# `marker_trait_attr`

The tracking issue for this feature is: [#29864]

[#29864]: https://github.com/rust-lang/rust/issues/29864

------------------------

Normally, Rust keeps you from adding trait implementations that could
overlap with each other, as it would be ambiguous which to use.  This
feature, however, carves out an exception to that rule: a trait can
opt-in to having overlapping implementations, at the cost that those
implementations are not allowed to override anything (and thus the
trait itself cannot have any associated items, as they're pointless
when they'd need to do the same thing for every type anyway).

```rust
#![feature(marker_trait_attr)]

use std::fmt::{Debug, Display};

#[marker] trait MyMarker {}

impl<T: Debug> MyMarker for T {}
impl<T: Display> MyMarker for T {}

fn foo<T: MyMarker>(t: T) -> T {
    t
}
```

This is expected to replace the unstable `overlapping_marker_traits`
feature, which applied to all empty traits (without needing an opt-in).

Add marker_trait_attr to the unstable book
Add marker_trait_attr to the unstable book
