PhantomData

A zero-size field that tells the compiler a type uses a type parameter it doesn't store.

PhantomData is zero-sized and makes the containing type act as if it owns or references T, for variance, drop checking, and auto traits.

Examples

struct Id<T> { raw: u64 } fails with E0392 (T is never used); adding _for: PhantomData<T> fixes it and adds 0 bytes.