Improve diagnostic for E0691

compiler/rustc_typeck/src/check/check.rs

@@ -20,7 +20,9 @@ use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::layout::{LayoutError, MAX_SIMD_LANES};
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::util::{Discr, IntTypeExt};
-use rustc_middle::ty::{self, ParamEnv, ToPredicate, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable};
+use rustc_middle::ty::{
+ self, Array, ParamEnv, ToPredicate, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable,
+};
 use rustc_session::lint::builtin::{UNINHABITED_STATIC, UNSUPPORTED_CALLING_CONVENTIONS};
 use rustc_span::symbol::sym;
 use rustc_span::{self, Span};
@@ -1204,17 +1206,17 @@ pub(super) fn check_packed(tcx: TyCtxt<'_>, sp: Span, def: ty::AdtDef<'_>) {
  for attr in tcx.get_attrs(def.did(), sym::repr) {
  for r in attr::parse_repr_attr(&tcx.sess, attr) {
  if let attr::ReprPacked(pack) = r
- && let Some(repr_pack) = repr.pack
- && pack as u64 != repr_pack.bytes()
- {
-  struct_span_err!(
-  tcx.sess,
-  sp,
-  E0634,
-  "type has conflicting packed representation hints"
-  )
-  .emit();
- }
+  && let Some(repr_pack) = repr.pack
+  && pack as u64 != repr_pack.bytes()
+  {
+ struct_span_err!(
+ tcx.sess,
+ sp,
+ E0634,
+ "type has conflicting packed representation hints"
+ )
+ .emit();
+  }
  }
  }
  if repr.align.is_some() {
@@ -1327,28 +1329,58 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, sp: Span, adt: ty::AdtD
  let layout = tcx.layout_of(param_env.and(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 zero_sized_array = match ty.kind() {
+ Array(_, len) => len.try_eval_usize(tcx, param_env) == Some(0),
+ _ => false,
+ };
  let zst = layout.map_or(false, |layout| layout.is_zst());
- let align1 = layout.map_or(false, |layout| layout.align.abi.bytes() == 1);
- (span, zst, align1)
+ let align = layout.ok().map(|layout| layout.align.abi.bytes());
+ (span, zst, zero_sized_array, align)
  });
 
  let non_zst_fields =
- field_infos.clone().filter_map(|(span, zst, _align1)| if !zst { Some(span) } else { None });
+ field_infos.clone().filter_map(
+ |(span, zst, zero_sized_array, _align)| {
+ if !zst && !zero_sized_array { 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, sp);
  }
- for (span, zst, align1) in field_infos {
- if zst && !align1 {
- struct_span_err!(
+
+ for (span, zst, zero_sized_array, align) in field_infos {
+ if (zst || zero_sized_array) && 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) = align {
+ err.span_label(span, format!("has alignment of {align}, which is larger than 1"));
+ } else {
+ err.span_label(span, "may have alignment larger than 1");
+ };
+
+ if let Some(item_list) =
+ tcx.get_attr(adt.did(), sym::repr).and_then(|attr| attr.meta_item_list())
+ {
+ for item in item_list {
+ if item.name_or_empty() == sym::transparent {
+ err.span_suggestion_short(
+ item.span(),
+ "Try using `repr(C)` instead",
+ "C",
+ Applicability::MachineApplicable,
+ );
+ }
+ }
+ }
+
+ err.emit();
  }
  }
 }
@@ -1475,7 +1507,7 @@ fn format_discriminant_overflow<'tcx>(
  && let rustc_ast::LitKind::Int(lit_value, _int_kind) = &lit.node
  && dis.val != *lit_value
  {
-  return format!("`{dis}` (overflowed from `{lit_value}`)");
+ return format!("`{dis}` (overflowed from `{lit_value}`)");
  }
  }
 

src/test/ui/repr/repr-transparent.rs

@@ -41,6 +41,9 @@ struct ZstAlign32<T>(PhantomData<T>);
 #[repr(transparent)]
 struct GenericAlign<T>(ZstAlign32<T>, u32); //~ ERROR alignment larger than 1
 
