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further extend ?sized alternatives
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davidtwco committed Nov 19, 2024
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Expand Up @@ -779,10 +779,19 @@ pre-eRFC][pre_erfc_fix_dsts] and then a [blog post][blog_dynsized_unsized] which
on those ideas, and the feedback to this RFC's prior art, but is not a load-bearing part
of this RFC.

In the following tables, "implicit relaxation" is used to refer to the behaviour that
this RFC proposes where adding any other sizedness bound removes the default bound.

To preserve backwards compatibility, `Sized` bounds must be migrated to `const Sized` (see
[the `size_of` and `size_of_val` section][size_of-and-size_of_val] for rationale), but
`?Sized` bounds could retain their existing behaviour of removing the default `Sized` bound:

### Keeping only `?Sized`
[keeping-only-sized]: #keeping-only-sized

Without adding any additional default bounds or relaxed forms, keeping `?Sized` could be
compatible with this proposal as follows:

In the current edition, `?Sized` would need to be equivalent to `?Sized + const ValueSized` to
maintain backwards compatibility (see [the `size_of` and `size_of_val`
section][size_of-and-size_of_val] for rationale). In the next edition, `?Sized` would be
Expand All @@ -792,30 +801,52 @@ bound) and bare `?Sized` would only remove the `Sized` default bound.
Prior to the edition migration, the default bound is `Sized`, which could be changed using
the following syntax:

| With "implicit relaxation" | Keeping `?Sized` | Notes |
| -------------------------- | ---------------------- | ------------------------------------------------------------------------------------------------ |
| `const Sized` | `?Sized + const Sized` | |
| `const ValueSized` | `?Sized` | `?Sized` is equivalent to `const ValueSized` for backwards compatibility |
| `ValueSized` | N/A | Not possible to write this, `?Sized` would remove the default `Sized` but add `const ValueSized` |
| `Pointee` | N/A | Not possible to write this, `?Sized` would remove the default `Sized` but add `const ValueSized` |
| With "implicit relaxation" | Keeping `?Sized` |
| -------------------------- | ---------------- |
| `const Sized` | `const Sized` |
| `Sized` | `Sized` |
| `const ValueSized` | `?Sized` |
| `ValueSized` | Not possible |
| `Pointee` | Not possible |

After the edition migration, the default bound is `const Sized`, which could be changed
using the following syntax:

| With "implicit relaxation" | Keeping `?Sized` | Notes |
| -------------------------- | ------------------------------------------------------------------- | ------------------------------------------------------------------------------------ |
| `Sized` | `?Sized + Sized` (maybe `?const Sized + Sized`?) | It isn't obvious how `?Sized` syntax should be used to remove the constness modifier |
| `const ValueSized` | `?Sized + const ValueSized` (or `?const Sized + const ValueSized`?) | |
| `ValueSized` | `?Sized + ValueSized` (or `?const Sized + ValueSized`?) | |
| `Pointee` | `?Sized` (or `?const Sized`?) | |

"Implicit relaxation" is used to refer to the behaviour that this RFC proposes where adding
any other sizedness bound removes the default bound.

Another alternative to "implicit relaxation" and `?Sized` could be to introduce new relaxed bounds
for `ValueSized` (i.e. `?ValueSized`), but this would only make sense if `ValueSized` were a default
bound (i.e. `T: Sized + ValueSized` was the default, rather than `T: Sized` which only implies
`T: Sized + ValueSized`).
| With "implicit relaxation" | Keeping `?Sized` |
| -------------------------- | --------------------------------- |
| `const Sized` | `const Sized` |
| `Sized` | `?const Sized + Sized` |
| `const ValueSized` | `?const Sized + const ValueSized` |
| `ValueSized` | `?const Sized + ValueSized` |
| `Pointee` | `?const Sized` |

### Adding `?ValueSized`
[adding-valuesized]: #adding-valuesized

Another alternative is to make `ValueSized` a default bound in addition to `Sized` and establish
that relaxing a supertrait bound also implies relaxing subtrait bounds:

Prior to the edition migration, the default bound is `ValueSized + const ValueSized + Sized`,
which could be changed using:

| With "implicit relaxation" | Adding `?ValueSized` |
|----------------------------|----------------------------------|
| `const Sized` | `const Sized` |
| `Sized` | `Sized` |
| `const ValueSized` | `?Sized` |
| `ValueSized` | `?const ValueSized` |
| `Pointee` | `?ValueSized` |

After the edition migration, the default bound is
`ValueSized + const ValueSized + Sized + const Sized`, which could be changed using:

| With "implicit relaxation" | Adding `?ValueSized` |
|----------------------------|----------------------------------|
| `const Sized` | `const Sized` |
| `Sized` | `?const Sized` |
| `const ValueSized` | `?Sized` |
| `ValueSized` | `?const ValueSized` |
| `Pointee` | `?ValueSized` |

# Why not re-use `std::ptr::Pointee`?
[why-not-re-use-stdptrpointee]: #why-not-re-use-stdptrpointee
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