Allow impl Fn() -> impl Trait

This allows writing the following function signatures:
```rust
fn f0() -> impl Fn() -> impl Trait;
fn f1(_: impl Fn() -> impl Trait);
fn f2<F: Fn() -> impl Trait>(_: F);
fn f3() -> &'static dyn Fn() -> impl Trait;
fn f4(_: &dyn Fn() -> impl Trait);
```

All of the above are already allowed with 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

@@ -222,7 +222,9 @@ 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 => {
                         let mut err = struct_span_err!(self.sess, data.span, E0214, "{}", msg);
                         err.span_label(data.span, "only `Fn` traits may use parentheses");
@@ -384,6 +386,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_parenthesized_parameter_data(
         &mut self,
         data: &ParenthesizedArgs,
+        itctx: ImplTraitContext<'_, 'hir>,
     ) -> (GenericArgsCtor<'hir>, bool) {
         // Switch to `PassThrough` mode for anonymous lifetimes; this
         // means that we permit things like `&Ref<T>`, where `Ref` has
@@ -396,7 +399,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 inputs.iter().map(|ty| this.lower_ty_direct(ty, ImplTraitContext::disallowed())),
             );
             let output_ty = match output {
-                FnRetTy::Ty(ty) => this.lower_ty(&ty, ImplTraitContext::disallowed()),
+                FnRetTy::Ty(ty) => this.lower_ty(&ty, itctx),
                 FnRetTy::Default(_) => this.arena.alloc(this.ty_tup(*span, &[])),
             };
             let args = smallvec![GenericArg::Type(this.ty_tup(*inputs_span, inputs))];

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

@@ -23,8 +23,7 @@ fn allowed_in_assoc_type() -> impl Iterator<Item=impl Fn()> {
 }
 
 fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
-//~^ `impl Trait` not allowed
-    || 5
+    || 5u8
 }
 
 fn main() {}

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

@@ -40,13 +40,7 @@ error[E0562]: `impl Trait` not allowed outside of function and method return typ
 LL | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
    |                                ^^^^^^^^^^^^^^^^^^^^^
 
-error[E0562]: `impl Trait` not allowed outside of function and method return types
-  --> $DIR/nested_impl_trait.rs:25:42
-   |
-LL | fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
-   |                                          ^^^^^^^^^^^^^^
-
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors
 
 Some errors have detailed explanations: E0562, E0666.
 For more information about an error, try `rustc --explain E0562`.

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

@@ -31,44 +31,38 @@ fn in_fn_return_in_return() -> fn() -> impl Debug { panic!() }
 fn in_dyn_Fn_parameter_in_parameters(_: &dyn Fn(impl Debug)) { panic!() }
 //~^ ERROR `impl Trait` not allowed outside of function and method return types
 
-// Disallowed
+// Allowed
 fn in_dyn_Fn_return_in_parameters(_: &dyn Fn() -> impl Debug) { panic!() }
-//~^ ERROR `impl Trait` not allowed outside of function and method return types
 
 // Disallowed
 fn in_dyn_Fn_parameter_in_return() -> &'static dyn Fn(impl Debug) { panic!() }
 //~^ ERROR `impl Trait` not allowed outside of function and method return types
 
-// Disallowed
+// Allowed
 fn in_dyn_Fn_return_in_return() -> &'static dyn Fn() -> impl Debug { panic!() }
-//~^ ERROR `impl Trait` not allowed outside of function and method return types
 
 // Disallowed
 fn in_impl_Fn_parameter_in_parameters(_: &impl Fn(impl Debug)) { panic!() }
 //~^ ERROR `impl Trait` not allowed outside of function and method return types
 //~^^ ERROR nested `impl Trait` is not allowed
 
-// Disallowed
+// Allowed
 fn in_impl_Fn_return_in_parameters(_: &impl Fn() -> impl Debug) { panic!() }
-//~^ ERROR `impl Trait` not allowed outside of function and method return types
 
 // Disallowed
 fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic!() }
 //~^ ERROR `impl Trait` not allowed outside of function and 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` not allowed outside of function and method return types
 
 // Disallowed
 fn in_Fn_parameter_in_generics<F: Fn(impl Debug)> (_: F) { panic!() }
 //~^ ERROR `impl Trait` not allowed outside of function and method return types
 
-// Disallowed
+// Allowed
 fn in_Fn_return_in_generics<F: Fn() -> impl Debug> (_: F) { panic!() }
-//~^ ERROR `impl Trait` not allowed outside of function and method return types
-
 
 // Allowed
 fn in_impl_Trait_in_parameters(_: impl Iterator<Item = impl Iterator>) { panic!() }