Reject generic self types.

[adetaylor]

The RFC for arbitrary self types v2 declares that we should reject "generic" self types. This commit does so.

The definition of "generic" was unclear in the RFC, but has been explored in
#129147
and the conclusion is that "generic" means any self type which is a type parameter defined on the method itself, or references to such a type.

This approach was chosen because other definitions of "generic" don't work. Specifically,

• we can't filter out generic type arguments, because that would filter out Rc and all the other types of smart pointer we want to support;
• we can't filter out all type params, because Self itself is a type param, and because existing Rust code depends on other type params declared on the type (as opposed to the method).

This PR decides to make a new error code for this case, instead of reusing the existing E0307 error. This makes the code a bit more complex, but it seems we have an opportunity to provide specific diagnostics for this case so we should do so.

This PR filters out generic self types whether or not the 'arbitrary self types' feature is enabled. However, it's believed that it can't have any effect on code which uses stable Rust, since there are no stable traits which can be used to indicate a valid generic receiver type, and thus it would have been impossible to write code which could trigger this new error case. It is however possible that this could break existing code which uses either of the unstable arbitrary_self_types or receiver_trait features. This breakage is intentional; as we move arbitrary self types towards stabilization we don't want to continue to support generic such types.

This PR adds lots of extra tests to arbitrary-self-from-method-substs. Most of these are ways to trigger a "type mismatch" error which

rust/compiler/rustc_hir_typeck/src/method/confirm.rs

Line 519 in 9b82580

// FIXME(arbitrary_self_types): We probably should limit the

hopes can be minimized by filtering out generics in this way. We remove a FIXME from confirm.rs suggesting that we make this change. It's still possible to cause type mismatch errors, and a subsequent PR may be able to improve diagnostics in this area, but it's harder to cause these errors without contrived uses of the turbofish.

This is a part of the arbitrary self types v2 project, rust-lang/rfcs#3519
#44874

r? @wesleywiser

[adetaylor]

@rustbot label +S-waiting-on-author -S-waiting-on-review

[adetaylor]

@wesleywiser Over to you for opinions here. See this comment for why I think this is the least bad approach for filtering out generics - to be explicit, we know it's still possible to have generic self types, but this probably reduces the risk of them being common and also resolves a FIXME.

I'd appreciate your help and adjudication about whether this is the right overall approach. We should probably get to the bottom of that before you review the PR in detail, but it's up to you!

@rustbot label -S-waiting-on-author +S-waiting-on-review

[rustbot]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

[bors]

☔ The latest upstream changes (presumably #130414) made this pull request unmergeable. Please resolve the merge conflicts.