1 files changed, 76 insertions, 0 deletions
diff --git a/tests/ui/lint/fn_must_use.rs b/tests/ui/lint/fn_must_use.rs
new file mode 100644
index 00000000000..b4e9da0fc84
--- /dev/null
+++ b/tests/ui/lint/fn_must_use.rs
@@ -0,0 +1,76 @@
+// 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
+}