[rustc.git] / compiler / rustc_error_codes / src / error_codes / E0785.md

An inherent `impl` was written on a dyn auto trait.

Erroneous code example:

```compile_fail,E0785
#![feature(auto_traits)]

auto trait AutoTrait {}

impl dyn AutoTrait {}
```

Dyn objects allow any number of auto traits, plus at most one non-auto trait.
The non-auto trait becomes the "principal trait".

When checking if an impl on a dyn trait is coherent, the principal trait is
normally the only one considered. Since the erroneous code has no principal
trait, it cannot be implemented at all.

Working example:

```
#![feature(auto_traits)]

trait PrincipalTrait {}

auto trait AutoTrait {}

impl dyn PrincipalTrait + AutoTrait + Send {}
```
Commit	Line	Data
94222f64 XL	1	An inherent `impl` was written on a dyn auto trait.
	2
	3	Erroneous code example:
	4
	5	```compile_fail,E0785
	6	#![feature(auto_traits)]
	7
	8	auto trait AutoTrait {}
	9
	10	impl dyn AutoTrait {}
	11	```
	12
	13	Dyn objects allow any number of auto traits, plus at most one non-auto trait.
	14	The non-auto trait becomes the "principal trait".
	15
	16	When checking if an impl on a dyn trait is coherent, the principal trait is
	17	normally the only one considered. Since the erroneous code has no principal
	18	trait, it cannot be implemented at all.
	19
	20	Working example:
	21
	22	```
	23	#![feature(auto_traits)]
	24
	25	trait PrincipalTrait {}
	26
	27	auto trait AutoTrait {}
	28
	29	impl dyn PrincipalTrait + AutoTrait + Send {}
	30	```