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specify that CoerceUnsized should ignore PhantomData fields #1234

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Sep 4, 2015
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specify that CoerceUnsized should ignore PhantomData fields #1234

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specify that CoerceUnsized should ignore PhantomData fields
Gankra Aug 3, 2015
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add updates section
Gankra Aug 4, 2015
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add unresolved question
Gankra Aug 20, 2015
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16 changes: 13 additions & 3 deletions text/0982-dst-coercion.md
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Expand Up @@ -113,7 +113,7 @@ 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 field that is coerced.
* 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.

Expand All @@ -135,7 +135,7 @@ 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.
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).
Expand Down Expand Up @@ -175,4 +175,14 @@ indicate the field type which is coerced, for example).

# Unresolved questions

None
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:

1. `CoerceUnsized` was specified to ingore PhantomData fields.