Summary

Custom coercions allow smart pointers to fully participate in the DST system. In particular, they allow practical use of Rc<T> and Arc<T> where T is unsized.

This RFC subsumes part of RFC 401 coercions.

Motivation

DST is not really finished without this, in particular there is a need for types like reference counted trait objects (Rc<Trait>) which are not currently well- supported (without coercions, it is pretty much impossible to create such values with such a type).

Detailed design

There is an Unsize trait and lang item. This trait signals that a type can be converted using the compiler's coercion machinery from a sized to an unsized type. All implementations of this trait are implicit and compiler generated. It is an error to implement this trait. If &T can be coerced to &U then there will be an implementation of Unsize<U> for T. E.g, [i32; 42]: Unsize<[i32]>. Note that the existence of an Unsize impl does not signify a coercion can itself can take place, it represents an internal part of the coercion mechanism (it corresponds with coerce_inner from RFC 401). The trait is defined as:

#[lang="unsize"]
trait Unsize<T: ?Sized>: ::std::marker::PhantomFn<Self, T> {}

There are implementations for any fixed size array to the corresponding unsized array, for any type to any trait that that type implements, for structs and tuples where the last field can be unsized, and for any pair of traits where Self is a sub-trait of T (see RFC 401 for more details).

There is a CoerceUnsized trait which is implemented by smart pointer types to opt-in to DST coercions. It is defined as:

#[lang="coerce_unsized"]
trait CoerceUnsized<Target>: ::std::marker::PhantomFn<Self, Target> + Sized {}

An example implementation:

impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
impl<T: Zeroable+CoerceUnsized<U>, U: Zeroable> CoerceUnsized<NonZero<U>> for NonZero<T> {}

// For reference, the definitions of Rc and NonZero:
pub struct Rc<T: ?Sized> {
    _ptr: NonZero<*mut RcBox<T>>,
}
pub struct NonZero<T: Zeroable>(T);

Implementing CoerceUnsized indicates that the self type should be able to be coerced to the Target type. E.g., the above implementation means that Rc<[i32; 42]> can be coerced to Rc<[i32]>. There will be CoerceUnsized impls for the various pointer kinds available in Rust and which allow coercions, therefore CoerceUnsized when used as a bound indicates coercible types. E.g.,

fn foo<T: CoerceUnsized<U>, U>(x: T) -> U {
    x
}

Built-in pointer impls:

impl<'a, 'b: 'aT: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b mut T {}
impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<&'a mut U> for &'a mut T {}
impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for &'a mut T {}
impl<'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for &'a mut T {}

impl<'a, 'b: 'a, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<&'a U> for &'b T {}
impl<'b, T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for &'b T {}

impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *mut T {}
impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*mut U> for *mut T {}

impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<*const U> for *const T {}

Note that there are some coercions which are not given by CoerceUnsized, e.g., from safe to unsafe function pointers, so it really is a CoerceUnsized trait, not a general Coerce trait.

Compiler checking

On encountering an implementation of `CoerceUnsized` (type collection phase)

If the impl is for a built-in pointer type, we check nothing, otherwise...
The compiler checks that the Self type is a struct or tuple struct and that the Target type is a simple substitution of type parameters from the Self type (i.e., That Self is Foo<Ts>, Target is Foo<Us> and that there exist Vs and Xs (where Xs are all type parameters) such that Target = [Vs/Xs]Self. One day, with HKT, this could be a regular part of type checking, for now it must be an ad hoc check). We might enforce that this substitution is of the form X/Y where X and Y are both formal type parameters of the implementation (I don't think this is necessary, but it makes checking coercions easier and is satisfied for all smart pointers).
The compiler checks each field in the Self type against the corresponding field in the Target type. Assuming Fs is the type of a field in Self and Ft is the type of the corresponding field in Target, then either Ft <: Fs or Fs: CoerceUnsized<Ft> (note that this includes some built-in coercions, coercions unrelated to unsizing are excluded, these could probably be added later, if needed).
There must be only one non-PhantomData field that is coerced.
We record for each impl, the index of the field in the Self type which is coerced.

On encountering a potential coercion (type checking phase)

If we have an expression with type E where the type F is required during type checking and E is not a subtype of F, nor is it coercible using the built-in coercions, then we search for a bound of E: CoerceUnsized<F>. Note that we may not at this stage find the actual impl, but finding the bound is good enough for type checking.
If we require a coercion in the receiver of a method call or field lookup, we perform the same search that we currently do, except that where we currently check for coercions, we check for built-in coercions and then for CoerceUnsized bounds. We must also check for Unsize bounds for the case where the receiver is auto-deref'ed, but not autoref'ed.

On encountering an adjustment (translation phase)

In trans (which is post-monomorphisation) we should always be able to find an impl for any CoerceUnsized bound.
If the impl is for a built-in pointer type, then we use the current coercion code for the various pointer kinds (Box<T> has different behaviour than & and * pointers).
Otherwise, we lookup which field is coerced due to the opt-in coercion, move the object being coerced and coerce the field in question by recursing (the built-in pointers are the base cases).

Adjustment types

We add AdjustCustom to the AutoAdjustment enum as a placeholder for coercions due to a CoerceUnsized bound. I don't think we need the UnsizeKind enum at all now, since all checking is postponed until trans or relies on traits and impls.

Drawbacks

Not as flexible as the previous proposal.

Alternatives

The original DST5 proposal contains a similar proposal with no opt-in trait, i.e., coercions are completely automatic and arbitrarily deep. This is a little too magical and unpredictable. It violates some 'soft abstraction boundaries' by interefering with the deep structure of objects, sometimes even automatically (and implicitly) allocating.

RFC 401 proposed a scheme for proposals where users write their own coercion using intrinsics. Although more flexible, this allows for implicit execution of arbitrary code. If we need the increased flexibility, I believe we can add a manual option to the CoerceUnsized trait backwards compatibly.

The proposed design could be tweaked: for example, we could change the CoerceUnsized trait in many ways (we experimented with an associated type to indicate the field type which is coerced, for example).

Unresolved questions

It is unclear to what extent DST coercions should support multiple fields that refer to the same type parameter. PhantomData<T> should definitely be supported as an "extra" field that's skipped, but can all zero-sized fields be skipped? Are there cases where this would enable by-passing the abstractions that make some API safe?

Updates since being accepted

Since it was accepted, the RFC has been updated as follows:

CoerceUnsized was specified to ignore PhantomData fields.

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