Auto merge of #129073 - compiler-errors:receiver-variance, r=lcnr

Relate receiver invariantly in method probe for `Mode::Path`

Effectively reverts part of #126128
Fixes #126227

This PR changes method probing to use equality for fully path-based method lookup, and subtyping for receiver `.` method lookup.

r? lcnr

tests/ui/methods/receiver-equality.rs

@@ -0,0 +1,16 @@
+// Tests that we probe receivers invariantly when using path-based method lookup.
+
+struct B<T>(T);
+
+impl B<fn(&'static ())> {
+    fn method(self) {
+        println!("hey");
+    }
+}
+
+fn foo(y: B<fn(&'static ())>) {
+    B::<for<'a> fn(&'a ())>::method(y);
+    //~^ ERROR no function or associated item named `method` found
+}
+
+fn main() {}

tests/ui/methods/receiver-equality.stderr

@@ -0,0 +1,12 @@
+error[E0599]: no function or associated item named `method` found for struct `B<for<'a> fn(&'a ())>` in the current scope
+  --> $DIR/receiver-equality.rs:12:30
+   |
+LL | struct B<T>(T);
+   | ----------- function or associated item `method` not found for this struct
+...
+LL |     B::<for<'a> fn(&'a ())>::method(y);
+   |                              ^^^^^^ function or associated item not found in `B<fn(&())>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0599`.