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miri: explain why we use static alignment in ref-to-place conversion #58615

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Feb 23, 2019
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miri: explain why we use static alignment in ref-to-place conversion #58615

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0f6d363
explain why we use static alignment in ref-to-place conversion
RalfJung Feb 21, 2019
b01f81b
remark about the one place where place computation calls size_and_ali…
RalfJung Feb 21, 2019
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10 changes: 8 additions & 2 deletions src/librustc_mir/interpret/place.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -326,6 +326,10 @@ where

let mplace = MemPlace {
ptr: val.to_scalar_ptr()?,
// We could use the run-time alignment here. For now, we do not, because
// the point of tracking the alignment here is to make sure that the *static*
// alignment information emitted with the loads is correct. The run-time
// alignment can only be more restrictive.
align: layout.align.abi,
meta: val.to_meta()?,
};
Expand Down Expand Up @@ -385,9 +389,11 @@ where
// above). In that case, all fields are equal.
let field_layout = base.layout.field(self, usize::try_from(field).unwrap_or(0))?;

// Offset may need adjustment for unsized fields
// Offset may need adjustment for unsized fields.
let (meta, offset) = if field_layout.is_unsized() {
// re-use parent metadata to determine dynamic field layout
// Re-use parent metadata to determine dynamic field layout.
// With custom DSTS, this *will* execute user-defined code, but the same
// happens at run-time so that's okay.
let align = match self.size_and_align_of(base.meta, field_layout)? {
Some((_, align)) => align,
None if offset == Size::ZERO =>
Expand Down