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

_This was originally proposed as part of rust-lang#93082 which was [closed](rust-lang#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 }
}
```
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Dylan-DPC authored Oct 28, 2022
2 parents a9ef100 + e93982a commit 38abbcf
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Showing 18 changed files with 295 additions and 76 deletions.
16 changes: 15 additions & 1 deletion compiler/rustc_ast_lowering/src/path.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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() {
Expand Down Expand Up @@ -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
Expand All @@ -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))
}
Expand Down
2 changes: 2 additions & 0 deletions compiler/rustc_feature/src/active.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_span/src/symbol.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand Down
Original file line number Diff line number Diff line change
@@ -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() {}
Original file line number Diff line number Diff line change
@@ -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`.
8 changes: 8 additions & 0 deletions src/test/ui/impl-trait/impl-fn-hrtb-bounds-2.rs
Original file line number Diff line number Diff line change
@@ -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() {}
11 changes: 11 additions & 0 deletions src/test/ui/impl-trait/impl-fn-hrtb-bounds-2.stderr
Original file line number Diff line number Diff line change
@@ -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`.
24 changes: 24 additions & 0 deletions src/test/ui/impl-trait/impl-fn-hrtb-bounds.rs
Original file line number Diff line number Diff line change
@@ -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() {}
51 changes: 51 additions & 0 deletions src/test/ui/impl-trait/impl-fn-hrtb-bounds.stderr
Original file line number Diff line number Diff line change
@@ -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`.
15 changes: 15 additions & 0 deletions src/test/ui/impl-trait/impl-fn-parsing-ambiguities.rs
Original file line number Diff line number Diff line change
@@ -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() {}
26 changes: 26 additions & 0 deletions src/test/ui/impl-trait/impl-fn-parsing-ambiguities.stderr
Original file line number Diff line number Diff line change
@@ -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

15 changes: 15 additions & 0 deletions src/test/ui/impl-trait/impl-fn-predefined-lifetimes.rs
Original file line number Diff line number Diff line change
@@ -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() {}
24 changes: 24 additions & 0 deletions src/test/ui/impl-trait/impl-fn-predefined-lifetimes.stderr
Original file line number Diff line number Diff line change
@@ -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`.
26 changes: 26 additions & 0 deletions src/test/ui/impl-trait/impl_fn_associativity.rs
Original file line number Diff line number Diff line change
@@ -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);
}
4 changes: 2 additions & 2 deletions src/test/ui/impl-trait/nested_impl_trait.rs
Original file line number Diff line number Diff line change
@@ -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 }
Expand Down Expand Up @@ -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() {}
22 changes: 8 additions & 14 deletions src/test/ui/impl-trait/nested_impl_trait.stderr
Original file line number Diff line number Diff line change
@@ -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 }
| ----------^^^^^^^^^^-
Expand All @@ -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>) {}
| ----------^^^^^^^^^^-
Expand All @@ -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>) { }
| ----------^^^^^^^^^^-
Expand All @@ -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 }
| ----------^^^^^^^^^^-
Expand All @@ -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`
Expand All @@ -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`.
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