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.

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
+    || 5u8
-    || 5
 }
 
 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!() }

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

@@ -1,5 +1,5 @@
 error[E0666]: nested `impl Trait` is not allowed
-  --> $DIR/where-allowed.rs:47:51
+  --> $DIR/where-allowed.rs:46:51
    |
 LL | fn in_impl_Fn_parameter_in_parameters(_: &impl Fn(impl Debug)) { panic!() }
    |                                           --------^^^^^^^^^^-
@@ -8,7 +8,7 @@ LL | fn in_impl_Fn_parameter_in_parameters(_: &impl Fn(impl Debug)) { panic!() }
    |                                           outer `impl Trait`
 
 error[E0666]: nested `impl Trait` is not allowed
-  --> $DIR/where-allowed.rs:56:57
+  --> $DIR/where-allowed.rs:55:57
    |
 LL | fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic!() }
    |                                                 --------^^^^^^^^^^-
@@ -17,7 +17,7 @@ LL | fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic
    |                                                 outer `impl Trait`
 
 error[E0658]: `impl Trait` in type aliases is unstable
-  --> $DIR/where-allowed.rs:119:16
+  --> $DIR/where-allowed.rs:117:16
    |
 LL |     type Out = impl Debug;
    |                ^^^^^^^^^^
@@ -26,7 +26,7 @@ LL |     type Out = impl Debug;
    = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
 
 error[E0658]: `impl Trait` in type aliases is unstable
-  --> $DIR/where-allowed.rs:154:23
+  --> $DIR/where-allowed.rs:152:23
    |
 LL | type InTypeAlias<R> = impl Debug;
    |                       ^^^^^^^^^^
@@ -35,7 +35,7 @@ LL | type InTypeAlias<R> = impl Debug;
    = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
 
 error[E0658]: `impl Trait` in type aliases is unstable
-  --> $DIR/where-allowed.rs:157:39
+  --> $DIR/where-allowed.rs:155:39
    |
 LL | type InReturnInTypeAlias<R> = fn() -> impl Debug;
    |                                       ^^^^^^^^^^
@@ -85,80 +85,68 @@ error[E0562]: `impl Trait` only allowed in function and inherent method return t
 LL | fn in_dyn_Fn_parameter_in_return() -> &'static dyn Fn(impl Debug) { panic!() }
    |                                                       ^^^^^^^^^^
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
-  --> $DIR/where-allowed.rs:43:57
-   |
-LL | fn in_dyn_Fn_return_in_return() -> &'static dyn Fn() -> impl Debug { panic!() }
-   |                                                         ^^^^^^^^^^
-
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait param
-  --> $DIR/where-allowed.rs:47:51
+  --> $DIR/where-allowed.rs:46:51
    |
 LL | fn in_impl_Fn_parameter_in_parameters(_: &impl Fn(impl Debug)) { panic!() }
    |                                                   ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
-  --> $DIR/where-allowed.rs:52:53
+  --> $DIR/where-allowed.rs:51:53
    |
 LL | fn in_impl_Fn_return_in_parameters(_: &impl Fn() -> impl Debug) { panic!() }
    |                                                     ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait param
-  --> $DIR/where-allowed.rs:56:57
+  --> $DIR/where-allowed.rs:55:57
    |
 LL | fn in_impl_Fn_parameter_in_return() -> &'static impl Fn(impl Debug) { panic!() }
    |                                                         ^^^^^^^^^^
 
