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 <[email protected]>

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.

Without #![feature(impl_trait_in_bindings)]: error[E0562]: `impl Trait` is not allowed in paths --> tests/test.rs:1680:42 | 1680 | async fn method(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error> { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = note: `impl Trait` is only allowed in arguments and return types of functions and methods error[E0562]: `impl Trait` is not allowed in the type of variable bindings --> tests/test.rs:1680:42 | 1680 | async fn method(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error> { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = note: `impl Trait` is only allowed in arguments and return types of functions and methods = note: see issue #63065 <rust-lang/rust#63065> for more information = help: add `#![feature(impl_trait_in_bindings)]` to the crate attributes to enable = note: this compiler was built on 2025-01-02; consider upgrading it if it is out of date With #![feature(impl_trait_in_bindings)]: error[E0562]: `impl Trait` is not allowed in paths --> tests/test.rs:1680:42 | 1680 | async fn method(&self) -> Result<impl AsRef<str> + Send + Sync, Self::Error> { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = note: `impl Trait` is only allowed in arguments and return types of functions and methods

JohnScience · 2025-05-29T05:31:53Z

The question I opened on Stack Overflow on this topic: https://stackoverflow.com/questions/79643282/why-errore0121-the-placeholder-is-not-allowed-within-types-on-item-signat.

A workaround can be found there too.

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

Closed

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

9 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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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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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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