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Document soundness of Integer -> Pointer -> Integer conversions in const contexts. #113510

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Sep 5, 2023
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Document soundness of Integer -> Pointer -> Integer conversions in const contexts. #113510

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5 changes: 4 additions & 1 deletion library/core/src/intrinsics.rs
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Expand Up @@ -1130,7 +1130,10 @@ extern "rust-intrinsic" {
/// may lead to unexpected and unstable compilation results. This makes `transmute` **incredibly
/// unsafe**. `transmute` should be the absolute last resort.
///
/// Transmuting pointers to integers in a `const` context is [undefined behavior][ub].
/// Transmuting pointers *to* integers in a `const` context is [undefined behavior][ub],
/// unless the pointer was originally created *from* an integer.
/// (That includes this function specifically, integer-to-pointer casts, and helpers like [`invalid`][crate::ptr::invalid],
/// but also semantically-equivalent conversions such as punning through `repr(C)` union fields.)
/// Any attempt to use the resulting value for integer operations will abort const-evaluation.
/// (And even outside `const`, such transmutation is touching on many unspecified aspects of the
/// Rust memory model and should be avoided. See below for alternatives.)
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