Allow `impl Fn() -> impl Trait` in return position #93582

WaffleLapkin · 2022-02-02T12:22:13Z

This was originally proposed as part of #93082 which was closed due to allowing impl Fn() -> impl Trait in argument position.

This allows writing the following function signatures:

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.

There even is a test that f0 compiles:

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

Lines 25 to 28 in 2f004d2

    
           fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> { 
        
           //~^ `impl Trait` not allowed 
        
               || 5 
        
           }

But it was changed in PR 48084 (lines) to test the opposite, probably unintentionally given PR 48084 (lines).

r? @nikomatsakis

This limitation is especially annoying with async code, since it forces one to write this:

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:

fn async_closure() -> impl Fn(i32, i32, i32) -> impl Future<Output = u32> {
    |a, b, c| async move { (a + b + c) as u32 }
}

bors · 2022-02-08T03:25:13Z

☔ The latest upstream changes (presumably #92007) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2022-02-11T20:40:55Z

☔ The latest upstream changes (presumably #93893) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2022-02-19T05:42:24Z

☔ The latest upstream changes (presumably #94134) made this pull request unmergeable. Please resolve the merge conflicts.

nikomatsakis

Hi @WaffleLapkin, sorry for the very slow review. Here are some tests that would be good!

Come to think of it, @WaffleLapkin, here are more tests related to '_: ---

we should show that fn foo() -> impl Fn() -> (impl Debug + '_) is an error

and that impl for<'a> Fn(&'a u8) -> (impl Debug + '_) correctly matches the '_ to 'a

related: impl Fn(&'a u8) -> (impl Debug + '_) correctly matches the '_ to some pre-existing 'a

nikomatsakis · 2022-03-14T21:35:26Z

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

+    || ()
+}
+
+fn ff_debug() -> impl Fn() -> impl Fn() -> impl Debug {


Let's add some tests about + bounds:

// This should error because the returned closure is capturing // its argument, which was not declared as part of the signature. fn f_debug() -> impl Fn(&u8) -> impl Debug { |x| x }

// This should (maybe) error because or parsing ambiguities fn f_debug() -> impl Fn(&u8) -> impl Debug + '_ { |x| x }

// This should (maybe) error because or parsing ambiguities fn f_debug() -> impl Fn() -> impl Debug + Send { || () }

// This should work fn f_debug() -> impl Fn(&u8) -> (impl Debug + '_) { |x| x }

// This should work fn f_debug() -> impl Fn() -> (impl Debug + Send) { || () }

nikomatsakis · 2022-03-14T21:41:34Z

Nominating for @rust-lang/lang meeting. We've discussed a predecessor of this PR before, and this is the follow-up. It extends -> impl Trait so that it works in "RPITIRPIT" position:

fn foo() -> impl Fn() -> impl Debug { }

There are a few interesting questions. On the prior PR, @cramertj pointed out that parsing has some syntactical ambiguities here:

fn foo() -> impl Fn() -> impl Debug + Send { }

// Is this `-> impl Fn() -> (impl Debug) + Send` or `impl Fn() -> (impl Debug + Send)` ?

The same above also implies the '_ bound, which ought to capture lifetimes from the arguments:

fn foo() -> impl Fn(&u8) -> (impl Debug + '_)

nikomatsakis · 2022-03-14T21:44:58Z

@rustbot team

bors · 2022-03-30T08:04:08Z

☔ The latest upstream changes (presumably #94081) made this pull request unmergeable. Please resolve the merge conflicts.

nikomatsakis · 2022-03-31T14:29:22Z

Hmm, we've been slow in getting to this! I won't be at the next triage meeting, but I'll try to ensure we discuss it soon! Apologies @WaffleLapkin

WaffleLapkin · 2022-03-31T15:22:53Z

No worries, this was blocked on #94081 anyway (without it -> impl for<'a> Fn(&'a u8) -> (impl Debug + 'a) crushed the compiler). I'll try adding suggested tests now that #94081 was merged :)

nikomatsakis · 2022-05-03T17:37:05Z

@WaffleLapkin -- this is still blocked on you making some adjustments, right? We are going to remove the nomination in the meantime.

nikomatsakis · 2022-05-03T17:37:12Z

@rustbot author

WaffleLapkin · 2022-05-03T17:40:56Z

@nikomatsakis the problem is not necessarily with this feature by itself, but more with the general HRTB+RPIT issues like #95647 which stop me from adding proposed tests (because they ICE).

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.

WaffleLapkin · 2022-10-25T13:50:30Z

@compiler-errors rebased ;)

compiler-errors · 2022-10-27T20:20:19Z

@bors r+

See my comment #93582 (comment) -- this may be something we want to revisit before stabilizing this, but it shouldn't block landing the feature gate IMO.

bors · 2022-10-27T20:20:21Z

📌 Commit e93982a has been approved by compiler-errors

It is now in the queue for this repository.

…errors 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 } } ```

…errors 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 } } ```

Rollup of 5 pull requests Successful merges: - rust-lang#93582 (Allow `impl Fn() -> impl Trait` in return position) - rust-lang#103560 (Point only to the identifiers in the typo suggestions of shadowed names instead of the entire struct) - rust-lang#103588 (rustdoc: add missing URL redirect) - rust-lang#103689 (Do fewer passes and generally be more efficient when filtering tests) - rust-lang#103740 (rustdoc: remove unnecessary CSS `.search-results { padding-bottom }`) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

WaffleLapkin · 2022-10-31T11:26:41Z

Yay!

