Update tests and add known problems to docs

tests/ui/undropped_manually_drops.rs

@@ -3,16 +3,24 @@
 struct S;
 
 fn main() {
-    let f = drop;
-    let manual = std::mem::ManuallyDrop::new(S);
+    let f = std::mem::drop;
+    let g = std::mem::ManuallyDrop::drop;
+    let mut manual1 = std::mem::ManuallyDrop::new(S);
+    let mut manual2 = std::mem::ManuallyDrop::new(S);
+    let mut manual3 = std::mem::ManuallyDrop::new(S);
+    let mut manual4 = std::mem::ManuallyDrop::new(S);
 
-    // These lines will not drop `S`
+    // These lines will not drop `S` and should be linted
     drop(std::mem::ManuallyDrop::new(S));
-    f(manual);
+    drop(manual1);
 
-    // These lines will
+    // FIXME: this line is not linted, though it should be
+    f(manual2);
+
+    // These lines will drop `S` and should be okay.
     unsafe {
-        std::mem::ManuallyDrop::drop(std::mem::ManuallyDrop::new(S));
-        std::mem::ManuallyDrop::drop(manual);
+        std::mem::ManuallyDrop::drop(&mut std::mem::ManuallyDrop::new(S));
+        std::mem::ManuallyDrop::drop(&mut manual3);
+        g(&mut manual4);
     }
 }

tests/ui/undropped_manually_drops.stderr

@@ -0,0 +1,19 @@
+error: the inner value of this ManuallyDrop will not be dropped
+  --> $DIR/undropped_manually_drops.rs:14:5
+   |
+LL |     drop(std::mem::ManuallyDrop::new(S));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::undropped-manually-drops` implied by `-D warnings`
+   = help: to drop a `ManuallyDrop<T>`, use std::mem::ManuallyDrop::drop
+
+error: the inner value of this ManuallyDrop will not be dropped
+  --> $DIR/undropped_manually_drops.rs:15:5
+   |
+LL |     drop(manual1);
+   |     ^^^^^^^^^^^^^
+   |
+   = help: to drop a `ManuallyDrop<T>`, use std::mem::ManuallyDrop::drop
+
+error: aborting due to 2 previous errors
+