Matthias Krüger 4e510daed7 Rollup merge of #130866 - compiler-errors:dyn-instantiate-binder, r=lcnr ... Allow instantiating object trait binder when upcasting This PR fixes two bugs (that probably need an FCP). ### We use equality rather than subtyping for upcasting dyn conversions This code should be valid: ```rust #![feature(trait_upcasting)] trait Foo: for<'h> Bar<'h> {} trait Bar<'a> {} fn foo(x: &dyn Foo) { let y: &dyn Bar<'static> = x; } ``` But instead: ``` error[E0308]: mismatched types --> src/lib.rs:7:32 | 7 | let y: &dyn Bar<'static> = x; | ^ one type is more general than the other | = note: expected existential trait ref `for<'h> Bar<'h>` found existential trait ref `Bar<'_>` ``` And so should this: ```rust #![feature(trait_upcasting)] fn foo(x: &dyn for<'h> Fn(&'h ())) { let y: &dyn FnOnce(&'static ()) = x; } ``` But instead: ``` error[E0308]: mismatched types --> src/lib.rs:4:39 | 4 | let y: &dyn FnOnce(&'static ()) = x; | ^ one type is more general than the other | = note: expected existential trait ref `for<'h> FnOnce<(&'h (),)>` found existential trait ref `FnOnce<(&(),)>` ``` Specifically, both of these fail because we use *equality* when comparing the supertrait to the *target* of the unsize goal. For the first example, since our supertrait is `for<'h> Bar<'h>` but our target is `Bar<'static>`, there's a higher-ranked type mismatch even though we *should* be able to instantiate that supertrait binder when upcasting. Similarly for the second example. ### New solver uses equality rather than subtyping for no-op (i.e. non-upcasting) dyn conversions This code should be valid in the new solver, like it is with the old solver: ```rust // -Znext-solver fn foo<'a>(x: &mut for<'h> dyn Fn(&'h ())) { let _: &mut dyn Fn(&'a ()) = x; } ``` But instead: ``` error: lifetime may not live long enough --> <source>:2:11 | 1 | fn foo<'a>(x: &mut dyn for<'h> Fn(&'h ())) { | -- lifetime `'a` defined here 2 | let _: &mut dyn Fn(&'a ()) = x; | ^^^^^^^^^^^^^^^^^^^ type annotation requires that `'a` must outlive `'static` | = note: requirement occurs because of a mutable reference to `dyn Fn(&())` ``` Specifically, this fails because we try to coerce `&mut dyn for<'h> Fn(&'h ())` to `&mut dyn Fn(&'a ())`, which registers an `dyn for<'h> Fn(&'h ()): dyn Fn(&'a ())` goal. This fails because the new solver uses *equating* rather than *subtyping* in `Unsize` goals. This is *mostly* not a problem... You may wonder why the same code passes on the new solver for immutable references: ``` // -Znext-solver fn foo<'a>(x: &dyn Fn(&())) { let _: &dyn Fn(&'a ()) = x; // works } ``` That's because in this case, we first try to coerce via `Unsize`, but due to the leak check the goal fails. Then, later in coercion, we fall back to a simple subtyping operation, which *does* work. Since `&T` is covariant over `T`, but `&mut T` is invariant, that's where the discrepancy between these two examples crops up. --- r? lcnr or reassign :D		2024-09-28 09:35:09 +02:00
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src	Rollup merge of #130866 - compiler-errors:dyn-instantiate-binder, r=lcnr	2024-09-28 09:35:09 +02:00
Cargo.toml	Auto merge of #127042 - GrigorenkoPV:derivative, r=compiler-errors	2024-07-25 22:50:58 +00:00