Auto merge of #132961 - adetaylor:arbitrary-self-types-the-big-bit, r=compiler-errors,wesleywiser

Arbitrary self types v2: main compiler changes

This is the main PR in a series of PRs related to Arbitrary Self Types v2, tracked in #44874. Specifically this is step 7 of the plan [described here](#44874 (comment)), for [RFC 3519](rust-lang/rfcs#3519).

Overall this PR:
* Switches from the `Deref` trait to the new `Receiver` trait when the unstable `arbitrary_self_types` feature is enabled (the simple bit)
* Introduces new algorithms to spot "shadowing"; that is, the case where a newly-added method in an outer smart pointer might end up overriding a pre-existing method in the pointee (the complex bit). Most of this bit was explored in [this earlier perf-testing PR](#127812 (comment)).
* Lots of tests

This should not break compatibility for:
* Stable users, where it should have no effect
* Users of the existing `arbitrary_self_types` feature (because we implement `Receiver` for `T: Deref`) _unless_ those folks have added methods which may shadow methods in inner types, which we no longer want to allow

Subsequent PRs will add better diagnostics.

It's probably easiest to review this commit-by-commit.

r? `@wesleywiser`

Author: bors

compiler/rustc_error_codes/src/error_codes/E0307.md

@@ -65,8 +65,10 @@ impl Trait for Foo {
 ```
 
 The nightly feature [Arbitrary self types][AST] extends the accepted
-set of receiver types to also include any type that can dereference to
-`Self`:
+set of receiver types to also include any type that implements the
+`Receiver` trait and can follow its chain of `Target` types to `Self`.
+There's a blanket implementation of `Receiver` for `T: Deref`, so any
+type which dereferences to `Self` can be used.
 
 ```
 #![feature(arbitrary_self_types)]

compiler/rustc_hir_analysis/messages.ftl

@@ -241,10 +241,10 @@ hir_analysis_invalid_generic_receiver_ty_help =
     use a concrete type such as `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`)
 
 hir_analysis_invalid_receiver_ty = invalid `self` parameter type: `{$receiver_ty}`
-    .note = type of `self` must be `Self` or a type that dereferences to it
+    .note = type of `self` must be `Self` or some type implementing `Receiver`
 
 hir_analysis_invalid_receiver_ty_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`)
+    consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 hir_analysis_invalid_union_field =
     field must implement `Copy` or be wrapped in `ManuallyDrop<...>` to be used in a union