Rollup merge of #69048 - estebank:hrlt-assoc, r=nagisa

Suggestion when encountering assoc types from hrtb

When encountering E0212, detect whether this is a representable case or
not, i.e. if it's happening on an `fn` or on an ADT. If the former,
provide a structured suggestion, otherwise note that this can't be
represented in Rust.

Fix #69000.

src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.fixed

@@ -0,0 +1,37 @@
+#![allow(dead_code, unused_variables)]
+// run-rustfix
+// Check projection of an associated type out of a higher-ranked trait-bound
+// in the context of a function signature.
+
+pub trait Foo<T> {
+    type A;
+
+    fn get(&self, t: T) -> Self::A;
+}
+
+fn foo2<I : for<'x> Foo<&'x isize>>(
+    x: <I as Foo<&isize>>::A)
+    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+{
+    // This case is illegal because we have to instantiate `'x`, and
+    // we don't know what region to instantiate it with.
+    //
+    // This could perhaps be made equivalent to the examples below,
+    // specifically for fn signatures.
+}
+
+fn foo3<I : for<'x> Foo<&'x isize>>(
+    x: <I as Foo<&isize>>::A)
+{
+    // OK, in this case we spelled out the precise regions involved, though we left one of
+    // them anonymous.
+}
+
+fn foo4<'a, I : for<'x> Foo<&'x isize>>(
+    x: <I as Foo<&'a isize>>::A)
+{
+    // OK, in this case we spelled out the precise regions involved.
+}
+
+
+pub fn main() {}

src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.rs

@@ -1,3 +1,5 @@
+#![allow(dead_code, unused_variables)]
+// run-rustfix
 // Check projection of an associated type out of a higher-ranked trait-bound
 // in the context of a function signature.
 

src/test/ui/associated-types/associated-types-project-from-hrtb-in-fn.stderr

@@ -1,8 +1,8 @@
 error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
-  --> $DIR/associated-types-project-from-hrtb-in-fn.rs:11:8
+  --> $DIR/associated-types-project-from-hrtb-in-fn.rs:13:8
    |
 LL |     x: I::A)
-   |        ^^^^
+   |        ^^^^ help: use a fully qualified path with inferred lifetimes: `<I as Foo<&isize>>::A`
 
 error: aborting due to previous error
 

src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.rs

@@ -7,18 +7,25 @@ pub trait Foo<T> {
     fn get(&self, t: T) -> Self::A;
 }
 
-struct SomeStruct<I : for<'x> Foo<&'x isize>> {
+struct SomeStruct<I: for<'x> Foo<&'x isize>> {
     field: I::A
     //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
 }
 
+enum SomeEnum<I: for<'x> Foo<&'x isize>> {
+    TupleVariant(I::A),
+    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+    StructVariant { field: I::A },
+    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+}
+
 // FIXME(eddyb) This one doesn't even compile because of the unsupported syntax.
 
 // struct AnotherStruct<I : for<'x> Foo<&'x isize>> {
 //     field: <I as for<'y> Foo<&'y isize>>::A
 // }
 
-struct YetAnotherStruct<'a, I : for<'x> Foo<&'x isize>> {
+struct YetAnotherStruct<'a, I: for<'x> Foo<&'x isize>> {
     field: <I as Foo<&'a isize>>::A
 }
 

src/test/ui/associated-types/associated-types-project-from-hrtb-in-struct.stderr

@@ -3,6 +3,38 @@ error[E0212]: cannot extract an associated type from a higher-ranked trait bound
    |
 LL |     field: I::A
    |            ^^^^
+   |
+help: use a fully qualified path with explicit lifetimes
+   |
+LL | struct SomeStruct<'a, I: for<'x> Foo<&'x isize>> {
+LL |     field: <I as Foo<&'a isize>>::A
+   |
 
