Static 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

A generic trait bound of the form T: Drop is most likely misleading and not what the programmer intended (they probably should have used std::mem::needs_drop instead).

Drop bounds do not actually indicate whether a type can be trivially dropped or not, because a composite type containing Drop types does not necessarily implement Drop itself. Naïvely, one might be tempted to write an implementation that assumes that a type can be trivially dropped while also supplying a specialization for T: Drop that actually calls the destructor. However, this breaks down e.g. when T is String, which does not implement Drop itself but contains a Vec, which does implement Drop, so assuming T can be trivially dropped would lead to a memory leak here.

Furthermore, the Drop trait only contains one method, Drop::drop, which may not be called explicitly in user code (E0040), so there is really no use case for using Drop in trait bounds, save perhaps for some obscure corner cases, which can use #[allow(drop_bounds)].