Allow impl Fn() -> impl Trait in return position

This allows writing the following function signatures:
```rust
fn f0() -> impl Fn() -> impl Trait;
fn f3() -> &'static dyn Fn() -> impl Trait;
```

These signatures were already allowed for common traits and associated
types, there is no reason why `Fn*` traits should be special in this
regard.

Author: WaffleLapkin

compiler/rustc_ast_lowering/src/path.rs

@@ -191,7 +191,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     self.lower_angle_bracketed_parameter_data(data, param_mode, itctx)
                 }
                 GenericArgs::Parenthesized(ref data) => match parenthesized_generic_args {
-                    ParenthesizedGenericArgs::Ok => self.lower_parenthesized_parameter_data(data),
+                    ParenthesizedGenericArgs::Ok => {
+                        self.lower_parenthesized_parameter_data(data, itctx)
+                    }
                     ParenthesizedGenericArgs::Err => {
                         // Suggest replacing parentheses with angle brackets `Trait(params...)` to `Trait<params...>`
                         let sub = if !data.inputs.is_empty() {
@@ -344,6 +346,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_parenthesized_parameter_data(
         &mut self,
         data: &ParenthesizedArgs,
+        itctx: ImplTraitContext,
     ) -> (GenericArgsCtor<'hir>, bool) {
         // Switch to `PassThrough` mode for anonymous lifetimes; this
         // means that we permit things like `&Ref<T>`, where `Ref` has
@@ -355,6 +358,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             self.lower_ty_direct(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitParam))
         }));
         let output_ty = match output {
+            // Only allow `impl Trait` in return position. i.e.:
+            // ```rust
+            // fn f(_: impl Fn() -> impl Debug) -> impl Fn() -> impl Debug
+            // //      disallowed --^^^^^^^^^^        allowed --^^^^^^^^^^
+            // ```
+            FnRetTy::Ty(ty) if matches!(itctx, ImplTraitContext::ReturnPositionOpaqueTy { .. }) => {
+                self.lower_ty(&ty, itctx)
+            }
             FnRetTy::Ty(ty) => {
                 self.lower_ty(&ty, &ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitReturn))
             }

src/test/ui/impl-trait/impl_fn_associativity.rs

@@ -0,0 +1,16 @@
+// run-pass
+use std::fmt::Debug;
+
+fn f_debug() -> impl Fn() -> impl Debug {
+    || ()
+}
+
+fn ff_debug() -> impl Fn() -> impl Fn() -> impl Debug {
+    || f_debug()
+}
+
+fn main() {
+    // Check that `ff_debug` is `() -> (() -> Debug)` and not `(() -> ()) -> Debug`
+    let debug = ff_debug()()();
+    assert_eq!(format!("{:?}", debug), "()");
+}

src/test/ui/impl-trait/nested_impl_trait.rs

@@ -25,8 +25,7 @@ fn allowed_in_assoc_type() -> impl Iterator<Item=impl Fn()> {
 }
 
 fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
-//~^ `impl Trait` only allowed in function and inherent method return types
-    || 5
+    || 5u8
 }
 
 fn main() {}

src/test/ui/impl-trait/nested_impl_trait.stderr

@@ -40,12 +40,6 @@ error[E0562]: `impl Trait` only allowed in function and inherent method return t
 LL | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
    |                                ^^^^^^^^^^^^^^^^^^^^^
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
-  --> $DIR/nested_impl_trait.rs:27:42
-   |
-LL | fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
-   |                                          ^^^^^^^^^^^^^^
-
 error[E0277]: the trait bound `impl Debug: From<impl Into<u32>>` is not satisfied
   --> $DIR/nested_impl_trait.rs:5:46
    |
@@ -64,7 +58,7 @@ LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    = help: the trait `Into<U>` is implemented for `T`
    = note: required for `impl Into<u32>` to implement `Into<impl Debug>`
 
-error: aborting due to 8 previous errors
+error: aborting due to 7 previous errors
 
 Some errors have detailed explanations: E0277, E0562, E0666.
 For more information about an error, try `rustc --explain E0277`.

src/test/ui/impl-trait/where-allowed.rs

@@ -39,9 +39,8 @@ fn in_dyn_Fn_return_in_parameters(_: &dyn Fn() -> impl Debug) { panic!() }
 fn in_dyn_Fn_parameter_in_return() -> &'static dyn Fn(impl Debug) { panic!() }
 //~^ ERROR `impl Trait` only allowed in function and inherent method return types
 
-// Disallowed
+// Allowed
 fn in_dyn_Fn_return_in_return() -> &'static dyn Fn() -> impl Debug { panic!() }
-//~^ ERROR `impl Trait` only allowed in function and inherent method return types
 
 // Disallowed
 fn in_impl_Fn_parameter_in_parameters(_: &impl Fn(impl Debug)) { panic!() }
@@ -57,9 +56,8 @@ fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic!() }
 //~^ ERROR `impl Trait` only allowed in function and inherent method return types
 //~| ERROR nested `impl Trait` is not allowed
 
-// Disallowed
+// Allowed
 fn in_impl_Fn_return_in_return() -> &'static impl Fn() -> impl Debug { panic!() }
-//~^ ERROR `impl Trait` only allowed in function and inherent method return types
 
 // Disallowed
 fn in_Fn_parameter_in_generics<F: Fn(impl Debug)> (_: F) { panic!() }