Tracking issue for `impl Trait` in `const` and `static` items and `let` bindings #63065

Centril · 2019-07-28T06:36:47Z

As provided for by rust-lang/rfcs#2071, this is a tracking issue for impl Trait in:

const items
static items
let bindings

Steps:

Implement the RFC
- In let: (Re-)Implement impl_trait_in_bindings #134185
Adjust documentation (see instructions on rustc-guide)
Stabilization PR (see instructions on rustc-guide)

Unresolved questions:

Should the part about let bindings be recast more generally to be about coercions? This would allow foo as impl Debug and foo: impl Debug as well.

Blocking issues:

Incomplete. Search the F-impl_trait_in_bindings issue.

The text was updated successfully, but these errors were encountered:

@varkor

…varkor Update `impl Trait` gate issues cc rust-lang#63065 cc rust-lang#63063 r? @varkor cc @alexreg

The existing issue rust-lang#34511 has been closed and replaced by meta-issue rust-lang#63066. The concrete part of the new meta-issue for this feature should be rust-lang#63065.

=== stdout === === stderr === warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes --> <anon>:1:12 | 1 | #![feature(impl_trait_in_bindings)] | ^^^^^^^^^^^^^^^^^^^^^^ | = note: `#[warn(incomplete_features)]` on by default = note: see issue #63065 <rust-lang/rust#63065> for more information warning: function is never used: `foo` --> <anon>:16:8 | 16 | fn foo<T: Trait<Assoc=u32>>() { | ^^^ | = note: `#[warn(dead_code)]` on by default warning: 2 warnings emitted ==============

=== stdout === === stderr === /home/runner/work/glacier/glacier/ices/75962.sh: line 3: impl: command not found warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes --> <anon>:3:12 | 3 | #![feature(impl_trait_in_bindings)] | ^^^^^^^^^^^^^^^^^^^^^^ | = note: `#[warn(incomplete_features)]` on by default = note: see issue #63065 <rust-lang/rust#63065> for more information warning: function is never used: `foo` --> <anon>:29:8 | 29 | fn foo<T: Trait<Assoc=u32>>() { | ^^^ | = note: `#[warn(dead_code)]` on by default warning: 2 warnings emitted ==============

=== stdout === === stderr === warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes --> /home/runner/work/glacier/glacier/ices/78721.rs:1:12 | 1 | #![feature(impl_trait_in_bindings)] | ^^^^^^^^^^^^^^^^^^^^^^ | = note: `#[warn(incomplete_features)]` on by default = note: see issue #63065 <rust-lang/rust#63065> for more information error: `async` blocks are not allowed in constants --> /home/runner/work/glacier/glacier/ices/78721.rs:5:57 | 5 | let f: impl core::future::Future<Output = u8> = async { 1 }; | ^^^^^^^^^^^ error[E0493]: destructors cannot be evaluated at compile-time --> /home/runner/work/glacier/glacier/ices/78721.rs:5:13 | 5 | let f: impl core::future::Future<Output = u8> = async { 1 }; | ^ constants cannot evaluate destructors 6 | 1 7 | }], | - value is dropped here error: aborting due to 2 previous errors; 1 warning emitted For more information about this error, try `rustc --explain E0493`. ==============

=== stdout === === stderr === warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes --> /home/runner/work/glacier/glacier/ices/78722.rs:2:12 | 2 | #![feature(impl_trait_in_bindings)] | ^^^^^^^^^^^^^^^^^^^^^^ | = note: `#[warn(incomplete_features)]` on by default = note: see issue #63065 <rust-lang/rust#63065> for more information error: `async` blocks are not allowed in constants --> /home/runner/work/glacier/glacier/ices/78722.rs:11:20 | 11 | let f: F = async { 1 }; | ^^^^^^^^^^^ error[E0493]: destructors cannot be evaluated at compile-time --> /home/runner/work/glacier/glacier/ices/78722.rs:11:13 | 11 | let f: F = async { 1 }; | ^ constants cannot evaluate destructors 12 | 1 13 | }], | - value is dropped here error: aborting due to 2 previous errors; 1 warning emitted For more information about this error, try `rustc --explain E0493`. ==============

Rua · 2021-01-12T09:48:16Z

What's the status on this now? Is it ready to be stabilised?

