CoerceUnsized

CoerceUnsized is primarily concerned with data containers. When a struct (typically, a smart pointer) implements CoerceUnsized, that means that the data it points to is being unsized.

Some implementors of CoerceUnsized include:

• &T
• Arc<T>
• Box<T>

This trait is (eventually) intended to be implemented by user-written smart pointers, and there are rules about when a type is allowed to implement CoerceUnsized that are explained in the trait's documentation.

Unsize

To contrast, the Unsize trait is concerned the actual types that are allowed to be unsized.

This is not intended to be implemented by users ever, since Unsize does not instruct the compiler (namely codegen) how to unsize a type, just whether it is allowed to be unsized. This is paired somewhat intimately with codegen which must understand how types are represented and unsized.

Primitive unsizing implementations

Built-in implementations are provided for:

• T -> dyn Trait + 'a when T: Trait (and T: Sized + 'a, and Trait is object safe).
• [T; N] -> [T]

Structural implementations

There are two implementations of Unsize which can be thought of as structural:

• (A1, A2, .., An): Unsize<(A1, A2, .., U)> given An: Unsize<U>, which allows the tail field of a tuple to be unsized. This is gated behind the unsized_tuple_coercion feature.
• Struct<.., Pi, .., Pj, ..>: Unsize<Struct<.., Ui, .., Uj, ..>> given TailField<Pi, .., Pj>: Unsize<Ui, .. Uj>, which allows the tail field of a struct to be unsized if it is the only field that mentions generic parameters Pi, .., Pj (which don't need to be contiguous).

The rules for the latter implementation are slightly complicated, since they may allow more than one parameter to be changed (not necessarily unsized) and are best stated in terms of the tail field of the struct.

Upcasting implementations

Two things are called "upcasting" internally:

1. True upcasting dyn SubTrait -> dyn SuperTrait (this also allows dropping auto traits and adjusting lifetimes, as below).
2. Dropping auto traits and adjusting the lifetimes of dyn trait without changing the principal¹: dyn Trait + AutoTraits... + 'a -> dyn Trait + NewAutoTraits... + 'b when AutoTraits ⊇ NewAutoTraits, and 'a: 'b.

These may seem like different operations, since (1.) includes adjusting the vtable of a dyn trait, while (2.) is a no-op. However, to the type system, these are handled with much the same code.

This built-in implementation of Unsize is the most involved, particularly after it was reworked to support the complexities of associated types.

Specifically, the upcasting algorithm involves: For each supertrait of the source dyn trait's principal (including itself)...

1. Unify the super trait ref with the principal of the target (making sure we only ever upcast to a true supertrait, and never via an impl).
2. For every auto trait in the target, check that it's present in the source (allowing us to drop auto traits, but never gain new ones).
3. For every projection in the target, check that it unifies with a single projection in the source (since there may be more than one given trait Sub: Sup<.., A = i32> + Sup<.., A = u32>).

Specifically, (3.) prevents a choice of projection bound to guide inference unnecessarily, though it may guide inference when it is unambiguous.