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library: core: document layout guarantee of TypeId #148265

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@dakr dakr commented Oct 29, 2025

Document the layout guarantee of TypeId not to exceed 16 bytes.

This is useful for FFI use-cases where a TypeId may be stored within a foreign data structure or buffer.

An example for such a use-case can be found in [1].

Link: https://git.kernel.org/pub/scm/linux/kernel/git/driver-core/driver-core.git/commit/?id=6f61a2637abe4f89877da3280775565baedb60e0 [1]
Suggested-by: Alice Ryhl aliceryhl@google.com
@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 29, 2025
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rustbot commented Oct 29, 2025

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dakr commented Oct 29, 2025

Cc: @Darksonn
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Do you have a reference to a place where there was an agreement to commit to this? Or is this a new libs-api/libs/compiler request?
@scottmcm scottmcm added the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Oct 29, 2025
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Do you have a reference to a place where there was an agreement to commit to this? Or is this a new libs-api/libs/compiler request?

This PR is a request for the stdlib to make this commitment. There is some prior discussion at #rust-for-linux > TypeId layout.
library: core: document layout guarantee of TypeId
460e92e 
Document the layout guarantee of `TypeId` not to exceed 16 bytes. This is useful for FFI use-cases where a `TypeId` may be stored within a foreign data structure or buffer. An example for such a use-case can be found in [1]. Link: https://git.kernel.org/pub/scm/linux/kernel/git/driver-core/driver-core.git/commit/?id=6f61a2637abe4f89877da3280775565baedb60e0 [1] Suggested-by: Alice Ryhl <aliceryhl@google.com> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
@scottmcm scottmcm added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. I-libs-api-nominated Nominated for discussion during a libs-api team meeting. I-compiler-nominated Nominated for discussion during a compiler team meeting. and removed T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 30, 2025
Noratrieb
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library/core/src/any.rs
///
/// # Layout
///
/// The size of `TypeId` is guaranteed not to exceed 16 bytes.
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can you make it repr(C) to ensure there can't be additional padding if layout randomization improved?

I think that should be hidden from the docs by default now, but you should double-check that it's not shown, since that's not a guarantee.
Noratrieb
Noratrieb reviewed Oct 30, 2025
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library/core/src/any.rs
///
/// # Layout
///
/// The size of `TypeId` is guaranteed not to exceed 16 bytes.
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Also please mention explicitly that these are the only guarantees, and that users must not make any assumptions about the contents.
@ojeda ojeda mentioned this pull request Oct 30, 2025
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I-compiler-nominated Nominated for discussion during a compiler team meeting. I-libs-api-nominated Nominated for discussion during a libs-api team meeting. needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue.

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