=== stdout === === stderr === error[E0658]: `impl Trait` in type aliases is unstable --> /home/runner/work/glacier/glacier/ices/53092.rs:21:18 | 21 | type Bug<T, U> = impl Fn(T) -> U + Copy; | ^^^^^^^^^^^^^^^^^^^^^^ | = note: see issue #63063 <rust-lang/rust#63063> for more information = help: add `#![feature(min_type_alias_impl_trait)]` to the crate attributes to enable warning: the feature `type_alias_impl_trait` is incomplete and may not be safe to use and/or cause compiler crashes --> /home/runner/work/glacier/glacier/ices/53092.rs:3:12 | 3 | #![feature(type_alias_impl_trait)] | ^^^^^^^^^^^^^^^^^^^^^ | = note: `#[warn(incomplete_features)]` on by default = note: see issue #63063 <rust-lang/rust#63063> for more information error[E0658]: type alias impl trait is not permitted here --> /home/runner/work/glacier/glacier/ices/53092.rs:23:32 | 23 | const CONST_BUG: Bug<u8, ()> = transmute(|_: u8| ()); | ^^^^^^^^^^^^^^^^^^^^^ | = note: see issue #63065 <rust-lang/rust#63065> for more information = help: add `#![feature(impl_trait_in_bindings)]` to the crate attributes to enable error: concrete type differs from previous defining opaque type use --> /home/runner/work/glacier/glacier/ices/53092.rs:25:1 | 25 | fn make_bug<T, U: From<T>>() -> Bug<T, U> { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `[type error]`, got `[closure@/home/runner/work/glacier/glacier/ices/53092.rs:26:5: 26:17]` | note: previous use here --> /home/runner/work/glacier/glacier/ices/53092.rs:23:1 | 23 | const CONST_BUG: Bug<u8, ()> = transmute(|_: u8| ()); | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ error: aborting due to 3 previous errors; 1 warning emitted For more information about this error, try `rustc --explain E0658`. ==============

=== stdout === === stderr === error[E0557]: feature has been removed --> /home/runner/work/glacier/glacier/ices/53092.rs:1:12 | 1 | #![feature(const_fn)] | ^^^^^^^^ feature has been removed | = note: split into finer-grained feature gates warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes --> /home/runner/work/glacier/glacier/ices/53092.rs:3:12 | 3 | #![feature(impl_trait_in_bindings)] | ^^^^^^^^^^^^^^^^^^^^^^ | = note: `#[warn(incomplete_features)]` on by default = note: see issue #63065 <rust-lang/rust#63065> for more information warning: the feature `type_alias_impl_trait` is incomplete and may not be safe to use and/or cause compiler crashes --> /home/runner/work/glacier/glacier/ices/53092.rs:5:12 | 5 | #![feature(type_alias_impl_trait)] | ^^^^^^^^^^^^^^^^^^^^^ | = note: see issue #63063 <rust-lang/rust#63063> for more information error: aborting due to previous error; 2 warnings emitted For more information about this error, try `rustc --explain E0557`. ==============

=== stdout === === stderr === warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes --> /home/runner/work/glacier/glacier/ices/85113.rs:2:12 | 2 | #![feature(impl_trait_in_bindings)] | ^^^^^^^^^^^^^^^^^^^^^^ | = note: `#[warn(incomplete_features)]` on by default = note: see issue #63065 <rust-lang/rust#63065> for more information error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds --> /home/runner/work/glacier/glacier/ices/85113.rs:4:29 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^^^^^^^^^^^^^^^^^^^ | note: hidden type `&'<empty> str` captures lifetime smaller than the function body --> /home/runner/work/glacier/glacier/ices/85113.rs:4:29 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^^^^^^^^^^^^^^^^^^^ error[E0477]: the type `&'<empty> str` does not fulfill the required lifetime --> /home/runner/work/glacier/glacier/ices/85113.rs:4:29 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^^^^^^^^^^^^^^^^^^^ | note: type must outlive the lifetime `'a` as defined on the item at 4:23 --> /home/runner/work/glacier/glacier/ices/85113.rs:4:23 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^ error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements --> /home/runner/work/glacier/glacier/ices/85113.rs:4:29 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^^^^^^^^^^^^^^^^^^^ | = note: first, the lifetime cannot outlive the empty lifetime... note: ...but the lifetime must also be valid for the lifetime `'a` as defined on the item at 4:23... --> /home/runner/work/glacier/glacier/ices/85113.rs:4:23 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^ note: ...so that the types are compatible --> /home/runner/work/glacier/glacier/ices/85113.rs:4:29 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^^^^^^^^^^^^^^^^^^^ = note: expected `Output<'a>` found `Output<'_>` error: aborting due to 3 previous errors; 1 warning emitted Some errors have detailed explanations: E0477, E0495, E0700. For more information about an error, try `rustc --explain E0477`. ==============

