Add AliasKind::Weak for type aliases.

Only use it when the type alias contains an opaque type.

Also does wf-checking on such type aliases.

clippy_lints/src/dereference.rs

@@ -1424,6 +1424,7 @@ fn ty_auto_deref_stability<'tcx>(
                 continue;
             },
             ty::Param(_) => TyPosition::new_deref_stable_for_result(precedence, ty),
+            ty::Alias(ty::Weak, _) => unreachable!("should have been normalized away above"),
             ty::Alias(ty::Inherent, _) => unreachable!("inherent projection should have been normalized away above"),
             ty::Alias(ty::Projection, _) if ty.has_non_region_param() => {
                 TyPosition::new_deref_stable_for_result(precedence, ty)

tests/ui/from_over_into.fixed

@@ -82,10 +82,4 @@ fn msrv_1_41() {
     }
 }
 
-type Opaque = impl Sized;
-struct IntoOpaque;
-impl Into<Opaque> for IntoOpaque {
-    fn into(self) -> Opaque {}
-}
-
 fn main() {}

tests/ui/from_over_into.rs

@@ -82,10 +82,4 @@ fn msrv_1_41() {
     }
 }
 
-type Opaque = impl Sized;
-struct IntoOpaque;
-impl Into<Opaque> for IntoOpaque {
-    fn into(self) -> Opaque {}
-}
-
 fn main() {}

tests/ui/from_over_into_unfixable.rs

@@ -32,4 +32,10 @@ impl Into<u8> for ContainsVal {
     }
 }
 
+type Opaque = impl Sized;
+struct IntoOpaque;
+impl Into<Opaque> for IntoOpaque {
+    fn into(self) -> Opaque {}
+}
+
 fn main() {}

tests/ui/from_over_into_unfixable.stderr

@@ -1,29 +1,12 @@
-error: an implementation of `From` is preferred since it gives you `Into<_>` for free where the reverse isn't true
-  --> $DIR/from_over_into_unfixable.rs:11:1
+error[E0658]: `impl Trait` in type aliases is unstable
+  --> $DIR/from_over_into_unfixable.rs:35:15
    |
-LL | impl Into<InMacro> for String {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | type Opaque = impl Sized;
+   |               ^^^^^^^^^^
    |
-   = help: replace the `Into` implementation with `From<std::string::String>`
-   = note: `-D clippy::from-over-into` implied by `-D warnings`
+   = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
+   = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
 
-error: an implementation of `From` is preferred since it gives you `Into<_>` for free where the reverse isn't true
-  --> $DIR/from_over_into_unfixable.rs:19:1
-   |
-LL | impl Into<WeirdUpperSelf> for &'static [u8] {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: replace the `Into` implementation with `From<&'static [u8]>`
-
-error: an implementation of `From` is preferred since it gives you `Into<_>` for free where the reverse isn't true
-  --> $DIR/from_over_into_unfixable.rs:28:1
-   |
-LL | impl Into<u8> for ContainsVal {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = help: `impl From<Local> for Foreign` is allowed by the orphan rules, for more information see
-           https://doc.rust-lang.org/reference/items/implementations.html#trait-implementation-coherence
-   = help: replace the `Into` implementation with `From<ContainsVal>`
-
-error: aborting due to 3 previous errors
+error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0658`.

tests/ui/new_ret_no_self.rs

@@ -401,25 +401,3 @@ mod issue7344 {
         }
     }
 }
-
-mod issue10041 {
-    struct Bomb;
-
-    impl Bomb {
-        // Hidden <Rhs = Self> default generic parameter.
-        pub fn new() -> impl PartialOrd {
-            0i32
-        }
-    }
-
-    // TAIT with self-referencing bounds
-    type X = impl std::ops::Add<Output = X>;
-
-    struct Bomb2;
-
-    impl Bomb2 {
-        pub fn new() -> X {
-            0i32
-        }
-    }
-}

tests/ui/new_ret_no_self.stderr

@@ -92,21 +92,5 @@ LL | |             unimplemented!()
 LL | |         }
    | |_________^
 
-error: methods called `new` usually return `Self`
-  --> $DIR/new_ret_no_self.rs:410:9
-   |
-LL | /         pub fn new() -> impl PartialOrd {
-LL | |             0i32
-LL | |         }
-   | |_________^
-
-error: methods called `new` usually return `Self`
-  --> $DIR/new_ret_no_self.rs:421:9
-   |
-LL | /         pub fn new() -> X {
-LL | |             0i32
-LL | |         }
-   | |_________^
-
-error: aborting due to 14 previous errors
+error: aborting due to 12 previous errors
 

tests/ui/new_ret_no_self_overflow.rs

@@ -0,0 +1,26 @@
+#![feature(type_alias_impl_trait)]
+#![warn(clippy::new_ret_no_self)]
+
+mod issue10041 {
+    struct Bomb;
+
+    impl Bomb {
+        // Hidden <Rhs = Self> default generic parameter.
+        pub fn new() -> impl PartialOrd {
+            0i32
+        }
+    }
+
+    // TAIT with self-referencing bounds
+    type X = impl std::ops::Add<Output = X>;
+
+    struct Bomb2;
+
+    impl Bomb2 {
+        pub fn new() -> X {
+            0i32
+        }
+    }
+}
+
+fn main() {}

tests/ui/new_ret_no_self_overflow.stderr

@@ -0,0 +1,9 @@
+error[E0275]: overflow evaluating the requirement `<i32 as std::ops::Add>::Output == issue10041::X`
+  --> $DIR/new_ret_no_self_overflow.rs:20:25
+   |
+LL |         pub fn new() -> X {
+   |                         ^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0275`.