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the unnormalized goal is in the callers inference context, while
anything inside of the `CanonicalGoalEvaluation` is inside of
a new one.
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When encountering code like
```rust
fn foo() -> i32 {
match 0 {
1 => return 0,
2 => "",
_ => 1,
}
}
```
Point at the return type and not at the prior arm, as that arm has type
`!` which isn't influencing the arm corresponding to arm `2`.
Fix #78124.
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update overflow handling in the new trait solver
implements https://hackmd.io/QY0dfEOgSNWwU4oiGnVRLw?view. I want to clean up this doc and add it to the rustc-dev-guide, but I think this PR is ready for merge as is, even without the dev-guide entry.
r? `@compiler-errors`
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Rework upcasting confirmation to support upcasting to fewer projections in target bounds
This PR implements a modified trait upcasting algorithm that is resilient to changes in the number of associated types in the bounds of the source and target trait objects.
It does this by equating each bound of the target trait ref individually against the bounds of the source trait ref, rather than doing them all together by constructing a new trait object.
#### The new way we do trait upcasting confirmation
1. Equate the target trait object's principal trait ref with one of the supertraits of the source trait object's principal.
https://github.com/rust-lang/rust/blob/fdcab310b2a57a4e9cc0b2629abd27afda49cd80/compiler/rustc_trait_selection/src/traits/select/mod.rs#L2509-L2525
2. Make sure that every auto trait in the *target* trait object is present in the source trait ref's bounds.
https://github.com/rust-lang/rust/blob/fdcab310b2a57a4e9cc0b2629abd27afda49cd80/compiler/rustc_trait_selection/src/traits/select/mod.rs#L2559-L2562
3. For each projection in the *target* trait object, make sure there is exactly one projection that equates with it in the source trait ref's bound. If there is more than one, bail with ambiguity.
https://github.com/rust-lang/rust/blob/fdcab310b2a57a4e9cc0b2629abd27afda49cd80/compiler/rustc_trait_selection/src/traits/select/mod.rs#L2526-L2557
* Since there may be more than one that applies, we probe first to check that there is exactly one, then we equate it outside of a probe once we know that it's unique.
4. Make sure the lifetime of the source trait object outlives the lifetime of the target.
<details>
<summary>Meanwhile, this is how we used to do upcasting:</summary>
1. For each supertrait of the source trait object, take that supertrait, append the source object's projection bounds, and the *target* trait object's auto trait bounds, and make this into a new object type:
https://github.com/rust-lang/rust/blob/d12c6e947ceacf3b22c154caf9532b390d8dc88a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs#L915-L929
2. Then equate it with the target trait object:
https://github.com/rust-lang/rust/blob/d12c6e947ceacf3b22c154caf9532b390d8dc88a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs#L936
This will be a type mismatch if the target trait object has fewer projection bounds, since we compare the bounds structurally in relate:
https://github.com/rust-lang/rust/blob/d12c6e947ceacf3b22c154caf9532b390d8dc88a/compiler/rustc_middle/src/ty/relate.rs#L696-L698
</details>
Fixes #114035
Also fixes #114113, because I added a normalize call in the old solver.
r? types
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refactor builtin unsize handling, extend comments
r? `@compiler-errors`
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More `ImplSource` nits
Even more clean-ups, I'll put this up in parallel with the `select_in_new_trait_solver` PR.
r? ``@lcnr``
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Only use it when the type alias contains an opaque type.
Also does wf-checking on such type aliases.
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Rename `impl_defaultness` to `defaultness`
Since this isn't just about the `impl`.
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fix recursion depth handling after confirmation
fixes #111729
I think having to use `Obligation::with_depth` correctly everywhere is very hard because e.g. the nested obligations from `eq` currently do not have the correct obligation depth.
The new solver [completely removes `recursion_depth` from obligations](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/traits/solve/struct.Goal.html) and instead tracks the depth in the solver itself which is far easier to get right. Moving the old solver towards this shouldn't be that hard but is probably somewhat annoying.
r? `@matthewjasper`
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Use `OpaqueTypeKey` in query response
Makes it a bit clearer that at least one of the types being returned from a canonical query is an opaque.
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`SelectionError` used to be 80 bytes (on 64 bit). That's quite big.
Especially because the selection cache contained `Result<_,
SelectionError>. The Ok type is only 32 bytes, so the 80 bytes
significantly inflate the size of the cache.
Most variants of the `SelectionError` seem to be hard errors, only
`Unimplemented` shows up in practice (for cranelift-codegen, it occupies
23.4% of all cache entries). We can just box away the biggest variant,
`OutputTypeParameterMismatch`, to get the size down to 16 bytes, well
within the size of the Ok type inside the cache.
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