diff --git a/compiler/rustc_borrowck/src/region_infer/mod.rs b/compiler/rustc_borrowck/src/region_infer/mod.rs index 70124dba809..f2693bded59 100644 --- a/compiler/rustc_borrowck/src/region_infer/mod.rs +++ b/compiler/rustc_borrowck/src/region_infer/mod.rs @@ -746,20 +746,33 @@ impl<'tcx> RegionInferenceContext<'tcx> { } debug!(?choice_regions, "after ub"); - // If we ruled everything out, we're done. - if choice_regions.is_empty() { - return false; - } - - // Otherwise, we need to find the minimum remaining choice, if - // any, and take that. - debug!("choice_regions remaining are {:#?}", choice_regions); - let Some(&min_choice) = choice_regions.iter().find(|&r1| { + // At this point we can pick any member of `choice_regions`, but to avoid potential + // non-determinism we will pick the *unique minimum* choice. + // + // Because universal regions are only partially ordered (i.e, not every two regions are + // comparable), we will ignore any region that doesn't compare to all others when picking + // the minimum choice. + // For example, consider `choice_regions = ['static, 'a, 'b, 'c, 'd, 'e]`, where + // `'static: 'a, 'static: 'b, 'a: 'c, 'b: 'c, 'c: 'd, 'c: 'e`. + // `['d, 'e]` are ignored because they do not compare - the same goes for `['a, 'b]`. + let totally_ordered_subset = choice_regions.iter().copied().filter(|&r1| { choice_regions.iter().all(|&r2| { - self.universal_region_relations.outlives(r2, *r1) + self.universal_region_relations.outlives(r1, r2) + || self.universal_region_relations.outlives(r2, r1) }) + }); + // Now we're left with `['static, 'c]`. Pick `'c` as the minimum! + let Some(min_choice) = totally_ordered_subset.reduce(|r1, r2| { + let r1_outlives_r2 = self.universal_region_relations.outlives(r1, r2); + let r2_outlives_r1 = self.universal_region_relations.outlives(r2, r1); + match (r1_outlives_r2, r2_outlives_r1) { + (true, true) => r1.min(r2), + (true, false) => r2, + (false, true) => r1, + (false, false) => bug!("incomparable regions in total order"), + } }) else { - debug!("no choice region outlived by all others"); + debug!("no unique minimum choice"); return false; }; diff --git a/tests/ui/async-await/multiple-lifetimes/member-constraints-min-choice-issue-63033.rs b/tests/ui/async-await/multiple-lifetimes/member-constraints-min-choice-issue-63033.rs new file mode 100644 index 00000000000..614f1897291 --- /dev/null +++ b/tests/ui/async-await/multiple-lifetimes/member-constraints-min-choice-issue-63033.rs @@ -0,0 +1,10 @@ +// Regression test for #63033. + +// check-pass +// edition: 2018 + +async fn test1(_: &'static u8, _: &'_ u8, _: &'_ u8) {} + +async fn test2<'s>(_: &'s u8, _: &'_ &'s u8, _: &'_ &'s u8) {} + +fn main() {} diff --git a/tests/ui/nll/member-constraints/min-choice-reject-ambiguous.rs b/tests/ui/nll/member-constraints/min-choice-reject-ambiguous.rs new file mode 100644 index 00000000000..52ea0f28d69 --- /dev/null +++ b/tests/ui/nll/member-constraints/min-choice-reject-ambiguous.rs @@ -0,0 +1,43 @@ +// ... continued from ./min-choice.rs + +// check-fail + +trait Cap<'a> {} +impl Cap<'_> for T {} + +fn type_test<'a, T: 'a>() -> &'a u8 { &0 } + +// Make sure we don't pick `'b`. +fn test_b<'a, 'b, 'c, T>() -> impl Cap<'a> + Cap<'b> + Cap<'c> +where + 'a: 'b, + 'a: 'c, + T: 'b, +{ + type_test::<'_, T>() // This should pass if we pick 'b. + //~^ ERROR the parameter type `T` may not live long enough +} + +// Make sure we don't pick `'c`. +fn test_c<'a, 'b, 'c, T>() -> impl Cap<'a> + Cap<'b> + Cap<'c> +where + 'a: 'b, + 'a: 'c, + T: 'c, +{ + type_test::<'_, T>() // This should pass if we pick 'c. + //~^ ERROR the parameter type `T` may not live long enough +} + +// We need to pick min_choice from `['b, 'c]`, but it's ambiguous which one to pick because +// they're incomparable. +fn test_ambiguous<'a, 'b, 'c>(s: &'a u8) -> impl Cap<'b> + Cap<'c> +where + 'a: 'b, + 'a: 'c, +{ + s + //~^ ERROR captures lifetime that does not appear in bounds +} + +fn main() {} diff --git