Rollup merge of #56043 - nikomatsakis:issue-55756-via-outlives, r=eddyb
remove "approx env bounds" if we already know from trait Alternative to https://github.com/rust-lang/rust/pull/55988 that fixes #55756 -- smaller fix that I cannot see having (correctness) repercussions beyond the test at hand, and hence better for backporting. (Famous last words, I know.) r? @eddyb
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commit
10565c45ac
2 changed files with 63 additions and 10 deletions
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@ -389,16 +389,6 @@ where
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// rule might not apply (but another rule might). For now, we err
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// on the side of adding too few edges into the graph.
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// Compute the bounds we can derive from the environment. This
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// is an "approximate" match -- in some cases, these bounds
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// may not apply.
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let approx_env_bounds = self.verify_bound
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.projection_approx_declared_bounds_from_env(projection_ty);
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debug!(
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"projection_must_outlive: approx_env_bounds={:?}",
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approx_env_bounds
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);
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// Compute the bounds we can derive from the trait definition.
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// These are guaranteed to apply, no matter the inference
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// results.
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@ -406,6 +396,32 @@ where
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.projection_declared_bounds_from_trait(projection_ty)
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.collect();
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// Compute the bounds we can derive from the environment. This
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// is an "approximate" match -- in some cases, these bounds
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// may not apply.
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let mut approx_env_bounds = self.verify_bound
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.projection_approx_declared_bounds_from_env(projection_ty);
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debug!(
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"projection_must_outlive: approx_env_bounds={:?}",
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approx_env_bounds
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);
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// Remove outlives bounds that we get from the environment but
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// which are also deducable from the trait. This arises (cc
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// #55756) in cases where you have e.g. `<T as Foo<'a>>::Item:
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// 'a` in the environment but `trait Foo<'b> { type Item: 'b
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// }` in the trait definition.
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approx_env_bounds.retain(|bound| {
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match bound.0.sty {
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ty::Projection(projection_ty) => {
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self.verify_bound.projection_declared_bounds_from_trait(projection_ty)
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.all(|r| r != bound.1)
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}
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_ => panic!("expected only projection types from env, not {:?}", bound.0),
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}
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});
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// If declared bounds list is empty, the only applicable rule is
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// OutlivesProjectionComponent. If there are inference variables,
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// then, we can break down the outlives into more primitive
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37
src/test/ui/nll/ty-outlives/issue-55756.rs
Normal file
37
src/test/ui/nll/ty-outlives/issue-55756.rs
Normal file
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@ -0,0 +1,37 @@
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// Regression test for #55756.
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//
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// In this test, the result of `self.callee` is a projection `<D as
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// Database<'?0>>::Guard`. As it may contain a destructor, the dropck
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// rules require that this type outlivess the scope of `state`. Unfortunately,
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// our region inference is not smart enough to figure out how to
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// translate a requirement like
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//
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// <D as Database<'0>>::guard: 'r
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//
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// into a requirement that `'0: 'r` -- in particular, it fails to do
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// so because it *also* knows that `<D as Database<'a>>::Guard: 'a`
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// from the trait definition. Faced with so many choices, the current
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// solver opts to do nothing.
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//
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// Fixed by tweaking the solver to recognize that the constraint from
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// the environment duplicates one from the trait.
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//
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// compile-pass
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#![crate_type="lib"]
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pub trait Database<'a> {
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type Guard: 'a;
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}
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pub struct Stateful<'a, D: 'a>(&'a D);
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impl<'b, D: for <'a> Database<'a>> Stateful<'b, D> {
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pub fn callee<'a>(&'a self) -> <D as Database<'a>>::Guard {
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unimplemented!()
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}
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pub fn caller<'a>(&'a self) -> <D as Database<'a>>::Guard {
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let state = self.callee();
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unimplemented!()
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}
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}
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