This does not yet handle the case of mixed deref patterns with normal
constructors; it'll ICE in `Constructor::is_covered_by`. That'll be
fixed in a later commit.
It's similar to the other limits, e.g. obtained via `get_limit`. So it
makes sense to handle it consistently with the other limits. We now use
`Limit`/`usize` in most places instead of `Option<usize>`, so we use
`Limit::new(usize::MAX)`/`usize::MAX` to emulate how `None` used to work.
The commit also adds `Limit::unlimited`.
pattern_analysis: Move constructor selection logic to `PlaceInfo`
This is a small refactor PR. There was a dense bit of constructor-related logic in `compute_exhaustiveness_and_usefulness`. I'm moving it out into a `PlaceInfo` method to make it easier to follow both separately. I also have plans that will complicate it further so it's good that it's somewhat encapsulated.
r? `@compiler-errors`
pattern_analysis: track usefulness without interior mutability
Because of or-patterns, exhaustiveness needs to be able to lint if a sub-pattern is redundant, e.g. in `Some(_) | Some(true)`. So far the only sane solution I had found was interior mutability. This is a bit of an abstraction leak, and would become a footgun if we ever reused the same `DeconstructedPat`. This PR replaces interior mutability with an address-indexed hashmap, which is logically equivalent.
pattern_analysis: gather up place-relevant info
We track 3 things about each place during exhaustiveness: its type, its (data) validity, and whether it's the scrutinee place. This PR gathers all three into a single struct.
r? `````@compiler-errors`````
pattern_analysis: use a plain `Vec` in `DeconstructedPat`
The use of an arena-allocated slice in `DeconstructedPat` dates to when we needed the arena anyway for lifetime reasons. Now that we don't, I'm thinking that if `thir::Pat` can use plain old `Vec`s, maybe so can I.
r? ```@ghost```
pattern_analysis: Gracefully abort on type incompatibility
This leaves the option for a consumer of the crate to return `Err` instead of panicking on type error. rust-analyzer could use that (e.g. https://github.com/rust-lang/rust-analyzer/issues/15808).
Since the only use of `TypeCx::bug` is in `Constructor::is_covered_by`, it is tempting to return `false` instead of `Err()`, but that would cause "non-exhaustive match" false positives.
r? `@compiler-errors`
Since the only use of `TypeCx::bug` is in `Constructor::is_covered_by`,
it is tempting to return `false` instead of `Err()`, but that would
cause "non-exhaustive match" false positives.
