tests/ui/lint/fn_must_use.rs

   1 // check-pass
   2
   3 #![warn(unused_must_use)]
   4
   5 #[derive(PartialEq, Eq)]
   6 struct MyStruct {
   7     n: usize,
   8 }
   9
  10 impl MyStruct {
  11     #[must_use]
  12     fn need_to_use_this_method_value(&self) -> usize {
  13         self.n
  14     }
  15
  16     #[must_use]
  17     fn need_to_use_this_associated_function_value() -> isize {
  18         -1
  19     }
  20 }
  21
  22 trait EvenNature {
  23     #[must_use = "no side effects"]
  24     fn is_even(&self) -> bool;
  25 }
  26
  27 impl EvenNature for MyStruct {
  28     fn is_even(&self) -> bool {
  29         self.n % 2 == 0
  30     }
  31 }
  32
  33 trait Replaceable {
  34     fn replace(&mut self, substitute: usize) -> usize;
  35 }
  36
  37 impl Replaceable for MyStruct {
  38     // ↓ N.b.: `#[must_use]` attribute on a particular trait implementation
  39     // method won't work; the attribute should be on the method signature in
  40     // the trait's definition.
  41     #[must_use]
  42     fn replace(&mut self, substitute: usize) -> usize {
  43         let previously = self.n;
  44         self.n = substitute;
  45         previously
  46     }
  47 }
  48
  49 #[must_use = "it's important"]
  50 fn need_to_use_this_value() -> bool {
  51     false
  52 }
  53
  54 fn main() {
  55     need_to_use_this_value(); //~ WARN unused return value
  56
  57     let mut m = MyStruct { n: 2 };
  58     let n = MyStruct { n: 3 };
  59
  60     m.need_to_use_this_method_value(); //~ WARN unused return value
  61     m.is_even(); // trait method!
  62     //~^ WARN unused return value
  63
  64     MyStruct::need_to_use_this_associated_function_value();
  65     //~^ WARN unused return value
  66
  67     m.replace(3); // won't warn (annotation needs to be in trait definition)
  68
  69     // comparison methods are `must_use`
  70     2.eq(&3); //~ WARN unused return value
  71     m.eq(&n); //~ WARN unused return value
  72
  73     // lint includes comparison operators
  74     2 == 3; //~ WARN unused comparison
  75     m == n; //~ WARN unused comparison
  76 }