Uplift drop-bounds lint from clippy

src/test/ui/drop-bounds/drop-bounds-impl-drop.rs

@@ -0,0 +1,14 @@
+// run-pass
+#![deny(drop_bounds)]
+// As a special exemption, `impl Drop` in the return position raises no error.
+// This allows a convenient way to return an unnamed drop guard.
+fn voldemort_type() -> impl Drop {
+  struct Voldemort;
+  impl Drop for Voldemort {
+    fn drop(&mut self) {}
+  }
+  Voldemort
+}
+fn main() {
+  let _ = voldemort_type();
+}

src/test/ui/drop-bounds/drop-bounds.rs

@@ -0,0 +1,19 @@
+#![deny(drop_bounds)]
+fn foo<T: Drop>() {} //~ ERROR
+fn bar<U>()
+where
+    U: Drop, //~ ERROR
+{
+}
+fn baz(_x: impl Drop) {} //~ ERROR
+struct Foo<T: Drop> { //~ ERROR
+  _x: T
+}
+struct Bar<U> where U: Drop { //~ ERROR
+  _x: U
+}
+trait Baz: Drop { //~ ERROR
+}
+impl<T: Drop> Baz for T { //~ ERROR
+}
+fn main() {}

src/test/ui/drop-bounds/drop-bounds.stderr

@@ -0,0 +1,50 @@
+error: bounds on `T: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor
+  --> $DIR/drop-bounds.rs:2:11
+   |
+LL | fn foo<T: Drop>() {}
+   |           ^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/drop-bounds.rs:1:9
+   |
+LL | #![deny(drop_bounds)]
+   |         ^^^^^^^^^^^
+
+error: bounds on `U: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor
+  --> $DIR/drop-bounds.rs:5:8
+   |
+LL |     U: Drop,
+   |        ^^^^
+
+error: bounds on `impl Drop: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor
+  --> $DIR/drop-bounds.rs:8:17
+   |
+LL | fn baz(_x: impl Drop) {}
+   |                 ^^^^
+
+error: bounds on `T: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor
+  --> $DIR/drop-bounds.rs:9:15
+   |
+LL | struct Foo<T: Drop> {
+   |               ^^^^
+
+error: bounds on `U: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor
+  --> $DIR/drop-bounds.rs:12:24
+   |
+LL | struct Bar<U> where U: Drop {
+   |                        ^^^^
+
+error: bounds on `Self: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor
+  --> $DIR/drop-bounds.rs:15:12
+   |
+LL | trait Baz: Drop {
+   |            ^^^^
+
+error: bounds on `T: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor
+  --> $DIR/drop-bounds.rs:17:9
+   |
+LL | impl<T: Drop> Baz for T {
+   |         ^^^^
+
+error: aborting due to 7 previous errors
+