Fixes#67765
When reporting errors during MIR region inference, we sometimes use
`universal_upper_bound` to obtain a named universal region that we
can display to the user. However, this is not always possible - in a
case like `fn foo<'a, 'b>() { .. }`, the only upper bound for a region
containing `'a` and `'b` is `'static`. When displaying diagnostics, it's
usually better to display *some* named region (even if there are
multiple involved) rather than fall back to a generic error involving
`'static`.
This commit adds a new `approx_universal_upper_bound` method, which
uses the lowest-numbered universal region if the only alternative is to
return `'static`.
Detect type parameter that might require lifetime constraint.
Do not name `ReVar`s in expected/found output.
Reword text suggesting to check the lifetimes.
Add outlives suggestions for some lifetime errors
This PR implements suggestion diagnostics for some lifetime mismatch errors. When the borrow checker finds that some lifetime 'a doesn't outlive some other lifetime 'b that it should outlive, then in addition to the current lifetime error, we also emit a suggestion for how to fix the problem by adding a bound:
- If a and b are normal named regions, suggest to add the bound `'a: 'b`
- If b is static, suggest to replace a with static
- If b also needs to outlive a, they must be the same, so suggest unifying them
We start with a simpler implementation that avoids diagnostic regression or implementation complexity:
- We only makes suggestions for lifetimes the user can already name (eg not closure regions or elided regions)
- For now, we only emit a help note, not an actually suggestion because it is significantly easier.
Finally, there is one hack: it seems that implicit regions in async fn are given the name '_ incorrectly. To avoid suggesting '_: 'x, we simply filter out such lifetimes by name.
For more info, see this internals thread:
https://internals.rust-lang.org/t/mechanical-suggestions-for-some-borrow-checker-errors/9049/3
TL;DR Make suggestions to add a `where 'a: 'b` constraint for some lifetime errors. Details are in the paper linked from the internals thread above.
r? @estebank
TODO
- [x] Clean up code
- [x] Only make idiomatic suggestions
- [x] don't suggest naming `&'a self`
- [x] rather than `'a: 'static`, suggest replacing `'a` with `'static`
- [x] rather than `'a: 'b, 'b: 'a`, suggest replacing `'a` with `'b` or vice versa
- [x] Performance (maybe need a perf run when this is closer to the finish line?)
- perf run was clean...
- EDIT: perf run seems to only check non-error performance... How do we check that error performance didn't regress?
- [x] Needs ui tests
- [x] Integrate the `help` message into the main lifetime `error`
Object-lifetime-default elision is distinct from other forms of
elision; it always refers to some enclosing lifetime *present in the
surrounding type* (e.g., `&dyn Bar` expands to `&'a (dyn Bar + 'a)`.
If there is no enclosing lifetime, then it expands to `'static`.
Therefore, in an `impl Trait<Item = dyn Bar>` setting, we don't expand
to create a lifetime parameter for the `dyn Bar + 'X` bound. It will
just be resolved to `'static`.
Annoyingly, the responsibility for this resolution is spread across
multiple bits of code right now (`middle::resolve_lifetimes`,
`lowering`). The lowering code knows that the default is for an object
lifetime, but it doesn't know what the correct result would be.
Probably this should be fixed, but what we do now is a surgical fix:
we have it generate a different result for elided lifetimes in a
object context, and then we can ignore those results when figuring out
the lifetimes that are captured in the opaque type.
Fix#58270, fix off-by-one error in error diagnostics.
This fixes#58270 by checking if two diagnostics overlap completely when we're calculating the line offset for each message.
