Rollup merge of #93582 - WaffleLapkin:rpitirpit, r=compiler-errors

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

_This was originally proposed as part of #93082 which was [closed](#93082 (comment)) due to allowing `impl Fn() -> impl Trait` in argument 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.

`impl Trait` in both `f0` and `f3` means "new existential type", just like with `-> impl Iterator<Item = impl Trait>` and such.

Arrow in `impl Fn() ->` is right-associative and binds from right to left, it's tested by [this test](https://github.com/WaffleLapkin/rust/blob/a819fecb8dea438fc70488ddec30a61e52942672/src/test/ui/impl-trait/impl_fn_associativity.rs).

There even is a test that `f0` compiles:
https://github.com/rust-lang/rust/blob/2f004d2d401682e553af3984ebd9a3976885e752/src/test/ui/impl-trait/nested_impl_trait.rs#L25-L28

But it was changed in [PR 48084 (lines)](https://github.com/rust-lang/rust/pull/48084/files#diff-ccecca938872d65ffe8cd1c3ef1956e309fac83bcda547d8b16b89257e53a437R37)  to test the opposite, probably unintentionally given [PR 48084 (lines)](https://github.com/rust-lang/rust/pull/48084/files#diff-5a02f1ed43debed1fd24f7aad72490064f795b9420f15d847bac822aa4621a1cR476-R477).

r? `@nikomatsakis`

----

This limitation is especially annoying with async code, since it forces one to write this:
```rust
trait AsyncFn3<A, B, C>: Fn(A, B, C) -> <Self as AsyncFn3<A, B, C>>::Future {
    type Future: Future<Output = Self::Out>;

    type Out;
}

impl<A, B, C, Fut, F> AsyncFn3<A, B, C> for F
where
    F: Fn(A, B, C) -> Fut,
    Fut: Future,
{
    type Future = Fut;

    type Out = Fut::Output;
}

fn async_closure() -> impl AsyncFn3<i32, i32, i32, Out = u32> {
    |a, b, c| async move { (a + b + c) as u32 }
}
```
Instead of:
```rust
fn async_closure() -> impl Fn(i32, i32, i32) -> impl Future<Output = u32> {
    |a, b, c| async move { (a + b + c) as u32 }
}
```

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 +358,17 @@ 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.tcx.features().impl_trait_in_fn_trait_return =>
+            {
+                self.lower_ty(&ty, itctx)
+            }
             FnRetTy::Ty(ty) => {
                 self.lower_ty(&ty, &ImplTraitContext::Disallowed(ImplTraitPosition::FnTraitReturn))
             }

compiler/rustc_feature/src/active.rs

@@ -412,6 +412,8 @@ declare_features! (
     (active, half_open_range_patterns_in_slices, "CURRENT_RUSTC_VERSION", Some(67264), None),
     /// Allows `if let` guard in match arms.
     (active, if_let_guard, "1.47.0", Some(51114), None),
+    /// Allows `impl Trait` as output type in `Fn` traits in return position of functions.
+    (active, impl_trait_in_fn_trait_return, "1.64.0", Some(99697), None),
     /// Allows using imported `main` function
     (active, imported_main, "1.53.0", Some(28937), None),
     /// Allows associated types in inherent impls.

compiler/rustc_span/src/symbol.rs

@@ -813,6 +813,7 @@ symbols! {
         impl_lint_pass,
         impl_macros,
         impl_trait_in_bindings,
+        impl_trait_in_fn_trait_return,
         implied_by,
         import,
         import_name_type,

src/test/ui/feature-gates/feature-gate-impl_trait_in_fn_trait_return.rs

@@ -0,0 +1,6 @@
+fn f() -> impl Fn() -> impl Sized { || () }
+//~^ ERROR `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
+fn g() -> &'static dyn Fn() -> impl Sized { &|| () }
+//~^ ERROR `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-impl_trait_in_fn_trait_return.stderr

@@ -0,0 +1,15 @@
+error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
+  --> $DIR/feature-gate-impl_trait_in_fn_trait_return.rs:1:24
+   |
+LL | fn f() -> impl Fn() -> impl Sized { || () }
+   |                        ^^^^^^^^^^
+
+error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `Fn` trait return
+  --> $DIR/feature-gate-impl_trait_in_fn_trait_return.rs:3:32
+   |
+LL | fn g() -> &'static dyn Fn() -> impl Sized { &|| () }
+   |                                ^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0562`.

src/test/ui/impl-trait/impl-fn-hrtb-bounds-2.rs

@@ -0,0 +1,8 @@
+#![feature(impl_trait_in_fn_trait_return)]
+use std::fmt::Debug;
+
+fn a() -> impl Fn(&u8) -> impl Debug {
+    |x| x //~ ERROR hidden type for `impl Debug` captures lifetime that does not appear in bounds
+}
+
+fn main() {}

src/test/ui/impl-trait/impl-fn-hrtb-bounds-2.stderr

@@ -0,0 +1,11 @@
+error[E0700]: hidden type for `impl Debug` captures lifetime that does not appear in bounds
+  --> $DIR/impl-fn-hrtb-bounds-2.rs:5:9
+   |
+LL |     |x| x
+   |     --- ^
+   |     |
+   |     hidden type `&u8` captures the anonymous lifetime #1 defined here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.

