rust/compiler/rustc_hir_analysis
Jubilee 9510c7366d
Rollup merge of #130367 - compiler-errors:super-unconstrained, r=spastorino
Check elaborated projections from dyn don't mention unconstrained late bound lifetimes

Check that the projections that are *not* explicitly written but which we deduce from elaborating the principal of a `dyn` *also* do not reference unconstrained late-bound lifetimes, just like the ones that the user writes by hand.

That is to say, given:

```
trait Foo<T>: Bar<Assoc = T> {}

trait Bar {
    type Assoc;
}
```

The type `dyn for<'a> Foo<&'a T>` (basically) elaborates to `dyn for<'a> Foo<&'a T> + for<'a> Bar<Assoc = &'a T>`[^1]. However, the `Bar` projection predicate is not well-formed, since `'a` must show up in the trait's arguments to be referenced in the term of a projection. We must error in this situation[^well], or else `dyn for<'a> Foo<&'a T>` is unsound.

We already detect this for user-written projections during HIR->rustc_middle conversion, so this largely replicates that logic using the helper functions that were already conveniently defined.

---

I'm cratering this first to see the fallout; if it's minimal or zero, then let's land it as-is. If not, the way that this is implemented is very conducive to an FCW.

---

Fixes #130347

[^1]: We don't actually elaborate it like that in rustc; we only keep the principal trait ref `Foo<&'a T>` and the projection part of `Bar<Assoc = ...>`, but it's useful to be a bit verbose here for the purpose of explaining the issue.
[^well]: Well, we could also make `dyn for<'a> Foo<&'a T>` *not* implement `for<'a> Bar<Assoc = &'a T>`, but this is inconsistent with the case where the user writes `Assoc = ...` in the type itself, and it overly complicates the implementation of trait objects' built-in impls.
2024-10-04 19:19:22 -07:00
..
src Rollup merge of #130367 - compiler-errors:super-unconstrained, r=spastorino 2024-10-04 19:19:22 -07:00
Cargo.toml bump itertools to 0.12 2024-03-08 12:34:05 +03:00
messages.ftl Compiler: Rename "object safe" to "dyn compatible" 2024-09-25 13:26:48 +02:00
README.md

For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.