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Make DefId
repr(C)
, optimize big-endian field order
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@ -221,10 +221,17 @@ impl<D: Decoder> Decodable<D> for DefIndex {
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#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Copy)]
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// On below-64 bit systems we can simply use the derived `Hash` impl
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#[cfg_attr(not(target_pointer_width = "64"), derive(Hash))]
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// Note that the order is essential here, see below why
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#[repr(C)]
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// We guarantee field order. Note that the order is essential here, see below why.
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pub struct DefId {
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// cfg-ing the order of fields so that the `DefIndex` which is high entropy always ends up in
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// the lower bits no matter the endianness. This allows the compiler to turn that `Hash` impl
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// into a direct call to 'u64::hash(_)`.
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#[cfg(not(all(target_pointer_width = "64", target_endian = "big")))]
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pub index: DefIndex,
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pub krate: CrateNum,
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#[cfg(all(target_pointer_width = "64", target_endian = "big"))]
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pub index: DefIndex,
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}
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// On 64-bit systems, we can hash the whole `DefId` as one `u64` instead of two `u32`s. This
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