Define an unsized tail#2287
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We refer to the unsized tail of a type, but we hadn't defined it. Let's do that and link to the definition.
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This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed. Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers. |
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| > [Variables], function parameters, [const] items, and [static] items must be `Sized`. | ||
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| r[dynamic-sized.tail] | ||
| The *unsized tail* of a type is the dynamically sized component that the [metadata] of a pointer to the type describes. A [slice] (`[T]`) and a [`str`] are each their own unsized tail, described by a length; a [trait object] (`dyn Trait`) is its own unsized tail, described by a pointer to a vtable. When a struct (per [dynamic-sized.struct-field]) or a tuple has an unsized last field, its unsized tail is the unsized tail of that field. A sized type has no unsized tail. |
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This definition contains redundant information with the definition of "metadata" from #2286. That doesn't seem great, we should define these things only in one place and then reference them elsewhere.
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Agreed. I had the same thought. I didn't have any immediate good ideas for how to resolve it, though (other than ensuring they were cross-linked, as I did). Suggestions welcome.
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We refer to the unsized tail of a type, but we hadn't defined it. Let's do that and link to the definition.
I'm breaking this out from #2282 so that we can merge the prerequisites before considering the new lang guarantees.
This is stacked on #2286 and that should be merged first.
cc @ehuss @RalfJung @Mark-Simulacrum