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
https://github.com/rust-lang/rust/issues/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,
https://github.com/rust-lang/rfcs/pull/3519
https://github.com/rust-lang/rust/issues/44874

r? @wesleywiser

2 files changed, 52 insertions, 0 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0801.md b/compiler/rustc_error_codes/src/error_codes/E0801.md
new file mode 100644
index 00000000000..c89feb9b308
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0801.md
@@ -0,0 +1,51 @@
+The `self` parameter in a method has an invalid generic "receiver type".
+
+Erroneous code example:
+
+```compile_fail,E0801
+struct Foo;
+
+impl Foo {
+    fn foo<R: std::ops::Deref<Target=Self>>(self: R) {}
+}
+```
+
+or alternatively,
+
+```compile_fail,E0801
+struct Foo;
+
+impl Foo {
+    fn foo(self: impl std::ops::Deref<Target=Self>) {}
+}
+```
+
+Methods take a special first parameter, termed `self`. It's normal to
+use `self`, `&self` or `&mut self`, which are syntactic sugar for
+`self: Self`, `self: &Self`, and `self: &mut Self` respectively.
+But it's also possible to use more sophisticated types of `self`
+parameter, for instance `std::rc::Rc<Self>`. The set of allowable
+`Self` types is extensible using the nightly feature
+[Arbitrary self types][AST].
+This will extend the valid set of `Self` types to anything which implements
+`std::ops::Deref<Target=Self>`, for example `Rc<Self>`, `Box<Self>`, or
+your own smart pointers that do the same.
+
+However, even with that feature, the `self` type must be concrete.
+Generic `self` types are not permitted. Specifically, a `self` type will
+be rejected if it is a type parameter defined on the method.
+
+These are OK:
+
+```
+struct Foo;
+
+impl Foo {
+    fn foo(self) {}
+    fn foo2(self: std::rc::Rc<Self>) {} // or some other similar
+        // smart pointer if you enable arbitrary self types and
+        // the pointer implements Deref<Target=Self>
+}
+```
+
+[AST]: https://doc.rust-lang.org/unstable-book/language-features/arbitrary-self-types.html
diff --git a/compiler/rustc_error_codes/src/lib.rs b/compiler/rustc_error_codes/src/lib.rs
index 27a34d6003d..29f3277d399 100644
--- a/compiler/rustc_error_codes/src/lib.rs
+++ b/compiler/rustc_error_codes/src/lib.rs
@@ -540,6 +540,7 @@ E0797: 0797,
 E0798: 0798,
 E0799: 0799,
 E0800: 0800,
+E0801: 0801,
         );
     )
 }