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Auto merge of rust-lang#112422 - aliemjay:implied-bounds-placeholders…
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…, r=lcnr

ignore implied bounds with placeholders

given the following code:
```rust
trait Trait {
    type Ty<'a> where Self: 'a;
}

impl<T> Trait for T {
    type Ty<'a> = () where Self: 'a;
}

struct Foo<T: Trait>(T)
where
    for<'x> T::Ty<'x>: Sized;
```

when computing the implied bounds from `Foo<X>` we incorrectly get the bound `X: !x` from the normalization of ` for<'x> <X as Trait>::Ty::<'x>: Sized`. This is a a known bug! we shouldn't use the constraints that arise from normalization as implied bounds. See rust-lang#109628.

Ignore these bounds for now. This should prevent later ICEs.

Fixes rust-lang#112250
Fixes rust-lang#107409
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bors committed Nov 17, 2023
2 parents 78efca8 + af79fd1 commit 4d7f952
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Showing 4 changed files with 81 additions and 3 deletions.
Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ use rustc_infer::infer::region_constraints::GenericKind;
use rustc_infer::infer::InferCtxt;
use rustc_middle::mir::ConstraintCategory;
use rustc_middle::traits::query::OutlivesBound;
use rustc_middle::ty::{self, RegionVid, Ty};
use rustc_middle::ty::{self, RegionVid, Ty, TypeVisitableExt};
use rustc_span::{ErrorGuaranteed, Span, DUMMY_SP};
use rustc_trait_selection::traits::query::type_op::{self, TypeOp};
use std::rc::Rc;
Expand Down Expand Up @@ -321,6 +321,9 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
.map_err(|_: ErrorGuaranteed| debug!("failed to compute implied bounds {:?}", ty))
.ok()?;
debug!(?bounds, ?constraints);
// Because of #109628, we may have unexpected placeholders. Ignore them!
// FIXME(#109628): panic in this case once the issue is fixed.
let bounds = bounds.into_iter().filter(|bound| !bound.has_placeholders());
self.add_outlives_bounds(bounds);
constraints
}
Expand Down
8 changes: 6 additions & 2 deletions compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
Original file line number Diff line number Diff line change
Expand Up @@ -72,7 +72,7 @@ impl<'a, 'tcx: 'a> InferCtxtExt<'a, 'tcx> for InferCtxt<'tcx> {
};

let mut constraints = QueryRegionConstraints::default();
let Ok(InferOk { value, obligations }) = self
let Ok(InferOk { value: mut bounds, obligations }) = self
.instantiate_nll_query_response_and_region_obligations(
&ObligationCause::dummy(),
param_env,
Expand All @@ -85,6 +85,10 @@ impl<'a, 'tcx: 'a> InferCtxtExt<'a, 'tcx> for InferCtxt<'tcx> {
};
assert_eq!(&obligations, &[]);

// Because of #109628, we may have unexpected placeholders. Ignore them!
// FIXME(#109628): panic in this case once the issue is fixed.
bounds.retain(|bound| !bound.has_placeholders());

if !constraints.is_empty() {
let span = self.tcx.def_span(body_id);

Expand Down Expand Up @@ -114,7 +118,7 @@ impl<'a, 'tcx: 'a> InferCtxtExt<'a, 'tcx> for InferCtxt<'tcx> {
}
};

value
bounds
}

fn implied_bounds_tys(
Expand Down
15 changes: 15 additions & 0 deletions tests/ui/implied-bounds/normalization-placeholder-leak.fail.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
error[E0477]: the type `&'lt u8` does not fulfill the required lifetime
--> $DIR/normalization-placeholder-leak.rs:31:40
|
LL | fn test_lifetime<'lt, T: Trait>(_: Foo<&'lt u8>) {}
| ^^^^^^^^^^^^

error[E0477]: the type `<T as AnotherTrait>::Ty2<'lt>` does not fulfill the required lifetime
--> $DIR/normalization-placeholder-leak.rs:36:44
|
LL | fn test_alias<'lt, T: AnotherTrait>(_: Foo<T::Ty2::<'lt>>) {}
| ^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0477`.
56 changes: 56 additions & 0 deletions tests/ui/implied-bounds/normalization-placeholder-leak.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,56 @@
// Because of #109628, when we compute the implied bounds from `Foo<X>`,
// we incorrectly get `X: placeholder('x)`.
// Make sure we ignore these bogus bounds and not use them for anything useful.
//
// revisions: fail pass
// [fail] check-fail
// [pass] check-pass

trait Trait {
type Ty<'a> where Self: 'a;
}

impl<T> Trait for T {
type Ty<'a> = () where Self: 'a;
}

struct Foo<T: Trait>(T)
where
for<'x> T::Ty<'x>: Sized;

trait AnotherTrait {
type Ty2<'a>: 'a;
}

#[cfg(fail)]
mod fail {
use super::*;

// implied_bound: `'lt: placeholder('x)`.
// don't use the bound to prove `'lt: 'static`.
fn test_lifetime<'lt, T: Trait>(_: Foo<&'lt u8>) {}
//[fail]~^ ERROR `&'lt u8` does not fulfill the required lifetime

// implied bound: `T::Ty2<'lt>: placeholder('x)`.
// don't use the bound to prove `T::Ty2<'lt>: 'static`.
fn test_alias<'lt, T: AnotherTrait>(_: Foo<T::Ty2::<'lt>>) {}
//[fail]~^ ERROR `<T as AnotherTrait>::Ty2<'lt>` does not fulfill the required lifetime
}


mod pass {
use super::*;

// implied_bound: 'static: placeholder('x).
// don't ice.
fn test_lifetime<T: Trait>(_: Foo<&'static u8>) {}

// implied bound: T::Ty2<'static>: placeholder('x).
// don't add the bound to the environment,
// otherwise we would fail to infer a value for `'_`.
fn test_alias<T: AnotherTrait>(_: Foo<T::Ty2::<'static>>) {
None::<&'static T::Ty2<'_>>;
}
}

fn main() {}

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