a/tests/ui/nll/member-constraints/min-choice-reject-ambiguous.stderr b/tests/ui/nll/member-constraints/min-choice-reject-ambiguous.stderr new file mode 100644 index 00000000000..1e6ef614dee --- /dev/null +++ b/tests/ui/nll/member-constraints/min-choice-reject-ambiguous.stderr @@ -0,0 +1,40 @@ +error[E0309]: the parameter type `T` may not live long enough + --> $DIR/min-choice-reject-ambiguous.rs:17:5 + | +LL | type_test::<'_, T>() // This should pass if we pick 'b. + | ^^^^^^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds + | +help: consider adding an explicit lifetime bound... + | +LL | T: 'b + 'a, + | ++++ + +error[E0309]: the parameter type `T` may not live long enough + --> $DIR/min-choice-reject-ambiguous.rs:28:5 + | +LL | type_test::<'_, T>() // This should pass if we pick 'c. + | ^^^^^^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds + | +help: consider adding an explicit lifetime bound... + | +LL | T: 'c + 'a, + | ++++ + +error[E0700]: hidden type for `impl Cap<'b> + Cap<'c>` captures lifetime that does not appear in bounds + --> $DIR/min-choice-reject-ambiguous.rs:39:5 + | +LL | fn test_ambiguous<'a, 'b, 'c>(s: &'a u8) -> impl Cap<'b> + Cap<'c> + | -- hidden type `&'a u8` captures the lifetime `'a` as defined here +... +LL | s + | ^ + | +help: to declare that `impl Cap<'b> + Cap<'c>` captures `'a`, you can add an explicit `'a` lifetime bound + | +LL | fn test_ambiguous<'a, 'b, 'c>(s: &'a u8) -> impl Cap<'b> + Cap<'c> + 'a + | ++++ + +error: aborting due to 3 previous errors + +Some errors have detailed explanations: E0309, E0700. +For more information about an error, try `rustc --explain E0309`. diff --git a/tests/ui/nll/member-constraints/min-choice.rs b/tests/ui/nll/member-constraints/min-choice.rs new file mode 100644 index 00000000000..14b4dae7abf --- /dev/null +++ b/tests/ui/nll/member-constraints/min-choice.rs @@ -0,0 +1,34 @@ +// Assuming that the hidden type in these tests is `&'_#15r u8`, +// we have a member constraint: `'_#15r member ['static, 'a, 'b, 'c]`. +// +// Make sure we pick up the minimum non-ambiguous region among them. +// We will have to exclude `['b, 'c]` because they're incomparable, +// and then we should pick `'a` because we know `'static: 'a`. + +// check-pass + +trait Cap<'a> {} +impl Cap<'_> for T {} + +fn type_test<'a, T: 'a>() -> &'a u8 { &0 } + +// Basic test: make sure we don't bail out because 'b and 'c are incomparable. +fn basic<'a, 'b, 'c>() -> impl Cap<'a> + Cap<'b> + Cap<'c> +where + 'a: 'b, + 'a: 'c, +{ + &0 +} + +// Make sure we don't pick `'static`. +fn test_static<'a, 'b, 'c, T>() -> impl Cap<'a> + Cap<'b> + Cap<'c> +where + 'a: 'b, + 'a: 'c, + T: 'a, +{ + type_test::<'_, T>() // This will fail if we pick 'static +} + +fn main() {} diff --git a/tests/ui/nll/member-constraints/nested-impl-trait-fail.rs b/tests/ui/nll/member-constraints/nested-impl-trait-fail.rs new file mode 100644 index 00000000000..66ff828a84f --- /dev/null +++ b/tests/ui/nll/member-constraints/nested-impl-trait-fail.rs @@ -0,0 +1,33 @@ +// Nested impl-traits can impose different member constraints on the same region variable. + +// check-fail + +trait Cap<'a> {} +impl Cap<'_> for T {} + +// Assuming the hidden type is `[&'_#15r u8; 1]`, we have two distinct member constraints: +// - '_#15r member ['static, 'a, 'b] // from outer impl-trait +// - '_#15r member ['static, 'a, 'b] // from inner impl-trait +// To satisfy both we can choose 'a or 'b, so it's a failure due to ambiguity. +fn fail_early_bound<'s, 'a, 'b>(a: &'s u8) -> impl IntoIterator + Cap<'b>> +where + 's: 'a, + 's: 'b, +{ + [a] + //~^ E0700 + //~| E0700 +} + +// Same as the above but with late-bound regions. +fn fail_late_bound<'s, 'a, 'b>( + a: &'s u8, + _: &'a &'s u8, + _: &'b &'s u8, +) -> impl IntoIterator + Cap<'b>> { + [a] + //~^ E0700 + //~| E0700 +} + +fn main() {} diff --git a/tests/ui/nll/member-constraints/nested-impl-trait-fail.stderr b/tests/ui/nll/member-constraints/nested-impl-trait-fail.stderr new file mode 100644 index 00000000000..6824e27ead0 --- /dev/null +++ b/tests/ui/nll/member-constraints/nested-impl-trait-fail.stderr @@ -0,0 +1,75 @@ +error[E0700]: hidden type for `impl IntoIterator + Cap<'b>>` captures lifetime that does not appear in bounds + --> $DIR/nested-impl-trait-fail.rs:17:5 + | +LL | fn fail_early_bound<'s, 'a, 'b>(a: &'s u8) -> impl IntoIterator + Cap<'b>> + | -- hidden type `[&'s u8; 1]` captures the lifetime `'s` as defined here +... +LL | [a] + | ^^^ + | +help: to declare that `impl IntoIterator + Cap<'b>>` captures `'s`, you can add an explicit `'s` lifetime bound + | +LL | fn fail_early_bound<'s, 'a, 'b>(a: &'s u8) -> impl IntoIterator + Cap<'b>> + 's + | ++++ +help: to declare that `impl Cap<'a> + Cap<'b>` captures `'s`, you can add an explicit `'s` lifetime bound + | +LL | fn fail_early_bound<'s, 'a, 'b>(a: &'s u8) -> impl IntoIterator + Cap<'b> + 's> + | ++++ + +error[E0700]: hidden type for `impl Cap<'a> + Cap<'b>` captures lifetime that does not appear in bounds + --> $DIR/nested-impl-trait-fail.rs:17:5 + | +LL | fn fail_early_bound<'s, 'a, 'b>(a: &'s u8) -> impl IntoIterator + Cap<'b>> + | -- hidden type `&'s u8` captures the lifetime `'s` as defined here +... +LL | [a] + | ^^^ + | +help: to declare that `impl IntoIterator + Cap<'b>>` captures `'s`, you can add an explicit `'s` lifetime bound + | +LL | fn fail_early_bound<'s, 'a, 'b>(a: &'s u8) -> impl IntoIterator + Cap<'b>> + 's + | ++++ +help: to declare that `impl Cap<'a> + Cap<'b>` captures `'s`, you can add an explicit `'s` lifetime bound + | +LL | fn fail_early_bound<'s, 'a, 'b>(a: &'s u8) -> impl IntoIterator + Cap<'b> + 's> + | ++++ + +error[E0700]: hidden type for `impl IntoIterator + Cap<'b>>` captures lifetime that does not appear in bounds + --> $DIR/nested-impl-trait-fail.rs:28:5 + | +LL | fn fail_late_bound<'s, 'a, 'b>( + | -- hidden type `[&'s u8; 1]` captures the lifetime `'s` as defined here +... +LL | [a] + | ^^^ + | +help: to declare that `impl IntoIterator + Cap<'b>>` captures `'s`, you can add an explicit `'s` lifetime bound + | +LL | ) -> impl IntoIterator + Cap<'b>> + 's { + | ++++ +help: to declare that `impl Cap<'a> + Cap<'b>` captures `'s`, you can add an explicit `'s` lifetime bound + | +LL | ) -> impl IntoIterator + Cap<'b> + 's> { + | ++++ + +error[E0700]: hidden type for `impl Cap<'a> + Cap<'b>` captures lifetime that does not appear in bounds + --> $DIR/nested-impl-trait-fail.rs:28:5 + | +LL | fn fail_late_bound<'s, 'a, 'b>( + | -- hidden type `&'s u8` captures the lifetime `'s` as defined here +... +LL | [a] + | ^^^ + | +help: to declare that `impl IntoIterator + Cap<'b>>` captures `'s`, you can add an explicit `'s` lifetime bound + | +LL | ) -> impl IntoIterator + Cap<'b>> + 's { + | ++++ +help: to declare that `impl Cap<'a> + Cap<'b>` captures `'s`, you can add an explicit `'s` lifetime bound + | +LL | ) -> impl IntoIterator + Cap<'b> + 's> { + | ++++ + +error: aborting due to 4 previous errors + +For more information about this error, try `rustc --explain E0700`. diff --git a/tests/ui/nll/member-constraints/nested-impl-trait-pass.rs b/tests/ui/nll/member-constraints/nested-impl-trait-pass.rs new file mode 100644 index 00000000000..15540cb460e --- /dev/null +++ b/tests/ui/nll/member-constraints/nested-impl-trait-pass.rs @@ -0,0 +1,29 @@ +// Nested impl-traits can impose different member constraints on the same region variable. + +// check-pass + +trait Cap<'a> {} +impl Cap<'_> for T {} + +// Assuming the hidden type is `[&'_#15r u8; 1]`, we have two distinct member constraints: +// - '_#15r member ['static, 'a, 'b] // from outer impl-trait +// - '_#15r member ['static, 'a] // from inner impl-trait +// To satisfy both we can only choose 'a. +fn pass_early_bound<'s, 'a, 'b>(a: &'s u8) -> impl IntoIterator> + Cap<'b> +where + 's: 'a, + 's: 'b, +{ + [a] +} + +// Same as the above but with late-bound regions. +fn pass_late_bound<'s, 'a, 'b>( + a: &'s u8, + _: &'a &'s u8, + _: &'b &'s u8, +) -> impl IntoIterator> + Cap<'b> { + [a] +} + +fn main() {}