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Improve assembly of Contiguous derive (#200)

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`from_integer` and `into_integer` are usually provided by the trait's
default implementation. We override this implementation because it goes
through `transmute_copy`, which can lead to inefficient assembly as seen
in https://github.com/Lokathor/bytemuck/issues/175 .
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Valentin 2023-09-05 21:41:44 +02:00 committed by GitHub
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commit 3c1a0d9b35
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19
derive/src/traits.rs
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@ -378,12 +378,31 @@ impl Derivable for Contiguous {
let min_lit = LitInt::new(&format!("{}", min), input.span());
let max_lit = LitInt::new(&format!("{}", max), input.span());
// `from_integer` and `into_integer` are usually provided by the trait's default implementation.
// We override this implementation because it goes through `transmute_copy`, which can lead to
// inefficient assembly as seen in https://github.com/Lokathor/bytemuck/issues/175 .
Ok((
quote!(),
quote! {
type Int = #integer_ty;
const MIN_VALUE: #integer_ty = #min_lit;
const MAX_VALUE: #integer_ty = #max_lit;
#[inline]
fn from_integer(value: Self::Int) -> Option<Self> {
#[allow(clippy::manual_range_contains)]
if Self::MIN_VALUE <= value && value <= Self::MAX_VALUE {
Some(unsafe { ::core::mem::transmute(value) })
} else {
None
}
}
#[inline]
fn into_integer(self) -> Self::Int {
self as #integer_ty
}
},
))
}