src/test/ui/impl-trait/impl-fn-hrtb-bounds.rs

@@ -0,0 +1,24 @@
+#![feature(impl_trait_in_fn_trait_return)]
+use std::fmt::Debug;
+
+fn a() -> impl Fn(&u8) -> (impl Debug + '_) {
+    //~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+    |x| x
+}
+
+fn b() -> impl for<'a> Fn(&'a u8) -> (impl Debug + 'a) {
+    //~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+    |x| x
+}
+
+fn c() -> impl for<'a> Fn(&'a u8) -> (impl Debug + '_) {
+    //~^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+    |x| x
+}
+
+fn d() -> impl Fn() -> (impl Debug + '_) {
+    //~^ ERROR missing lifetime specifier
+    || ()
+}
+
+fn main() {}

src/test/ui/impl-trait/impl-fn-hrtb-bounds.stderr

@@ -0,0 +1,51 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/impl-fn-hrtb-bounds.rs:19:38
+   |
+LL | fn d() -> impl Fn() -> (impl Debug + '_) {
+   |                                      ^^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
+help: consider using the `'static` lifetime
+   |
+LL | fn d() -> impl Fn() -> (impl Debug + 'static) {
+   |                                      ~~~~~~~
+
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+  --> $DIR/impl-fn-hrtb-bounds.rs:4:41
+   |
+LL | fn a() -> impl Fn(&u8) -> (impl Debug + '_) {
+   |                                         ^^
+   |
+note: lifetime declared here
+  --> $DIR/impl-fn-hrtb-bounds.rs:4:19
+   |
+LL | fn a() -> impl Fn(&u8) -> (impl Debug + '_) {
+   |                   ^
+
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+  --> $DIR/impl-fn-hrtb-bounds.rs:9:52
+   |
+LL | fn b() -> impl for<'a> Fn(&'a u8) -> (impl Debug + 'a) {
+   |                                                    ^^
+   |
+note: lifetime declared here
+  --> $DIR/impl-fn-hrtb-bounds.rs:9:20
+   |
+LL | fn b() -> impl for<'a> Fn(&'a u8) -> (impl Debug + 'a) {
+   |                    ^^
+
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+  --> $DIR/impl-fn-hrtb-bounds.rs:14:52
+   |
+LL | fn c() -> impl for<'a> Fn(&'a u8) -> (impl Debug + '_) {
+   |                                                    ^^
+   |
+note: lifetime declared here
+  --> $DIR/impl-fn-hrtb-bounds.rs:14:20
+   |
+LL | fn c() -> impl for<'a> Fn(&'a u8) -> (impl Debug + '_) {
+   |                    ^^
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0106`.

src/test/ui/impl-trait/impl-fn-parsing-ambiguities.rs

@@ -0,0 +1,15 @@
+#![feature(impl_trait_in_fn_trait_return)]
+use std::fmt::Debug;
+
+fn a() -> impl Fn(&u8) -> impl Debug + '_ {
+    //~^ ERROR ambiguous `+` in a type
+    //~^^ ERROR higher kinded lifetime bounds on nested opaque types are not supported yet
+    |x| x
+}
+
+fn b() -> impl Fn() -> impl Debug + Send {
+    //~^ ERROR ambiguous `+` in a type
+    || ()
+}
+
+fn main() {}

src/test/ui/impl-trait/impl-fn-parsing-ambiguities.stderr

@@ -0,0 +1,26 @@
+error: ambiguous `+` in a type
+  --> $DIR/impl-fn-parsing-ambiguities.rs:4:27
+   |
+LL | fn a() -> impl Fn(&u8) -> impl Debug + '_ {
+   |                           ^^^^^^^^^^^^^^^ help: use parentheses to disambiguate: `(impl Debug + '_)`
+
+error: ambiguous `+` in a type
+  --> $DIR/impl-fn-parsing-ambiguities.rs:10:24
+   |
+LL | fn b() -> impl Fn() -> impl Debug + Send {
+   |                        ^^^^^^^^^^^^^^^^^ help: use parentheses to disambiguate: `(impl Debug + Send)`
+
+error: higher kinded lifetime bounds on nested opaque types are not supported yet
+  --> $DIR/impl-fn-parsing-ambiguities.rs:4:40
+   |
+LL | fn a() -> impl Fn(&u8) -> impl Debug + '_ {
+   |                                        ^^
+   |
+note: lifetime declared here
+  --> $DIR/impl-fn-parsing-ambiguities.rs:4:19
+   |
+LL | fn a() -> impl Fn(&u8) -> impl Debug + '_ {
+   |                   ^
+
+error: aborting due to 3 previous errors
+

src/test/ui/impl-trait/impl-fn-predefined-lifetimes.rs

@@ -0,0 +1,15 @@
+#![feature(impl_trait_in_fn_trait_return)]
+use std::fmt::Debug;
+
+fn a<'a>() -> impl Fn(&'a u8) -> (impl Debug + '_) {
+    //~^ ERROR cannot resolve opaque type
+
+    |x| x
+    //~^ ERROR concrete type differs from previous defining opaque type use
+}
+
+fn _b<'a>() -> impl Fn(&'a u8) -> (impl Debug + 'a) {
+    a()
+}
+
+fn main() {}

