unstable book section for fn_must_use
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src/doc/unstable-book/src/language-features/fn-must-use.md
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src/doc/unstable-book/src/language-features/fn-must-use.md
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# `fn_must_use`
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The tracking issue for this feature is [#43302].
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[#43302]: https://github.com/rust-lang/rust/issues/43302
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------------------------
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The `fn_must_use` feature allows functions and methods to be annotated with
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`#[must_use]`, indicating that the `unused_must_use` lint should require their
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return values to be used (similarly to how types annotated with `must_use`,
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most notably `Result`, are linted if not used).
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## Examples
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```rust
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#![feature(fn_must_use)]
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#[must_use]
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fn double(x: i32) -> i32 {
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2 * x
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}
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fn main() {
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double(4); // warning: unused return value of `double` which must be used
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let _ = double(4); // (no warning)
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}
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```
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