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Rollup merge of #130384 - workingjubilee:document-what-abi-and-pref-align-is, r=compiler-errors

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compiler: Document AbiAndPrefAlign

The mere existence of this struct is confusing, and the second field doubly so. It's easy to mistake the "preferred" alignment as semantically relevant somehow. Insofar as I am aware, it is not, and certainly not for Rust code.
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Matthias Krüger 2024-09-15 20:55:13 +02:00 committed by GitHub
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compiler/rustc_abi/src/lib.rs
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@ -781,6 +781,14 @@ pub fn restrict_for_offset(self, offset: Size) -> Align {
} }
/// A pair of alignments, ABI-mandated and preferred. /// A pair of alignments, ABI-mandated and preferred.
///
/// The "preferred" alignment is an LLVM concept that is virtually meaningless to Rust code:
/// it is not exposed semantically to programmers nor can they meaningfully affect it.
/// The only concern for us is that preferred alignment must not be less than the mandated alignment
/// and thus in practice the two values are almost always identical.
///
/// An example of a rare thing actually affected by preferred alignment is aligning of statics.
/// It is of effectively no consequence for layout in structs and on the stack.
#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)] #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
#[cfg_attr(feature = "nightly", derive(HashStable_Generic))] #[cfg_attr(feature = "nightly", derive(HashStable_Generic))]
pub struct AbiAndPrefAlign { pub struct AbiAndPrefAlign {