src/test/ui/impl-trait/impl-fn-predefined-lifetimes.stderr

@@ -0,0 +1,24 @@
+error: concrete type differs from previous defining opaque type use
+  --> $DIR/impl-fn-predefined-lifetimes.rs:7:9
+   |
+LL |     |x| x
+   |         ^ expected `impl Debug + '_`, got `&u8`
+   |
+note: previous use here
+  --> $DIR/impl-fn-predefined-lifetimes.rs:7:5
+   |
+LL |     |x| x
+   |     ^^^^^
+
+error[E0720]: cannot resolve opaque type
+  --> $DIR/impl-fn-predefined-lifetimes.rs:4:35
+   |
+LL | fn a<'a>() -> impl Fn(&'a u8) -> (impl Debug + '_) {
+   |                                   ^^^^^^^^^^^^^^^ recursive opaque type
+...
+LL |     |x| x
+   |     ----- returning here with type `[closure@$DIR/impl-fn-predefined-lifetimes.rs:7:5: 7:8]`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0720`.

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

@@ -0,0 +1,26 @@
+// run-pass
+#![feature(impl_trait_in_fn_trait_return)]
+use std::fmt::Debug;
+
+fn f_debug() -> impl Fn() -> impl Debug {
+    || ()
+}
+
+fn ff_debug() -> impl Fn() -> impl Fn() -> impl Debug {
+    || f_debug()
+}
+
+fn multi() -> impl Fn() -> (impl Debug + Send) {
+    || ()
+}
+
+fn main() {
+    // Check that `ff_debug` is `() -> (() -> Debug)` and not `(() -> ()) -> Debug`
+    let debug = ff_debug()()();
+    assert_eq!(format!("{:?}", debug), "()");
+
+    let x = multi()();
+    assert_eq!(format!("{:?}", x), "()");
+    fn assert_send(_: &impl Send) {}
+    assert_send(&x);
+}

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

@@ -1,3 +1,4 @@
+#![feature(impl_trait_in_fn_trait_return)]
 use std::fmt::Debug;
 
 fn fine(x: impl Into<u32>) -> impl Into<u32> { x }
@@ -25,8 +26,7 @@ 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

@@ -1,5 +1,5 @@
 error[E0666]: nested `impl Trait` is not allowed
-  --> $DIR/nested_impl_trait.rs:5:56
+  --> $DIR/nested_impl_trait.rs:6:56
    |
 LL | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                              ----------^^^^^^^^^^-
@@ -8,7 +8,7 @@ LL | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                              outer `impl Trait`
 
 error[E0666]: nested `impl Trait` is not allowed
-  --> $DIR/nested_impl_trait.rs:9:42
+  --> $DIR/nested_impl_trait.rs:10:42
    |
 LL | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
    |                                ----------^^^^^^^^^^-
@@ -17,7 +17,7 @@ LL | fn bad_in_fn_syntax(x: fn() -> impl Into<impl Debug>) {}
    |                                outer `impl Trait`
 
 error[E0666]: nested `impl Trait` is not allowed
-  --> $DIR/nested_impl_trait.rs:13:37
+  --> $DIR/nested_impl_trait.rs:14:37
    |
 LL | fn bad_in_arg_position(_: impl Into<impl Debug>) { }
    |                           ----------^^^^^^^^^^-
@@ -26,7 +26,7 @@ LL | fn bad_in_arg_position(_: impl Into<impl Debug>) { }
    |                           outer `impl Trait`
 
 error[E0666]: nested `impl Trait` is not allowed
-  --> $DIR/nested_impl_trait.rs:18:44
+  --> $DIR/nested_impl_trait.rs:19:44
    |
 LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                  ----------^^^^^^^^^^-
@@ -35,19 +35,13 @@ LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                  outer `impl Trait`
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `fn` pointer return
-  --> $DIR/nested_impl_trait.rs:9:32
+  --> $DIR/nested_impl_trait.rs:10:32
    |
 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
+  --> $DIR/nested_impl_trait.rs:6:46
    |
 LL | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                              ^^^^^^^^^^^^^^^^^^^^^ the trait `From<impl Into<u32>>` is not implemented for `impl Debug`
@@ -56,15 +50,15 @@ LL | fn bad_in_ret_position(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    = note: required for `impl Into<u32>` to implement `Into<impl Debug>`
 
 error[E0277]: the trait bound `impl Debug: From<impl Into<u32>>` is not satisfied
-  --> $DIR/nested_impl_trait.rs:18:34
+  --> $DIR/nested_impl_trait.rs:19:34
    |
 LL |     fn bad(x: impl Into<u32>) -> impl Into<impl Debug> { x }
    |                                  ^^^^^^^^^^^^^^^^^^^^^ the trait `From<impl Into<u32>>` is not implemented for `impl Debug`
    |
    = 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`.