=== stdout === === stderr === warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes --> /home/runner/work/glacier/glacier/ices/85113.rs:2:12 | 2 | #![feature(impl_trait_in_bindings)] | ^^^^^^^^^^^^^^^^^^^^^^ | = note: `#[warn(incomplete_features)]` on by default = note: see issue #63065 <rust-lang/rust#63065> for more information error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds --> /home/runner/work/glacier/glacier/ices/85113.rs:4:29 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^^^^^^^^^^^^^^^^^^^ | note: hidden type `&'<empty> str` captures lifetime smaller than the function body --> /home/runner/work/glacier/glacier/ices/85113.rs:4:29 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^^^^^^^^^^^^^^^^^^^ error[E0477]: the type `&'<empty> str` does not fulfill the required lifetime --> /home/runner/work/glacier/glacier/ices/85113.rs:4:29 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^^^^^^^^^^^^^^^^^^^ | note: type must outlive the lifetime `'a` as defined on the item at 4:23 --> /home/runner/work/glacier/glacier/ices/85113.rs:4:23 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^ error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements --> /home/runner/work/glacier/glacier/ices/85113.rs:4:29 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^^^^^^^^^^^^^^^^^^^ | = note: first, the lifetime cannot outlive the empty lifetime... note: ...but the lifetime must also be valid for the lifetime `'a` as defined on the item at 4:23... --> /home/runner/work/glacier/glacier/ices/85113.rs:4:23 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^ note: ...so that the types are compatible --> /home/runner/work/glacier/glacier/ices/85113.rs:4:29 | 4 | type OpaqueOutputImpl<'a> = impl Output<'a> + 'a; | ^^^^^^^^^^^^^^^^^^^^ = note: expected `Output<'a>` found `Output<'_>` error: aborting due to 3 previous errors; 1 warning emitted Some errors have detailed explanations: E0477, E0495, E0700. For more information about an error, try `rustc --explain E0477`. ============== Co-authored-by: rustbot <rustbot@users.noreply.github.com>

QuineDot · 2021-11-19T05:25:03Z

The implementation was removed as part of #86729 and the corresponding checkbox should be unchecked.

NobodyXu · 2022-10-23T00:38:16Z

Pinging, is there any progress on this feature?

ctrlcctrlv · 2023-03-28T05:35:36Z

Hopefully there will be soon, as it was the cleanest way for me to solve an issue just now.

…gs, r=<try> (Re-)Implement `impl_trait_in_bindings` This reimplements the `impl_trait_in_bindings` feature for local bindings. "`impl Trait` in bindings" serve as a form of *trait* ascription, where the type basically functions as an infer var but additionally registering the `impl Trait`'s trait bounds for the infer type. These trait bounds can be used to enforce that predicates hold, and can guide inference (e.g. for closure signature inference): ```rust let _: impl Fn(&u8) -> &u8 = |x| x; ``` They are implemented as an additional set of bounds that are registered when the type is lowered during typeck, and then these bounds are tied to a given `CanonicalUserTypeAscription` for borrowck. We enforce these `CanonicalUserTypeAscription` bounds during borrowck to make sure that the `impl Trait` types are sensitive to lifetimes: ```rust trait Static: 'static {} impl<T> Static for T where T: 'static {} let local = 1; let x: impl Static = &local; //~^ ERROR `local` does not live long enough ``` r? oli-obk cc rust-lang#63065 --- Why can't we just use TAIT inference or something? Well, TAITs in bodies have the problem that they cannot reference lifetimes local to a body. For example: ```rust type TAIT = impl Display; let local = 0; let x: TAIT = &local; //~^ ERROR `local` does not live long enough ``` That's because TAITs requires us to do *opaque type inference* which is pretty strict, since we need to remap all of the lifetimes of the hidden type to universal regions. This is simply not possible here. --- I consider this part of the "impl trait everywhere" experiment. I'm not certain if this needs yet another lang team experiment.

…gs, r=oli-obk (Re-)Implement `impl_trait_in_bindings` This reimplements the `impl_trait_in_bindings` feature for local bindings. "`impl Trait` in bindings" serve as a form of *trait* ascription, where the type basically functions as an infer var but additionally registering the `impl Trait`'s trait bounds for the infer type. These trait bounds can be used to enforce that predicates hold, and can guide inference (e.g. for closure signature inference): ```rust let _: impl Fn(&u8) -> &u8 = |x| x; ``` They are implemented as an additional set of bounds that are registered when the type is lowered during typeck, and then these bounds are tied to a given `CanonicalUserTypeAscription` for borrowck. We enforce these `CanonicalUserTypeAscription` bounds during borrowck to make sure that the `impl Trait` types are sensitive to lifetimes: ```rust trait Static: 'static {} impl<T> Static for T where T: 'static {} let local = 1; let x: impl Static = &local; //~^ ERROR `local` does not live long enough ``` r? oli-obk cc rust-lang#63065 --- Why can't we just use TAIT inference or something? Well, TAITs in bodies have the problem that they cannot reference lifetimes local to a body. For example: ```rust type TAIT = impl Display; let local = 0; let x: TAIT = &local; //~^ ERROR `local` does not live long enough ``` That's because TAITs requires us to do *opaque type inference* which is pretty strict, since we need to remap all of the lifetimes of the hidden type to universal regions. This is simply not possible here. --- I consider this part of the "impl trait everywhere" experiment. I'm not certain if this needs yet another lang team experiment.

