Change:
The following code should pass regardless of the declaration order of lifetimes: This is related to but it's different in that we do have a lower bound region, , but we fail to recognize that because , which is responsible for calculating from , assumes that the outlive relation is a total order, which is not true. Here is the graph of : ! So we need an efficient algorithm that works for partial order relations, but the one that comes to mind is . The perf may not be really important here but I'm curious to see the alternatives. label C-bug T-compiler A-borrow-checker E-mentor E-help-wanted <!-- TRIAGEBOTSTART --<!-- TRIAGEBOTASSIGNSTART --<!-- TRIAGEBOTASSIGNDATASTARTTRIAGEBOTASSIGNDATAEND --<!-- TRIAGEBOTASSIGNEND --<!-- TRIAGEBOTEND --
claim
Here is the code of concern The code should calculate the from a set of universal regions (). The relations are encoded in and are guaranteed to be transitive and reflexive. The relation graph may have cycles and can be sparse. The goal here is to find a unique minimum choice that is known to be outlived by all other regions. If none can be found, the function returns . Regions that have cycles between them are considered equivalent so we can pick either one of them as a unique choice. e.g. , we can pick either one of as a minimum choice.

Discussion:
} debug!(?choice_regions, "after ub"); <del> // 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| { </del> <ins> // 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| { </ins> choice_regions.iter().all(|&r2| { <del> self.universal_region_relations.outlives(r2, *r1) </del> <ins> self.universal_region_relations.outlives(r1, r2) || self.universal_region_relations.outlives(r2, r1) </ins> }) <ins> }); // 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"), } </ins> }) else { <del> debug!("no choice region outlived by all others"); </del> <ins> debug!("no unique minimum choice"); </ins> return false; };

<ins> // 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() {} </ins>

<ins> // ... continued from ./min-choice.rs // check-fail trait Cap<'a> {} impl<T> 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() {} </ins>

<ins> 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`. </ins>

<ins> // 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<T> 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() {} </ins>

<ins> // Nested impl-traits can impose different member constraints on the same region variable. // check-fail trait Cap<'a> {} impl<T> 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<Item = impl Cap<'a> + 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<Item = impl Cap<'a> + Cap<'b>> { [a] //~^ E0700 //~| E0700 } fn main() {} </ins>

<ins> error[E0700]: hidden type for `impl IntoIterator<Item = 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<Item = impl Cap<'a> + Cap<'b>> |                     -- hidden type `[&'s u8; 1]` captures the lifetime `'s` as defined here ... LL |     [a] |     ^^^ | help: to declare that `impl IntoIterator<Item = 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<Item = impl Cap<'a> + 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<Item = impl Cap<'a> + 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<Item = impl Cap<'a> + Cap<'b>> |                     -- hidden type `&'s u8` captures the lifetime `'s` as defined here ... LL |     [a] |     ^^^ | help: to declare that `impl IntoIterator<Item = 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<Item = impl Cap<'a> + 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<Item = impl Cap<'a> + Cap<'b> + 's> |                                                                                               ++++ error[E0700]: hidden type for `impl IntoIterator<Item = 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; 1]` captures the lifetime `'s` as defined here ... LL |     [a] |     ^^^ | help: to declare that `impl IntoIterator<Item = impl Cap<'a> + Cap<'b>>` captures `'s`, you can add an explicit `'s` lifetime bound | LL | ) -> impl IntoIterator<Item = impl Cap<'a> + 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<Item = impl Cap<'a> + 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<Item = impl Cap<'a> + Cap<'b>>` captures `'s`, you can add an explicit `'s` lifetime bound | LL | ) -> impl IntoIterator<Item = impl Cap<'a> + 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<Item = impl Cap<'a> + Cap<'b> + 's> { |                                                      ++++ error: aborting due to 4 previous errors For more information about this error, try `rustc --explain E0700`. </ins>

<ins> // Nested impl-traits can impose different member constraints on the same region variable. // check-pass trait Cap<'a> {} impl<T> 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<Item = impl Cap<'a>> + 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<Item = impl Cap<'a>> + Cap<'b> { [a] } fn main() {} </ins>