unused_must_use: Add extra test for types that are still generic

Author: niacdoial

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

@@ -91,3 +91,11 @@ fn main() {
     controlflow_infallible_unit();
     controlflow_never();
 }
+
+trait AssocTypeBeforeMonomorphisation {
+    type Error;
+    fn generate(&self) -> Result<(), Self::Error>;
+    fn process(&self) {
+        self.generate(); //~ ERROR: unused `Result` that must be used
+    }
+}

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

@@ -62,5 +62,17 @@ help: use `let _ = ...` to ignore the resulting value
 LL |     let _ = controlflow_unit();
    |     +++++++
 
-error: aborting due to 5 previous errors
+error: unused `Result` that must be used
+  --> $DIR/must_use-result-unit-uninhabited.rs:99:9
+   |
+LL |         self.generate();
+   |         ^^^^^^^^^^^^^^^
+   |
+   = note: this `Result` may be an `Err` variant, which should be handled
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |         let _ = self.generate();
+   |         +++++++
+
+error: aborting due to 6 previous errors