repr(transparent) check: do not compute check_unsuited more than once #148281
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RalfJung
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Oct 30, 2025
| let layout = tcx.layout_of(typing_env.as_query_input(ty)); | ||
| // We are currently checking the type this field came from, so it must be local | ||
| let span = tcx.hir_span_if_local(field.did).unwrap(); | ||
| let trivial = layout.is_ok_and(|layout| layout.is_1zst()); |
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There is still some code here we are running twice, in particular the layout_of query. The alternative would be to simply collect the results into a Vec, which will cost us an allocation but then avoids running the iterator twice.
hkBst
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Oct 30, 2025
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field_infosis an iterator that we execute multiple times. However, we usually ignore theunsuitedfield -- we only need it in the last iteration. So move the computation of that field to that iteration to avoid computing it multiple times. Computingunsuitedinvolves a recursive traversal over the types of all non-trivial fields, so there can be non-trivial amounts of work here.(I benchmarked this in #148243 and saw no changes, probably because we don't have a benchmark with many repr(transparent) types. But still, computing this each time just seemed silly.)