-error: aborting due to previous error
+error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+  --> $DIR/associated-types-project-from-hrtb-in-struct.rs:16:18
+   |
+LL |     TupleVariant(I::A),
+   |                  ^^^^
+   |
+help: use a fully qualified path with explicit lifetimes
+   |
+LL | enum SomeEnum<'a, I: for<'x> Foo<&'x isize>> {
+LL |     TupleVariant(<I as Foo<&'a isize>>::A),
+   |
+
+error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+  --> $DIR/associated-types-project-from-hrtb-in-struct.rs:18:28
+   |
+LL |     StructVariant { field: I::A },
+   |                            ^^^^
+   |
+help: use a fully qualified path with explicit lifetimes
+   |
+LL | enum SomeEnum<'a, I: for<'x> Foo<&'x isize>> {
+LL |     TupleVariant(I::A),
+LL |
+LL |     StructVariant { field: <I as Foo<&'a isize>>::A },
+   |
+
+error: aborting due to 3 previous errors
 

src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.fixed

@@ -0,0 +1,38 @@
+#![allow(dead_code)]
+// run-rustfix
+// Check projection of an associated type out of a higher-ranked trait-bound
+// in the context of a method definition in a trait.
+
+pub trait Foo<T> {
+    type A;
+
+    fn get(&self, t: T) -> Self::A;
+}
+
+trait SomeTrait<I : for<'x> Foo<&'x isize>> {
+    fn some_method(&self, arg: <I as Foo<&isize>>::A);
+    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+}
+
+trait AnotherTrait<I : for<'x> Foo<&'x isize>> {
+    fn some_method(&self, arg: <I as Foo<&isize>>::A);
+}
+
+trait YetAnotherTrait<I : for<'x> Foo<&'x isize>> {
+    fn some_method<'a>(&self, arg: <I as Foo<&'a isize>>::A);
+}
+
+trait Banana<'a> {
+    type Assoc: Default;
+}
+
+struct Peach<X>(std::marker::PhantomData<X>);
+
+impl<X: for<'a> Banana<'a>> Peach<X> {
+    fn mango(&self) -> <X as Banana<'_>>::Assoc {
+    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+        Default::default()
+    }
+}
+
+pub fn main() {}

src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.rs

@@ -1,3 +1,5 @@
+#![allow(dead_code)]
+// run-rustfix
 // Check projection of an associated type out of a higher-ranked trait-bound
 // in the context of a method definition in a trait.
 
@@ -20,4 +22,17 @@ trait YetAnotherTrait<I : for<'x> Foo<&'x isize>> {
     fn some_method<'a>(&self, arg: <I as Foo<&'a isize>>::A);
 }
 
+trait Banana<'a> {
+    type Assoc: Default;
+}
+
+struct Peach<X>(std::marker::PhantomData<X>);
+
+impl<X: for<'a> Banana<'a>> Peach<X> {
+    fn mango(&self) -> X::Assoc {
+    //~^ ERROR cannot extract an associated type from a higher-ranked trait bound in this context
+        Default::default()
+    }
+}
+
 pub fn main() {}

src/test/ui/associated-types/associated-types-project-from-hrtb-in-trait-method.stderr

@@ -1,8 +1,14 @@
 error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
-  --> $DIR/associated-types-project-from-hrtb-in-trait-method.rs:11:32
+  --> $DIR/associated-types-project-from-hrtb-in-trait-method.rs:13:32
    |
 LL |     fn some_method(&self, arg: I::A);
-   |                                ^^^^
+   |                                ^^^^ help: use a fully qualified path with inferred lifetimes: `<I as Foo<&isize>>::A`
 
-error: aborting due to previous error
+error[E0212]: cannot extract an associated type from a higher-ranked trait bound in this context
+  --> $DIR/associated-types-project-from-hrtb-in-trait-method.rs:32:24
+   |
+LL |     fn mango(&self) -> X::Assoc {
+   |                        ^^^^^^^^ help: use a fully qualified path with inferred lifetimes: `<X as Banana<'_>>::Assoc`
+
+error: aborting due to 2 previous errors