src/doc/unstable-book/src/language-features/marker-trait-attr.md

   1 # `marker_trait_attr`
   2
   3 The tracking issue for this feature is: [#29864]
   4
   5 [#29864]: https://github.com/rust-lang/rust/issues/29864
   6
   7 ------------------------
   8
   9 Normally, Rust keeps you from adding trait implementations that could
  10 overlap with each other, as it would be ambiguous which to use.  This
  11 feature, however, carves out an exception to that rule: a trait can
  12 opt-in to having overlapping implementations, at the cost that those
  13 implementations are not allowed to override anything (and thus the
  14 trait itself cannot have any associated items, as they're pointless
  15 when they'd need to do the same thing for every type anyway).
  16
  17 ```rust
  18 #![feature(marker_trait_attr)]
  19
  20 #[marker] trait CheapToClone: Clone {}
  21
  22 impl<T: Copy> CheapToClone for T {}
  23
  24 // These could potentially overlap with the blanket implementation above,
  25 // so are only allowed because CheapToClone is a marker trait.
  26 impl<T: CheapToClone, U: CheapToClone> CheapToClone for (T, U) {}
  27 impl<T: CheapToClone> CheapToClone for std::ops::Range<T> {}
  28
  29 fn cheap_clone<T: CheapToClone>(t: T) -> T {
  30     t.clone()
  31 }
  32 ```
  33
  34 This is expected to replace the unstable `overlapping_marker_traits`
  35 feature, which applied to all empty traits (without needing an opt-in).