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Make explicit that TypeId's layout and size are unstable #148394

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Nov 3, 2025
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Make explicit that TypeId's layout and size are unstable #148394

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9 changes: 9 additions & 0 deletions library/core/src/any.rs
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Expand Up @@ -611,6 +611,15 @@ impl dyn Any + Send + Sync {
/// noting that the hashes and ordering will vary between Rust releases. Beware
/// of relying on them inside of your code!
///
/// # Layout
///
/// Like other [`Rust`-representation][repr-rust] types, `TypeId`'s size and layout are unstable.
/// In particular, this means that you cannot rely on the size and layout of `TypeId` remaining the
/// same between Rust releases; they are subject to change without prior notice between Rust
/// releases.
///
/// [repr-rust]: https://doc.rust-lang.org/reference/type-layout.html#r-layout.repr.rust.unspecified
///
/// # Danger of Improper Variance
///
/// You might think that subtyping is impossible between two static types,
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