Define an unsized tail#2287
Merged
Merged
Conversation
rustbot
added
the
S-waiting-on-review
This was referenced Jun 16, 2026
ehuss
approved these changes
Jun 16, 2026
ehuss
removed
the
S-waiting-on-review
We refer to the unsized tail of a type, but we hadn't defined it. Let's do that and link to the definition.
traviscross
force-pushed
the
TC/define-unsized-tail
branch
from
e3cc399 to
39fa8df
Compare
Collaborator
|
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. |
RalfJung
reviewed
Jun 25, 2026
| > [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. |
Member
There was a problem hiding this comment.
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.
Contributor
Author
There was a problem hiding this comment.
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.
Member
There was a problem hiding this comment.
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, ...
JonathanBrouwer
added a commit
to JonathanBrouwer/rust
that referenced
this pull request
Jun 30, 2026
Update books ## rust-lang/nomicon 1 commits in cc6a6bae8c3bfa389974e533c54694662c1a9de6..5012a37c682b26c4e19433888ed2ca9b129696ca 2026-06-25 10:05:58 UTC to 2026-06-25 10:05:58 UTC - Clarify when safety may rely on correctness (rust-lang/nomicon#523) ## rust-lang/reference 10 commits in 2c27905c15a51983b54d84f050d3bda096194d27..86635e30bf861a038dc197d7e16fd09e7e514e7a 2026-06-25 17:00:32 UTC to 2026-06-16 18:56:35 UTC - use-declarations.md: move example to where it's described (rust-lang/reference#2295) - fix link target (rust-lang/reference#2299) - remove broken text (rust-lang/reference#2297) - type-layout: rewrite `#[repr(C)]` struct layout algorithm (rust-lang/reference#2243) - Fix grammar rules containing or pertaining to bounds (rust-lang/reference#2257) - Document metadata of ptrs to indirectly unsized types (rust-lang/reference#2289) - Generalize the wide-pointer total-size bound (rust-lang/reference#2290) - Cover `str` in the wide-pointer metadata rule (rust-lang/reference#2288) - Define an unsized tail (rust-lang/reference#2287) - Define pointer metadata (rust-lang/reference#2286)
rust-timer
added a commit
to rust-lang/rust
that referenced
this pull request
Jun 30, 2026
Rollup merge of #158571 - rustbot:docs-update, r=ehuss Update books ## rust-lang/nomicon 1 commits in cc6a6bae8c3bfa389974e533c54694662c1a9de6..5012a37c682b26c4e19433888ed2ca9b129696ca 2026-06-25 10:05:58 UTC to 2026-06-25 10:05:58 UTC - Clarify when safety may rely on correctness (rust-lang/nomicon#523) ## rust-lang/reference 10 commits in 2c27905c15a51983b54d84f050d3bda096194d27..86635e30bf861a038dc197d7e16fd09e7e514e7a 2026-06-25 17:00:32 UTC to 2026-06-16 18:56:35 UTC - use-declarations.md: move example to where it's described (rust-lang/reference#2295) - fix link target (rust-lang/reference#2299) - remove broken text (rust-lang/reference#2297) - type-layout: rewrite `#[repr(C)]` struct layout algorithm (rust-lang/reference#2243) - Fix grammar rules containing or pertaining to bounds (rust-lang/reference#2257) - Document metadata of ptrs to indirectly unsized types (rust-lang/reference#2289) - Generalize the wide-pointer total-size bound (rust-lang/reference#2290) - Cover `str` in the wide-pointer metadata rule (rust-lang/reference#2288) - Define an unsized tail (rust-lang/reference#2287) - Define pointer metadata (rust-lang/reference#2286)
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
4 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
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