src/doc/unstable-book/src/language-features/fn-must-use.md

   1 # `fn_must_use`
   2
   3 The tracking issue for this feature is [#43302].
   4
   5 [#43302]: https://github.com/rust-lang/rust/issues/43302
   6
   7 ------------------------
   8
   9 The `fn_must_use` feature allows functions and methods to be annotated with
  10 `#[must_use]`, indicating that the `unused_must_use` lint should require their
  11 return values to be used (similarly to how types annotated with `must_use`,
  12 most notably `Result`, are linted if not used).
  13
  14 ## Examples
  15
  16 ```rust
  17 #![feature(fn_must_use)]
  18
  19 #[must_use]
  20 fn double(x: i32) -> i32 {
  21     2 * x
  22 }
  23
  24 fn main() {
  25     double(4); // warning: unused return value of `double` which must be used
  26
  27     let _ = double(4); // (no warning)
  28 }
  29
  30 ```