Reject generic self types using impl defid: do not merge.

[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 is a second attempt at achieving this, based on producing a new well-formedness checking context which is only aware of the type params of the impl block, not of the method itself.

It doesn't currently work because it turns out we do need some method params under some circumstances, but raising it for completeness.

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

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling cfg-if v1.0.0
   Compiling adler v1.0.2
   Compiling rustc-demangle v0.1.24
   Compiling unwind v0.0.0 (/checkout/library/unwind)
error[E0658]: `Box<Node<T>, A>` cannot be used as the type of `self` without the `arbitrary_self_types` feature
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160 |     fn into_element<A: Allocator>(self: Box<Self, A>) -> T {
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    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
    = note: this compiler was built on 2024-09-08; consider upgrading it if it is out of date
    = note: this compiler was built on 2024-09-08; consider upgrading it if it is out of date
    = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)

error[E0658]: `Box<[T], A>` cannot be used as the type of `self` without the `arbitrary_self_types` feature
    |
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497 |     pub fn into_vec<A: Allocator>(self: Box<Self, A>) -> Vec<T, A> {
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    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
    = note: this compiler was built on 2024-09-08; consider upgrading it if it is out of date
    = note: this compiler was built on 2024-09-08; consider upgrading it if it is out of date
    = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
For more information about this error, try `rustc --explain E0658`.
error: could not compile `alloc` (lib) due to 2 previous errors
Build completed unsuccessfully in 0:05:16
  local time: Sun Sep  8 22:03:52 UTC 2024

[fmease]

_{(oops, I didn't mean to mark it as ready for review ^^)}