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Rust unstable book: basic desc and example for const_fn.
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frewsxcv committed Mar 14, 2017
1 parent fd182c4 commit e06c515
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19 changes: 19 additions & 0 deletions src/doc/unstable-book/src/const-fn.md
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Expand Up @@ -6,5 +6,24 @@ The tracking issue for this feature is: [#24111]

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

The `const_fn` feature allows marking free functions and inherent methods as
`const`, enabling them to be called in constants contexts, with constant
arguments.

## Examples

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

const fn double(x: i32) -> i32 {
x * 2
}

const FIVE: i32 = 5;
const TEN: i32 = double(FIVE);

fn main() {
assert_eq!(5, FIVE);
assert_eq!(10, TEN);
}
```

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