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// check-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
}
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