Rollup merge of #57501 - petrochenkov:highvar, r=alexreg

High priority resolutions for associated variants

In https://github.com/rust-lang/rust/pull/56225 variants were assigned lowest priority during name resolution to avoid crater run and potential breakage.

This PR changes the rules to give variants highest priority instead.
Some motivation:
- If variants (and their constructors) are treated as associated items, then they are obviously *inherent* associated items since they don't come from traits.
- Inherent associated items have higher priority during resolution than associated items from traits.
- The reason is that there is a way to disambiguate in favor of trait items (`<Type as Trait>::Ambiguous`), but there's no way to disambiguate in favor of inherent items, so they became unusable in case of ambiguities if they have low priority.
- It's technically problematic to fallback from associated types to anything until lazy normalization (?) is implemented.

Crater found some regressions from this change, but they are all in type positions, e.g.
```rust
fn f() -> Self::Ambiguos { ... } // Variant `Ambiguous` or associated type `Ambiguous`?
```
, so variants are not usable there right now, but they may become usable in the future if https://github.com/rust-lang/rfcs/pull/2593 is accepted.
This PR keeps code like this successfully resolving, but introduces a future-compatibility lint `ambiguous_associated_items` that recommends rewriting it as `<Self as Trait>::Ambiguous`.

src/test/ui/type-alias-enum-variants-priority-2.rs

@@ -0,0 +1,13 @@
+#![feature(type_alias_enum_variants)]
+
+enum E {
+    V(u8)
+}
+
+impl E {
+    fn V() {}
+}
+
+fn main() {
+    <E>::V(); //~ ERROR this function takes 1 parameter but 0 parameters were supplied
+}

src/test/ui/type-alias-enum-variants-priority-2.stderr

@@ -0,0 +1,12 @@
+error[E0061]: this function takes 1 parameter but 0 parameters were supplied
+  --> $DIR/type-alias-enum-variants-priority-2.rs:12:5
+   |
+LL |     V(u8)
+   |     ----- defined here
+...
+LL |     <E>::V(); //~ ERROR this function takes 1 parameter but 0 parameters were supplied
+   |     ^^^^^^^^ expected 1 parameter
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0061`.

src/test/ui/type-alias-enum-variants-priority-3.rs

@@ -0,0 +1,10 @@
+#![feature(type_alias_enum_variants)]
+
+enum E {
+    V
+}
+
+fn check() -> <E>::V {}
+//~^ ERROR expected type, found variant `V`
+
+fn main() {}

src/test/ui/type-alias-enum-variants-priority-3.stderr

@@ -0,0 +1,8 @@
+error: expected type, found variant `V`
+  --> $DIR/type-alias-enum-variants-priority-3.rs:7:15
+   |
+LL | fn check() -> <E>::V {}
+   |               ^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/type-alias-enum-variants-priority.rs

@@ -0,0 +1,20 @@
+#![feature(type_alias_enum_variants)]
+#![deny(ambiguous_associated_items)]
+
+enum E {
+    V
+}
+
+trait Tr {
+    type V;
+    fn f() -> Self::V;
+}
+
+impl Tr for E {
+    type V = u8;
+    fn f() -> Self::V { 0 }
+    //~^ ERROR ambiguous associated item
+    //~| WARN this was previously accepted
+}
+
+fn main() {}

src/test/ui/type-alias-enum-variants-priority.stderr

@@ -0,0 +1,26 @@
+error: ambiguous associated item
+  --> $DIR/type-alias-enum-variants-priority.rs:15:15
+   |
+LL |     fn f() -> Self::V { 0 }
+   |               ^^^^^^^ help: use fully-qualified syntax: `<E as Trait>::V`
+   |
+note: lint level defined here
+  --> $DIR/type-alias-enum-variants-priority.rs:2:9
+   |
+LL | #![deny(ambiguous_associated_items)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #57644 <https://github.com/rust-lang/rust/issues/57644>
+note: `V` could refer to variant defined here
+  --> $DIR/type-alias-enum-variants-priority.rs:5:5
+   |
+LL |     V
+   |     ^
+note: `V` could also refer to associated type defined here
+  --> $DIR/type-alias-enum-variants-priority.rs:9:5
+   |
+LL |     type V;
+   |     ^^^^^^^
+
+error: aborting due to previous error
+