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error/E0691: include alignment in error message

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Include the computed alignment of the violating field when rejecting
transparent types with non-trivially aligned ZSTs.

ZST member fields in transparent types must have an alignment of 1 (to
ensure it does not raise the layout requirements of the transparent
field). The current error message looks like this:

 LL | struct Foobar(u32, [u32; 0]);
    |                    ^^^^^^^^ has alignment larger than 1

This patch changes the report to include the alignment of the violating
field:

 LL | struct Foobar(u32, [u32; 0]);
    |                    ^^^^^^^^ has alignment of 4, which is larger than 1

In case of unknown alignments, it will yield:

 LL | struct Foobar<T>(u32, [T; 0]);
    |                       ^^^^^^ may have alignment larger than 1

This allows developers to get a better grasp why a specific field is
rejected. Knowing the alignment of the violating field makes it easier
to judge where that alignment-requirement originates, and thus hopefully
provide better hints on how to mitigate the problem.

This idea was proposed in 2022 in #98071 as part of a bigger change.
This commit simply extracts this error-message change, to decouple it
from the other diagnostic improvements.
This commit is contained in:
David Rheinsberg 2023-07-21 09:32:28 +02:00
parent 1a44b45987
commit b0dadff6de
3 changed files with 25 additions and 15 deletions
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29
compiler/rustc_hir_analysis/src/check/check.rs
View file

@ -1078,9 +1078,9 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
// 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 zst = layout.is_ok_and(|layout| layout.is_zst());
let align1 = layout.is_ok_and(|layout| layout.align.abi.bytes() == 1);
let align = layout.ok().map(|layout| layout.align.abi.bytes());
if !zst {
return (span, zst, align1, None);
return (span, zst, align, None);
}
fn check_non_exhaustive<'tcx>(
@ -1115,12 +1115,12 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
}
}
(span, zst, align1, check_non_exhaustive(tcx, ty).break_value())
(span, zst, align, check_non_exhaustive(tcx, ty).break_value())
});
let non_zst_fields = field_infos
.clone()
.filter_map(|(span, zst, _align1, _non_exhaustive)| if !zst { Some(span) } else { None });
.filter_map(|(span, zst, _align, _non_exhaustive)| if !zst { Some(span) } else { None });
let non_zst_count = non_zst_fields.clone().count();
if non_zst_count >= 2 {
bad_non_zero_sized_fields(tcx, adt, non_zst_count, non_zst_fields, tcx.def_span(adt.did()));
@ -1128,17 +1128,26 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
let incompatible_zst_fields =
field_infos.clone().filter(|(_, _, _, opt)| opt.is_some()).count();
let incompat = incompatible_zst_fields + non_zst_count >= 2 && non_zst_count < 2;
for (span, zst, align1, non_exhaustive) in field_infos {
if zst && !align1 {
struct_span_err!(
for (span, zst, align, non_exhaustive) in field_infos {
if zst && align != Some(1) {
let mut err = struct_span_err!(
tcx.sess,
span,
E0691,
"zero-sized field in transparent {} has alignment larger than 1",
adt.descr(),
)
.span_label(span, "has alignment larger than 1")
.emit();
);
if let Some(align_bytes) = align {
err.span_label(
span,
format!("has alignment of {align_bytes}, which is larger than 1"),
);
} else {
err.span_label(span, "may have alignment larger than 1");
}
err.emit();
}
if incompat && let Some((descr, def_id, args, non_exhaustive)) = non_exhaustive {
tcx.struct_span_lint_hir(