Commits on Feb 25, 2024

1. try to exclude packed attr for types that contain aligned types …

   This patch handles some (but not all) cases of types with a `packed`
   attribute that contain a type with an `align(N)` attribute.
   
   This uses information available in the Clang IR about types' layout to
   determine if a given type is likely to have an alignment attribute
   placed on it during the code generation phase. This is just a heuristic;
   the decision to actually place an alignment attribute depends on
   information that is not known until code generation, so the logic here
   may result in both false positives and false negatives.
   
   Using the real information from codegen on whether an alignment
   attribute was placed on a child type is not possible in general, because
   the order in which types are generated is not guaranteed. In some cases
   code may be generated for parent types before code is generated for
   child types contained in that parent. Then, it will be impossible to
   make an accurate decision regarding whether to remove the `packed`
   attribute from the parent type.
   
   The impact of a false negative is that an outer type may have a `packed`
   attr even when an inner type as an `align` attr. Such a type would not
   compile under rustc, but it already would not compile so this has no
   harmful impact.
   
   The impact of a false positive is that an outer type may have its
   `packed` attr stripped needlessly, because no inner types actually have
   an `align` attr. Because we only remove the `packed` attr when we can
   confirm that there will be no change to the type's layout, this also
   should have no harmful impact.

   

   bertschingert committed Feb 25, 2024
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