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Improve the unstable book example for #[marker] trait #56402

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Dec 3, 2018
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Improve the unstable book example for #[marker] trait #56402

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14 changes: 8 additions & 6 deletions src/doc/unstable-book/src/language-features/marker-trait-attr.md
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Original file line number Diff line number Diff line change
Expand Up @@ -17,15 +17,17 @@ 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 CheapToClone: Clone {}

#[marker] trait MyMarker {}
impl<T: Copy> CheapToClone for T {}

impl<T: Debug> MyMarker for T {}
impl<T: Display> MyMarker for T {}
// These could potentally overlap with the blanket implementation above,
// so are only allowed because CheapToClone is a marker trait.
impl<T: CheapToClone, U: CheapToClone> CheapToClone for (T, U) {}
impl<T: CheapToClone> CheapToClone for std::ops::Range<T> {}

fn foo<T: MyMarker>(t: T) -> T {
t
fn cheap_clone<T: CheapToClone>(t: T) -> T {
t.clone()
}
```

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