-error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
-  --> $DIR/where-allowed.rs:61:59
-   |
-LL | fn in_impl_Fn_return_in_return() -> &'static impl Fn() -> impl Debug { panic!() }
-   |                                                           ^^^^^^^^^^
-
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait param
-  --> $DIR/where-allowed.rs:65:38
+  --> $DIR/where-allowed.rs:63:38
    |
 LL | fn in_Fn_parameter_in_generics<F: Fn(impl Debug)> (_: F) { panic!() }
    |                                      ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
-  --> $DIR/where-allowed.rs:69:40
+  --> $DIR/where-allowed.rs:67:40
    |
 LL | fn in_Fn_return_in_generics<F: Fn() -> impl Debug> (_: F) { panic!() }
    |                                        ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:82:32
+  --> $DIR/where-allowed.rs:80:32
    |
 LL | struct InBraceStructField { x: impl Debug }
    |                                ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in path
-  --> $DIR/where-allowed.rs:86:41
+  --> $DIR/where-allowed.rs:84:41
    |
 LL | struct InAdtInBraceStructField { x: Vec<impl Debug> }
    |                                         ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:90:27
+  --> $DIR/where-allowed.rs:88:27
    |
 LL | struct InTupleStructField(impl Debug);
    |                           ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:95:25
+  --> $DIR/where-allowed.rs:93:25
    |
 LL |     InBraceVariant { x: impl Debug },
    |                         ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:97:20
+  --> $DIR/where-allowed.rs:95:20
    |
 LL |     InTupleVariant(impl Debug),
    |                    ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in trait method return
-  --> $DIR/where-allowed.rs:108:23
+  --> $DIR/where-allowed.rs:106:23
    |
 LL |     fn in_return() -> impl Debug;
    |                       ^^^^^^^^^^
@@ -167,7 +155,7 @@ LL |     fn in_return() -> impl Debug;
    = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `impl` method return
-  --> $DIR/where-allowed.rs:125:34
+  --> $DIR/where-allowed.rs:123:34
    |
 LL |     fn in_trait_impl_return() -> impl Debug { () }
    |                                  ^^^^^^^^^^
@@ -176,127 +164,127 @@ LL |     fn in_trait_impl_return() -> impl Debug { () }
    = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `extern fn` param
-  --> $DIR/where-allowed.rs:138:33
+  --> $DIR/where-allowed.rs:136:33
    |
 LL |     fn in_foreign_parameters(_: impl Debug);
    |                                 ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `extern fn` return
-  --> $DIR/where-allowed.rs:141:31
+  --> $DIR/where-allowed.rs:139:31
    |
 LL |     fn in_foreign_return() -> impl Debug;
    |                               ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `fn` pointer return
-  --> $DIR/where-allowed.rs:157:39
+  --> $DIR/where-allowed.rs:155:39
    |
 LL | type InReturnInTypeAlias<R> = fn() -> impl Debug;
    |                                       ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in trait
-  --> $DIR/where-allowed.rs:162:16
+  --> $DIR/where-allowed.rs:160:16
    |
 LL | impl PartialEq<impl Debug> for () {
    |                ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:167:24
+  --> $DIR/where-allowed.rs:165:24
    |
 LL | impl PartialEq<()> for impl Debug {
    |                        ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:172:6
+  --> $DIR/where-allowed.rs:170:6
    |
 LL | impl impl Debug {
    |      ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:178:24
+  --> $DIR/where-allowed.rs:176:24
    |
 LL | impl InInherentImplAdt<impl Debug> {
    |                        ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:184:11
+  --> $DIR/where-allowed.rs:182:11
    |
 LL |     where impl Debug: Debug
    |           ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:191:15
+  --> $DIR/where-allowed.rs:189:15
    |
 LL |     where Vec<impl Debug>: Debug
    |               ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in bound
-  --> $DIR/where-allowed.rs:198:24
+  --> $DIR/where-allowed.rs:196:24
    |
 LL |     where T: PartialEq<impl Debug>
    |                        ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait param
-  --> $DIR/where-allowed.rs:205:17
+  --> $DIR/where-allowed.rs:203:17
    |
 LL |     where T: Fn(impl Debug)
    |                 ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
-  --> $DIR/where-allowed.rs:212:22
+  --> $DIR/where-allowed.rs:210:22
    |
 LL |     where T: Fn() -> impl Debug
    |                      ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:218:40
+  --> $DIR/where-allowed.rs:216:40
    |
 LL | struct InStructGenericParamDefault<T = impl Debug>(T);
    |                                        ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:222:36
+  --> $DIR/where-allowed.rs:220:36
    |
 LL | enum InEnumGenericParamDefault<T = impl Debug> { Variant(T) }
    |                                    ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:226:38
+  --> $DIR/where-allowed.rs:224:38
    |
 LL | trait InTraitGenericParamDefault<T = impl Debug> {}
    |                                      ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:230:41
+  --> $DIR/where-allowed.rs:228:41
    |
 LL | type InTypeAliasGenericParamDefault<T = impl Debug> = T;
    |                                         ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:234:11
+  --> $DIR/where-allowed.rs:232:11
    |
 LL | impl <T = impl Debug> T {}
    |           ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in type
-  --> $DIR/where-allowed.rs:241:40
+  --> $DIR/where-allowed.rs:239:40
    |
 LL | fn in_method_generic_param_default<T = impl Debug>(_: T) {}
    |                                        ^^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in variable binding
-  --> $DIR/where-allowed.rs:247:29
+  --> $DIR/where-allowed.rs:245:29
    |
 LL |     let _in_local_variable: impl Fn() = || {};
    |                             ^^^^^^^^^
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in closure return
-  --> $DIR/where-allowed.rs:249:46
+  --> $DIR/where-allowed.rs:247:46
    |
 LL |     let _in_return_in_local_variable = || -> impl Fn() { || {} };
    |                                              ^^^^^^^^^
 
 error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
-  --> $DIR/where-allowed.rs:234:7
+  --> $DIR/where-allowed.rs:232:7
    |
 LL | impl <T = impl Debug> T {}
    |       ^^^^^^^^^^^^^^
@@ -306,7 +294,7 @@ LL | impl <T = impl Debug> T {}
    = note: `#[deny(invalid_type_param_default)]` on by default
 
 error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
-  --> $DIR/where-allowed.rs:241:36
+  --> $DIR/where-allowed.rs:239:36
    |
 LL | fn in_method_generic_param_default<T = impl Debug>(_: T) {}
    |                                    ^^^^^^^^^^^^^^
@@ -315,14 +303,14 @@ LL | fn in_method_generic_param_default<T = impl Debug>(_: T) {}
    = note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887>
 
 error[E0118]: no nominal type found for inherent implementation
-  --> $DIR/where-allowed.rs:234:23
+  --> $DIR/where-allowed.rs:232:23
    |
 LL | impl <T = impl Debug> T {}
    |                       ^ impl requires a nominal type
    |
    = note: either implement a trait on it or create a newtype to wrap it instead
 
-error: aborting due to 49 previous errors
+error: aborting due to 47 previous errors
 
 Some errors have detailed explanations: E0118, E0562, E0658, E0666.
 For more information about an error, try `rustc --explain E0118`.