unused_must_use: Add test for a method using an associated error type

Author: joshtriplett

tests/ui/lint/unused/must_use-result-unit-uninhabited.rs

@@ -44,6 +44,25 @@ fn f6<AT: AssocType>(_: AT) -> Result<(), AT::Error> {
     Ok(())
 }
 
+trait UsesAssocType {
+    type Error;
+    fn method(&self) -> Result<(), Self::Error>;
+}
+
+impl UsesAssocType for S1 {
+    type Error = !;
+    fn method(&self) -> Result<(), Self::Error> {
+        Ok(())
+    }
+}
+
+impl UsesAssocType for S2 {
+    type Error = ();
+    fn method(&self) -> Result<(), Self::Error> {
+        Err(())
+    }
+}
+
 fn main() {
     f1(); //~ ERROR: unused `Result` that must be used
     f2();
@@ -52,4 +71,6 @@ fn main() {
     f5(); //~ ERROR: unused `Result` that must be used
     f6(S1);
     f6(S2); //~ ERROR: unused `Result` that must be used
+    S1.method();
+    S2.method(); //~ ERROR: unused `Result` that must be used
 }

tests/ui/lint/unused/must_use-result-unit-uninhabited.stderr

@@ -1,5 +1,5 @@
 error: unused `Result` that must be used
-  --> $DIR/must_use-result-unit-uninhabited.rs:48:5
+  --> $DIR/must_use-result-unit-uninhabited.rs:67:5
    |
 LL |     f1();
    |     ^^^^
@@ -16,7 +16,7 @@ LL |     let _ = f1();
    |     +++++++
 
 error: unused `Result` that must be used
-  --> $DIR/must_use-result-unit-uninhabited.rs:52:5
+  --> $DIR/must_use-result-unit-uninhabited.rs:71:5
    |
 LL |     f5();
    |     ^^^^
@@ -28,7 +28,7 @@ LL |     let _ = f5();
    |     +++++++
 
 error: unused `Result` that must be used
-  --> $DIR/must_use-result-unit-uninhabited.rs:54:5
+  --> $DIR/must_use-result-unit-uninhabited.rs:73:5
    |
 LL |     f6(S2);
    |     ^^^^^^
@@ -39,5 +39,17 @@ help: use `let _ = ...` to ignore the resulting value
 LL |     let _ = f6(S2);
    |     +++++++
 
-error: aborting due to 3 previous errors
+error: unused `Result` that must be used
+  --> $DIR/must_use-result-unit-uninhabited.rs:75:5
+   |
+LL |     S2.method();
+   |     ^^^^^^^^^^^
+   |
+   = note: this `Result` may be an `Err` variant, which should be handled
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = S2.method();
+   |     +++++++
+
+error: aborting due to 4 previous errors