcast `Foo` to `Bar<i32>`,
// but `Foo` is declared as `trait Foo: Bar<u32>`.
let candidate_supertraits = util::supertraits(self.tcx(), poly_trait_ref)
let candidate_supertraits = util::supertraits(self.tcx(), principal_trait_ref)
.enumerate()
.filter(|&(_, upcast_trait_ref)| {
self.infcx.probe(|_| {
Expand Down
11 changes: 11 additions & 0 deletions tests/ui/traits/non-lifetime-via-dyn-builtin.current.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,11 @@
warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
--> $DIR/non-lifetime-via-dyn-builtin.rs:5:12
|
LL | #![feature(non_lifetime_binders)]
| ^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
= note: `#[warn(incomplete_features)]` on by default

warning: 1 warning emitted

11 changes: 11 additions & 0 deletions tests/ui/traits/non-lifetime-via-dyn-builtin.next.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,11 @@
warning: the feature `non_lifetime_binders` is incomplete and may not be safe to use and/or cause compiler crashes
--> $DIR/non-lifetime-via-dyn-builtin.rs:5:12
|
LL | #![feature(non_lifetime_binders)]
| ^^^^^^^^^^^^^^^^^^^^
|
= note: see issue #108185 <https://github.com/rust-lang/rust/issues/108185> for more information
= note: `#[warn(incomplete_features)]` on by default

warning: 1 warning emitted

16 changes: 16 additions & 0 deletions tests/ui/traits/non-lifetime-via-dyn-builtin.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,16 @@
// revisions: current next
//[next] compile-flags: -Ztrait-solver=next
// check-pass

#![feature(non_lifetime_binders)]
//~^ WARN the feature `non_lifetime_binders` is incomplete and may not be safe

fn trivial<A>()
where
for<B> dyn Fn(A, *const B): Fn(A, *const B),
{
}

fn main() {
trivial::<u8>();
}
Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
// check-pass

trait Foo {
type Bar<T>
where
dyn Send + 'static: Send;
}

impl Foo for () {
type Bar<T> = i32;
// We take `<() as Foo>::Bar<T>: Sized` and normalize it under the where clause
// of `for<S> <() as Foo>::Bar<S> = i32`. This gives us back `i32: Send` with
// the nested obligation `(dyn Send + 'static): Send`. However, during candidate
// assembly for object types, we disqualify any obligations that has non-region
// late-bound vars in the param env(!), rather than just the predicate. This causes
// the where clause to not hold even though it trivially should.
}

fn main() {}

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