error/E0691: include alignment in error message

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.

3 files changed, 25 insertions, 15 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0691.md b/compiler/rustc_error_codes/src/error_codes/E0691.md
index 60060cacbd6..483c74c0ff5 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0691.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0691.md
@@ -11,7 +11,8 @@ struct ForceAlign32;
 
 #[repr(transparent)]
 struct Wrapper(f32, ForceAlign32); // error: zero-sized field in transparent
-                                   //        struct has alignment larger than 1
+                                   //        struct has alignment of 32, which
+                                   //        is larger than 1
 ```
 
 A transparent struct, enum, or union is supposed to be represented exactly like
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index d9e14096954..e0698a72335 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -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(
diff --git a/tests/ui/repr/repr-transparent.stderr b/tests/ui/repr/repr-transparent.stderr
index cb1e2337776..028fc25db46 100644
--- a/tests/ui/repr/repr-transparent.stderr
+++ b/tests/ui/repr/repr-transparent.stderr
@@ -20,13 +20,13 @@ error[E0691]: zero-sized field in transparent struct has alignment larger than 1
   --> $DIR/repr-transparent.rs:36:32
    |
 LL | struct NontrivialAlignZst(u32, [u16; 0]);
-   |                                ^^^^^^^^ has alignment larger than 1
+   |                                ^^^^^^^^ has alignment of 2, which is larger than 1
 
 error[E0691]: zero-sized field in transparent struct has alignment larger than 1
   --> $DIR/repr-transparent.rs:42:24
    |
 LL | struct GenericAlign<T>(ZstAlign32<T>, u32);
-   |                        ^^^^^^^^^^^^^ has alignment larger than 1
+   |                        ^^^^^^^^^^^^^ has alignment of 32, which is larger than 1
 
 error[E0084]: unsupported representation for zero-variant enum
   --> $DIR/repr-transparent.rs:44:1
@@ -66,13 +66,13 @@ error[E0691]: zero-sized field in transparent enum has alignment larger than 1
   --> $DIR/repr-transparent.rs:71:14
    |
 LL |     Foo(u32, [u16; 0]),
-   |              ^^^^^^^^ has alignment larger than 1
+   |              ^^^^^^^^ has alignment of 2, which is larger than 1
 
 error[E0691]: zero-sized field in transparent enum has alignment larger than 1
   --> $DIR/repr-transparent.rs:76:11
    |
 LL |     Foo { bar: ZstAlign32<T>, baz: u32 }
-   |           ^^^^^^^^^^^^^^^^^^ has alignment larger than 1
+   |           ^^^^^^^^^^^^^^^^^^ has alignment of 32, which is larger than 1
 
 error[E0690]: transparent union needs at most one non-zero-sized field, but has 2
   --> $DIR/repr-transparent.rs:85:1