broken interaction between marker traits, lifetimes, and caching

[lcnr]

rust/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

Line 44 in d9297d2

let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate);

erases all lifetimes for caching, including 'static.

rust/compiler/rustc_trait_selection/src/traits/select/mod.rs

Line 221 in d9297d2

freshener: infcx.freshener_keep_static(),

rust/compiler/rustc_trait_selection/src/traits/select/mod.rs

Line 2289 in d9297d2

let fresh_trait_pred = obligation.predicate.fold_with(&mut self.freshener);

rust/compiler/rustc_trait_selection/src/traits/select/mod.rs

Lines 1026 to 1036 in d9297d2

	// If we erased any lifetimes, then we want to use
	// `EvaluatedToOkModuloRegions` instead of `EvaluatedToOk`
	// as your final result. The result will be cached using
	// the freshened trait predicate as a key, so we need
	// our result to be correct by *any* choice of original lifetimes,
	// not just the lifetime choice for this particular (non-erased)
	// predicate.
	// See issue #80691
	if stack.fresh_trait_pred.has_erased_regions() {
	result = result.max(EvaluatedToOkModuloRegions);
	}

changes the evaluation result depending on whether there are any erased regions in the predicate, excluding 'static.

rust/compiler/rustc_trait_selection/src/traits/select/mod.rs

Line 1753 in d9297d2

if other.evaluation.must_apply_considering_regions() {

selection only drops allowed-to-overlap candidate if the other resulted in EvaluatedToOk.

This means that depending on the order of evaluation, marker traits can either succeed or result in ambiguity

[lcnr]

this has been found in #102205 where changing caching to also keep 'static caused this test to fail:

#![allow(order_dependent_trait_objects)]
trait Trait {}

impl Trait for dyn Send + Sync {}
impl Trait for dyn Sync + Send {}
fn assert_trait<T: Trait + ?Sized>() {}

fn main() {
    // compiles right now as we first evaluate `dyn Send + Sync + 'static`,
    // but store `dyn Send + Sync + 'erased` in the cache.
    assert_trait::<dyn Send + Sync>();
}