bombless · 2022-11-28T01:35:47Z

Mention related tracking issue #34511 #63066

…errors 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 } } ```

Rollup of 5 pull requests Successful merges: - rust-lang#93582 (Allow `impl Fn() -> impl Trait` in return position) - rust-lang#103560 (Point only to the identifiers in the typo suggestions of shadowed names instead of the entire struct) - rust-lang#103588 (rustdoc: add missing URL redirect) - rust-lang#103689 (Do fewer passes and generally be more efficient when filtering tests) - rust-lang#103740 (rustdoc: remove unnecessary CSS `.search-results { padding-bottom }`) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

rust-highfive assigned nikomatsakis Feb 2, 2022

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Feb 2, 2022

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Feb 2, 2022

WaffleLapkin changed the title ~~Allow impl Fn() -> impl Trait in return possition~~ Allow impl Fn() -> impl Trait in return position Feb 2, 2022

WaffleLapkin mentioned this pull request Feb 2, 2022

Allow impl Fn() -> impl Trait #93082

Closed

WaffleLapkin force-pushed the rpitirpit branch from a819fec to 711a422 Compare February 8, 2022 13:52

WaffleLapkin force-pushed the rpitirpit branch from 711a422 to e478a1a Compare February 14, 2022 11:37

WaffleLapkin force-pushed the rpitirpit branch from e478a1a to 64489b2 Compare February 21, 2022 14:24

This comment has been minimized.

Sign in to view

WaffleLapkin force-pushed the rpitirpit branch from 64489b2 to 15e2324 Compare February 21, 2022 15:23

nikomatsakis requested changes Mar 14, 2022

View reviewed changes

nikomatsakis added the I-lang-nominated Nominated for discussion during a lang team meeting. label Mar 14, 2022

oli-obk added S-waiting-on-team Status: Awaiting decision from the relevant subteam (see the T-<team> label). and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 30, 2022

WaffleLapkin mentioned this pull request Apr 4, 2022

yet another ICE with HRTB and RPIT #95647

Closed

joshtriplett removed the I-lang-nominated Nominated for discussion during a lang team meeting. label May 3, 2022

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label May 3, 2022

nikomatsakis removed the S-waiting-on-team Status: Awaiting decision from the relevant subteam (see the T-<team> label). label May 3, 2022

JohnCSimon removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label May 22, 2022

jackh726 removed the I-types-nominated Nominated for discussion during a types team meeting. label Oct 21, 2022

WaffleLapkin added 7 commits October 25, 2022 13:25

Add more tests for impl Fn() -> impl Trait

7a4ba2f

Add even more tests for impl Fn() -> impl Trait

00f2277

Feature gate impl_trait_in_fn_trait_return

cc752f5

--bless

d116859

add a test for gate impl_trait_in_fn_trait_return

690e037

adopt to compiler changes

e93982a

WaffleLapkin force-pushed the rpitirpit branch from 92ebd2a to e93982a Compare October 25, 2022 13:49

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 27, 2022

Dylan-DPC mentioned this pull request Oct 28, 2022

Rollup of 6 pull requests #103685

Closed

Dylan-DPC mentioned this pull request Oct 28, 2022

Rollup of 6 pull requests #103696

Closed

Dylan-DPC mentioned this pull request Oct 30, 2022

Rollup of 5 pull requests #103755

Merged

bors merged commit b4cf523 into rust-lang:master Oct 30, 2022

rustbot added this to the 1.67.0 milestone Oct 30, 2022

WaffleLapkin deleted the rpitirpit branch October 31, 2022 11:26

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Allow `impl Fn() -> impl Trait` in return position #93582

Allow `impl Fn() -> impl Trait` in return position #93582

WaffleLapkin commented Feb 2, 2022

bors commented Feb 8, 2022

bors commented Feb 11, 2022

bors commented Feb 19, 2022

This comment has been minimized.

nikomatsakis left a comment •

edited

Loading

nikomatsakis Mar 14, 2022

nikomatsakis commented Mar 14, 2022

nikomatsakis commented Mar 14, 2022

bors commented Mar 30, 2022

nikomatsakis commented Mar 31, 2022

WaffleLapkin commented Mar 31, 2022

nikomatsakis commented May 3, 2022

nikomatsakis commented May 3, 2022

WaffleLapkin commented May 3, 2022

WaffleLapkin commented Oct 25, 2022 •

edited

Loading

compiler-errors commented Oct 27, 2022

bors commented Oct 27, 2022

WaffleLapkin commented Oct 31, 2022

bombless commented Nov 28, 2022

	fn allowed_in_ret_type() -> impl Fn() -> impl Into<u32> {
	//~^ `impl Trait` not allowed
	\|\| 5
	}

Allow impl Fn() -> impl Trait in return position #93582

Allow impl Fn() -> impl Trait in return position #93582

Conversation

WaffleLapkin commented Feb 2, 2022

bors commented Feb 8, 2022

bors commented Feb 11, 2022

bors commented Feb 19, 2022

This comment has been minimized.

nikomatsakis left a comment • edited Loading

Choose a reason for hiding this comment

nikomatsakis Mar 14, 2022

Choose a reason for hiding this comment

nikomatsakis commented Mar 14, 2022

nikomatsakis commented Mar 14, 2022

bors commented Mar 30, 2022

nikomatsakis commented Mar 31, 2022

WaffleLapkin commented Mar 31, 2022

nikomatsakis commented May 3, 2022

nikomatsakis commented May 3, 2022

WaffleLapkin commented May 3, 2022

WaffleLapkin commented Oct 25, 2022 • edited Loading

compiler-errors commented Oct 27, 2022

bors commented Oct 27, 2022

WaffleLapkin commented Oct 31, 2022

bombless commented Nov 28, 2022

Allow `impl Fn() -> impl Trait` in return position #93582

Allow `impl Fn() -> impl Trait` in return position #93582

nikomatsakis left a comment •

edited

Loading

WaffleLapkin commented Oct 25, 2022 •

edited

Loading