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Define an unsized tail #2287

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traviscross merged 1 commit into from
Jun 16, 2026
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Define an unsized tail #2287

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3 changes: 2 additions & 1 deletion src/behavior-considered-undefined.md
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Original file line number Diff line number Diff line change
Expand Up @@ -145,7 +145,7 @@ r[undefined.validity.reference-box]
* A reference or [`Box<T>`] must be aligned and non-null, it cannot be [dangling], and it must point to a valid value (in case of dynamically sized types, using the actual dynamic type of the pointee as determined by the [metadata]). Note that the last point (about pointing to a valid value) remains a subject of some debate.

r[undefined.validity.wide]
* The [metadata] of a wide reference, [`Box<T>`], or raw pointer must match the type of the unsized tail:
* The [metadata] of a wide reference, [`Box<T>`], or raw pointer must match the type of the [unsized tail]:
* `dyn Trait` metadata must be a pointer to a compiler-generated vtable for `Trait`. (For raw pointers, this requirement remains a subject of some debate.)
* Slice (`[T]`) metadata must be a valid `usize`. Furthermore, for wide references and [`Box<T>`], slice metadata is invalid if it makes the total size of the pointed-to value bigger than `isize::MAX`.

Expand Down Expand Up @@ -204,6 +204,7 @@ r[undefined.validity.undef]
[project-field]: expressions/field-expr.md
[project-tuple]: expressions/tuple-expr.md#tuple-indexing-expressions
[project-slice]: expressions/array-expr.md#array-and-slice-indexing-expressions
[unsized tail]: dynamic-sized.tail
[unwinding-ffi]: panic.md#unwinding-across-ffi-boundaries
[const-promoted]: destructors.md#constant-promotion
[lifetime-extended]: destructors.md#temporary-lifetime-extension
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7 changes: 7 additions & 0 deletions src/dynamically-sized-types.md
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Expand Up @@ -25,10 +25,17 @@ r[dynamic-sized.struct-field]
> [!NOTE]
> [Variables], function parameters, [const] items, and [static] items must be `Sized`.

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.

@RalfJung RalfJung Jun 25, 2026 •
edited by rustbot
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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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@traviscross traviscross Jun 25, 2026 •
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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.

@RalfJung RalfJung Jul 4, 2026

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We could define "unsized tail" without repeating the metadata definition. For instance:

The unsized tail of an unsized type is the unsized type at the "end" of the type, after recursing through compound types:

  • A [slice] ([T]), a [str], and a [trait object] (dyn Trait) are each their own unsized tail.
  • 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.

And then for metadata I'd say something like:
[Pointer types] to DSTs are sized but have twice the size of pointers to sized types, since they also store metadata.
The metadata carries by a pointer is determined by the [unsized tail] of its pointee type:

  • If the unsized tail is a slice or str, ...
  • If the unsized tail is a trait object, ...


[metadata]: dynamic-sized.pointer-types
[sized]: special-types-and-traits.md#sized
[Slices]: types/slice.md
[slice]: types/slice.md
[str]: types/str.md
[`str`]: types/str.md
[trait objects]: types/trait-object.md
[trait object]: types/trait-object.md
[Pointer types]: types/pointer.md
[Variables]: variables.md
[const]: items/constant-items.md
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