This was referenced Jul 28, 2019

Meta tracking issue for impl Trait #63066

Open

Tracking issue for impl trait in const/static/let #44686

Closed

Centril added F-impl_trait_in_bindings `#![feature(impl_trait_in_bindings)]` requires-nightly This issue requires a nightly compiler in some way. and removed requires-nightly This issue requires a nightly compiler in some way. labels Jul 28, 2019

Centril mentioned this issue Jul 28, 2019

Update impl Trait gate issues #63092

Merged

Centril added a commit to Centril/rust that referenced this issue Jul 29, 2019

Rollup merge of rust-lang#63092 - Centril:update-impl-trait-gates, r=…

f8321d0

…varkor Update `impl Trait` gate issues cc rust-lang#63065 cc rust-lang#63063 r? @varkor cc @alexreg

runiq mentioned this issue Sep 13, 2019

Things blocking or slowing progress of Ruma ruma/homeserver#189

Open

Centril mentioned this issue Oct 18, 2019

standard lazy types rust-lang/rfcs#2788

Merged

Ralith mentioned this issue Nov 25, 2019

Expose derived lenses as associated constants linebender/druid#352

Merged

nrxus mentioned this issue Jan 7, 2020

impl SomeType as a parameter on a mocked method does not compile nrxus/faux#15

Closed

Boscop mentioned this issue Feb 26, 2020

impl_trait_in_bindings doesn't interact well with associated types yet #69476

Open

strohel mentioned this issue Mar 2, 2020

Update issue reference for impl_trait_in_bindings feature #69648

Closed

This was referenced Apr 10, 2020

ICE: broken MIR with nested opaque types. #70971

Closed

Bug. Get doubled error message with impl_trait_in_bindings when no error must be #70997

Closed

ecstatic-morse mentioned this issue Oct 7, 2020

Tracking Issue for #![feature(const_impl_trait)] #77463

Closed

longtomjr mentioned this issue May 19, 2021

feat: trusted key-value store using sgxfs ntls-io/rtc-data#61

Merged

NobodyXu mentioned this issue Apr 4, 2022

Support specializing on trait sagebind/castaway#3

Open

joshtriplett added the S-tracking-impl-incomplete Status: The implementation is incomplete. label Jun 15, 2022

alice-i-cecile mentioned this issue Jul 22, 2022

Make library more usable in a const context DioxusLabs/taffy#203

Merged

NobodyXu mentioned this issue Oct 23, 2022

How do I initialize a Lazy with a closure that captures a variable? (With fun recursion question) matklad/once_cell#156

Closed

slanterns mentioned this issue Dec 16, 2022

Tracking issue for RFC 2515, "Permit impl Trait in type aliases" #63063

Open

8 tasks

mrnossiom mentioned this issue Mar 18, 2023

instrument macro sometimes triggers clippy::let_with_type_underscore tokio-rs/tracing#2503

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NobodyXu mentioned this issue Mar 31, 2023

Tracking Issue for lazy_cell #109736

Closed

5 tasks

coder137 mentioned this issue Apr 5, 2023

Rust Trait enhancements coder137/lowlevel_rust#9

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compiler-errors mentioned this issue Dec 11, 2024

(Re-)Implement impl_trait_in_bindings #134185

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Tracking issue for `impl Trait` in `const` and `static` items and `let` bindings #63065

Tracking issue for `impl Trait` in `const` and `static` items and `let` bindings #63065

Centril commented Jul 28, 2019 •

edited by Dylan-DPC

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Rua commented Jan 12, 2021

QuineDot commented Nov 19, 2021

NobodyXu commented Oct 23, 2022

ctrlcctrlv commented Mar 28, 2023

Tracking issue for impl Trait in const and static items and let bindings #63065

Tracking issue for impl Trait in const and static items and let bindings #63065

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Centril commented Jul 28, 2019 • edited by Dylan-DPC Loading

Rua commented Jan 12, 2021

QuineDot commented Nov 19, 2021

NobodyXu commented Oct 23, 2022

ctrlcctrlv commented Mar 28, 2023

Tracking issue for `impl Trait` in `const` and `static` items and `let` bindings #63065

Tracking issue for `impl Trait` in `const` and `static` items and `let` bindings #63065

Centril commented Jul 28, 2019 •

edited by Dylan-DPC

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