Commits on Sep 8, 2024

1. Reject generic self types. …

   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
   rust-lang#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<Self> 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
   https://github.com/rust-lang/rust/blob/9b82580c7347f800c2550e6719e4218a60a80b28/compiler/rustc_hir_typeck/src/method/confirm.rs#L519
   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
   rust-lang#44874
   
   r? @wesleywiser

   

   adetaylor committed Sep 8, 2024
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