Static rustc_lint::traits::DROP_BOUNDS

pub static DROP_BOUNDS: &Lint

The drop_bounds lint checks for generics with std::ops::Drop as bounds.

Example

fn foo<T: Drop>() {}

{{produces}}

Explanation

Drop bounds do not really accomplish anything. A type may have compiler-generated drop glue without implementing the Drop trait itself. The Drop trait also only has one method, Drop::drop, and that function is by fiat not callable in user code. So there is really no use case for using Drop in trait bounds.

The most likely use case of a drop bound is to distinguish between types that have destructors and types that don't. Combined with specialization, a naive coder would write an implementation that assumed a type could be trivially dropped, then write a specialization for T: Drop that actually calls the destructor. Except that doing so is not correct; String, for example, doesn't actually implement Drop, but because String contains a Vec, assuming it can be trivially dropped will leak memory.