Variance subtyping and understanding const vs mut pointers

[Yandros]

Or, in textual form, and in this very forum:

Looking for a deeper understanding of PhantomData

What is PhantomData<T> The two defining properties of PhantomData<T> are the following: At runtime, PhantomData<T> is exactly like (), i.e., nothing. This gives the peace of mind of it being (a) zero-cost (abstraction) For compile-time analysis, PhantomData<T> is exactly like T The usefulness of such a thing may not be obvious at first glance, but it turns out that types have properties (or more precisely, they can lack properties), which in turn leads to the containing struct trans…

• I've written a shorter version of that post (focusing on variance exclusively) here

PhantomData and dropck confusion

Imagine the following "lib" (I am creating a custom Box rather than a Vec for the sake of simplicity): A custom Box type use ::std::{*, ptr::NonNull, }; /// Simplified raw allocation interface pub trait IsAllocator { /// Allocates memory for a T fn alloc<T> () -> NonNull<T>; /// Frees the pointed memory, but does **not** drop the pointee unsafe fn free<T> (_: NonNull<T>); } pub enum Allocator {} impl IsAllocator for Allocator { /* ... */ } /// Box<T> is simpler than Ve…

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The TL,DRs:

• Variance (and subtyping) is related to "being able to shrink lifetime( parameter)s" – or in some cases (contravariance), expanding lifetime( parameter)s.

• The interaction between PhantomData and dropck only matters when an unsafe impl<#[may_dangle] …> is used, so it's more of a theoretical curiosity more than a knowledge with actual practical usages