| // Copyright 2017 The Rust Project Developers. See the COPYRIGHT |
| // file at the top-level directory of this distribution and at |
| // http://rust-lang.org/COPYRIGHT. |
| // |
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your |
| // option. This file may not be copied, modified, or distributed |
| // except according to those terms. |
| |
| // compile-pass |
| |
| #![warn(unused_must_use)] |
| |
| #[derive(PartialEq, Eq)] |
| struct MyStruct { |
| n: usize, |
| } |
| |
| impl MyStruct { |
| #[must_use] |
| fn need_to_use_this_method_value(&self) -> usize { |
| self.n |
| } |
| |
| #[must_use] |
| fn need_to_use_this_associated_function_value() -> isize { |
| -1 |
| } |
| } |
| |
| trait EvenNature { |
| #[must_use = "no side effects"] |
| fn is_even(&self) -> bool; |
| } |
| |
| impl EvenNature for MyStruct { |
| fn is_even(&self) -> bool { |
| self.n % 2 == 0 |
| } |
| } |
| |
| trait Replaceable { |
| fn replace(&mut self, substitute: usize) -> usize; |
| } |
| |
| impl Replaceable for MyStruct { |
| // ↓ N.b.: `#[must_use]` attribute on a particular trait implementation |
| // method won't work; the attribute should be on the method signature in |
| // the trait's definition. |
| #[must_use] |
| fn replace(&mut self, substitute: usize) -> usize { |
| let previously = self.n; |
| self.n = substitute; |
| previously |
| } |
| } |
| |
| #[must_use = "it's important"] |
| fn need_to_use_this_value() -> bool { |
| false |
| } |
| |
| fn main() { |
| need_to_use_this_value(); //~ WARN unused return value |
| |
| let mut m = MyStruct { n: 2 }; |
| let n = MyStruct { n: 3 }; |
| |
| m.need_to_use_this_method_value(); //~ WARN unused return value |
| m.is_even(); // trait method! |
| //~^ WARN unused return value |
| |
| MyStruct::need_to_use_this_associated_function_value(); |
| //~^ WARN unused return value |
| |
| m.replace(3); // won't warn (annotation needs to be in trait definition) |
| |
| // comparison methods are `must_use` |
| 2.eq(&3); //~ WARN unused return value |
| m.eq(&n); //~ WARN unused return value |
| |
| // lint includes comparison operators |
| 2 == 3; //~ WARN unused comparison |
| m == n; //~ WARN unused comparison |
| } |