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document -Zrandomize-layout in the unstable book
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src/doc/unstable-book/src/compiler-flags/randomize-layout.md
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# `randomize-layout` | ||
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The tracking issue for this feature is: [#106764](https://github.com/rust-lang/rust/issues/106764). | ||
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------------------------ | ||
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The `-Zrandomize-layout` flag changes the layout algorithm for `repr(Rust)` types defined in the current crate from its normal | ||
optimization goals to pseudorandomly rearranging fields within the degrees of freedom provided by the largely unspecified | ||
default representation. This also affects type sizes and padding. | ||
Downstream intantiations of generic types defined in a crate with randomization enabled will also be randomized. | ||
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It can be used to find unsafe code that accidentally relies on unspecified behavior. | ||
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Randomization is not guaranteed to use a different permutation for each compilation session. | ||
`-Zlayout-seed=<u64>` can be used to supply additional entropy. | ||
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Randomization only approximates the intended freedom of repr(Rust). Sometimes two distinct types may still consistently | ||
result in the same layout due to limitations of the current implementation. Randomization may become | ||
more aggressive over time as our coverage of the available degrees of freedoms improves. | ||
Corollary: Randomization is not a safety oracle. Two struct layouts being observably the same under different layout seeds | ||
on the current compiler version does not guarantee that future compiler versions won't give them distinct layouts. | ||
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Randomization may also become less aggressive in the future if additional guarantees get added to the default layout. |