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num::NonZero::get can be 1 transmute instead of 2 #123139

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Mar 28, 2024
+14 −12
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num::NonZero::get can be 1 transmute instead of 2 #123139

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26 changes: 14 additions & 12 deletions library/core/src/num/nonzero.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -20,7 +20,15 @@ use super::{IntErrorKind, ParseIntError};
///
/// # Safety
///
/// Types implementing this trait must be primitves that are valid when zeroed.
/// Types implementing this trait must be primitives that are valid when zeroed.
///
/// The associated `Self::NonZeroInner` type must have the same size+align as `Self`,
/// but with a niche and bit validity making it so the following `transmutes` are sound:
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@scottmcm scottmcm Mar 27, 2024 •
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I think these were already implied -- they certainly already hold for all the implementations that exist -- but figured I might as well write them out for anyone reading this stuff later.

(And it's a perma-unstable trait, so we can do whatever.)

///
/// - `Self::NonZeroInner` to `Option<Self::NonZeroInner>`
/// - `Option<Self::NonZeroInner>` to `Self`
///
/// (And, consequently, `Self::NonZeroInner` to `Self`.)
#[unstable(
feature = "nonzero_internals",
reason = "implementation detail which may disappear or be replaced at any time",
Expand Down Expand Up @@ -434,17 +442,11 @@ where
// of some not-inlined function, LLVM don't have range metadata
// to understand that the value cannot be zero.
//
// SAFETY: `Self` is guaranteed to have the same layout as `Option<Self>`.
match unsafe { intrinsics::transmute_unchecked(self) } {
None => {
// SAFETY: `NonZero` is guaranteed to only contain non-zero values, so this is unreachable.
unsafe { intrinsics::unreachable() }
}
Some(Self(inner)) => {
// SAFETY: `T::NonZeroInner` is guaranteed to have the same layout as `T`.
unsafe { intrinsics::transmute_unchecked(inner) }
}
}
// For now, using the transmute `assume`s the range at runtime.
//
// SAFETY: `ZeroablePrimitive` guarantees that the size and bit validity
// of `.0` is such that this transmute is sound.
unsafe { intrinsics::transmute_unchecked(self) }
}
}

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