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compiler: Document the offset invariant of OperandValue::Pair #142044

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Jun 6, 2025
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compiler: Document the offset invariant of OperandValue::Pair #142044

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10 changes: 8 additions & 2 deletions compiler/rustc_codegen_ssa/src/mir/operand.rs
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Expand Up @@ -45,9 +45,15 @@ pub enum OperandValue<V> {
Immediate(V),
/// A pair of immediate LLVM values. Used by wide pointers too.
///
/// An `OperandValue` *must* be this variant for any type for which
/// # Invariants
/// - For `Pair(a, b)`, `a` is always at offset 0, but may have `FieldIdx(1..)`
/// - `b` is not at offset 0, because `V` is not a 1ZST type.
/// - `a` and `b` will have a different FieldIdx, but otherwise `b`'s may be lower
/// or they may not be adjacent, due to arbitrary numbers of 1ZST fields that
/// will not affect the shape of the data which determines if `Pair` will be used.
/// - An `OperandValue` *must* be this variant for any type for which
/// [`LayoutTypeCodegenMethods::is_backend_scalar_pair`] returns `true`.
/// The backend values in this variant must be the *immediate* backend types,
/// - The backend values in this variant must be the *immediate* backend types,
/// as returned by [`LayoutTypeCodegenMethods::scalar_pair_element_backend_type`]
/// with `immediate: true`.
Pair(V, V),
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