Auto merge of #129317 - compiler-errors:expectation-subtyping, r=lcnr

Use equality when relating formal and expected type in arg checking

#129059 uncovered an interesting issue in argument checking. When we check arguments, we create three sets of types:
* Formals
* Expected
* Actuals

The **actuals** are the types of the argument expressions themselves. The **formals** are the types from the signature that we're checking. The **expected** types are the formal types, but passed through `expected_inputs_for_expected_outputs`:

https://github.com/rust-lang/rust/blob/a971212545766fdfe0dd68e5d968133f79944a19/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs#L691-L725

This method attempts to constrain the formal inputs by relating the [expectation](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_typeck/expectation/enum.Expectation.html) of the call expression and the formal output.

When we check an argument, we get the expression's actual type, and then we first attempt to coerce it to the expected type:

https://github.com/rust-lang/rust/blob/a971212545766fdfe0dd68e5d968133f79944a19/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs#L280-L293

Then we subtype the expected type and the formal type:

https://github.com/rust-lang/rust/blob/a971212545766fdfe0dd68e5d968133f79944a19/compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs#L299-L305

However, since we are now recording the right coercion target (since #129059), we now end up recording the expected type to the typeck results, rather than the actual.

Since that expected type was [fudged](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.fudge_inference_if_ok), it has fresh variables. And since the expected type is only subtyped against the formal type, if that expected type has a bivariant parameter, it will likely remain unconstrained since `Covariant * Bivariant = Bivariant` according to [xform](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.Variance.html#method.xform). This leads to an unconstrained type variable in writeback.

AFAICT, there's no reason for us to be using subtyping here, though. The expected output is already related to the expectation by subtyping:

https://github.com/rust-lang/rust/blob/a971212545766fdfe0dd68e5d968133f79944a19/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs#L713

So the formals don't need "another" indirection of subtyping in the argument checking... So I've changed it to use equality here. We could alternatively fix this by requiring WF for all the expected types to constrain their bivariant parameters, but this seems a bit overkill.

Fixes #129286

Author: bors