[rustc.git] / src / librustc_error_codes / error_codes / E0723.md

An unstable feature in `const` contexts was used.

Erroneous code example:

```compile_fail,E0723
trait T {}

impl T for () {}

const fn foo() -> impl T { // error: `impl Trait` in const fn is unstable
    ()
}
```

To enable this feature on a nightly version of rustc, add the `const_fn`
feature flag:

```
#![feature(const_fn)]

trait T {}

impl T for () {}

const fn foo() -> impl T {
    ()
}
```
Commit	Line	Data
3dfed10e	1	An unstable feature in `const` contexts was used.
60c5eb7d XL	2
	3	Erroneous code example:
	4
	5	```compile_fail,E0723
	6	trait T {}
	7
	8	impl T for () {}
	9
	10	const fn foo() -> impl T { // error: `impl Trait` in const fn is unstable
	11	()
	12	}
	13	```
	14
	15	To enable this feature on a nightly version of rustc, add the `const_fn`
	16	feature flag:
	17
	18	```
	19	#![feature(const_fn)]
	20
	21	trait T {}
	22
	23	impl T for () {}
	24
	25	const fn foo() -> impl T {
	26	()
	27	}
	28	```