+#[repr(transparent)]
+struct GenericAlignZeroArray<T>([T; 0], u32); //~ ERROR alignment larger than 1
+
 #[repr(transparent)] //~ ERROR unsupported representation for zero-variant enum
 enum Void {} //~ ERROR transparent enum needs exactly one variant, but has 0
 
@@ -76,6 +79,11 @@ enum GenericAlignEnum<T> {
  Foo { bar: ZstAlign32<T>, baz: u32 } //~ ERROR alignment larger than 1
 }
 
+#[repr(transparent)]
+enum GenericAlignEnumZeroArray<T> {
+ Foo { bar: [T; 0], baz: u32 } //~ ERROR alignment larger than 1
+}
+
 #[repr(transparent)]
 union UnitUnion {
  u: (),

src/test/ui/repr/repr-transparent.stderr

@@ -21,31 +21,43 @@ LL | pub struct StructWithProjection(f32, <f32 as Mirror>::It);
 error[E0691]: zero-sized field in transparent struct has alignment larger than 1
  --> $DIR/repr-transparent.rs:36:32
  |
+LL | #[repr(transparent)]
+ | ----------- help: Try using `repr(C)` instead
 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 | #[repr(transparent)]
+ | ----------- help: Try using `repr(C)` instead
 LL | struct GenericAlign<T>(ZstAlign32<T>, u32);
- | ^^^^^^^^^^^^^ has alignment larger than 1
+ | ^^^^^^^^^^^^^ has alignment of 32, which is larger than 1
+
+error[E0691]: zero-sized field in transparent struct has alignment larger than 1
+ --> $DIR/repr-transparent.rs:45:33
+ |
+LL | #[repr(transparent)]
+ | ----------- help: Try using `repr(C)` instead
+LL | struct GenericAlignZeroArray<T>([T; 0], u32);
+ | ^^^^^^ may have alignment larger than 1
 
 error[E0084]: unsupported representation for zero-variant enum
- --> $DIR/repr-transparent.rs:44:1
+ --> $DIR/repr-transparent.rs:47:1
  |
 LL | #[repr(transparent)]
  | ^^^^^^^^^^^^^^^^^^^^
 LL | enum Void {}
  | ------------ zero-variant enum
 
 error[E0731]: transparent enum needs exactly one variant, but has 0
- --> $DIR/repr-transparent.rs:45:1
+ --> $DIR/repr-transparent.rs:48:1
  |
 LL | enum Void {}
  | ^^^^^^^^^ needs exactly one variant, but has 0
 
 error[E0690]: the variant of a transparent enum needs at most one non-zero-sized field, but has 2
- --> $DIR/repr-transparent.rs:58:1
+ --> $DIR/repr-transparent.rs:61:1
  |
 LL | enum TooManyFieldsEnum {
  | ^^^^^^^^^^^^^^^^^^^^^^ needs at most one non-zero-sized field, but has 2
@@ -55,7 +67,7 @@ LL | Foo(u32, String),
  | this field is non-zero-sized
 
 error[E0731]: transparent enum needs exactly one variant, but has 2
- --> $DIR/repr-transparent.rs:64:1
+ --> $DIR/repr-transparent.rs:67:1
  |
 LL | enum MultipleVariants {
  | ^^^^^^^^^^^^^^^^^^^^^ needs exactly one variant, but has 2
@@ -65,19 +77,34 @@ LL | Bar,
  | --- too many variants in `MultipleVariants`
 
 error[E0691]: zero-sized field in transparent enum has alignment larger than 1
- --> $DIR/repr-transparent.rs:71:14
+ --> $DIR/repr-transparent.rs:74:14
  |
+LL | #[repr(transparent)]
+ | ----------- help: Try using `repr(C)` instead
+LL | enum NontrivialAlignZstEnum {
 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
+ --> $DIR/repr-transparent.rs:79:11
  |
+LL | #[repr(transparent)]
+ | ----------- help: Try using `repr(C)` instead
+LL | enum GenericAlignEnum<T> {
 LL | Foo { bar: ZstAlign32<T>, baz: u32 }
- | ^^^^^^^^^^^^^^^^^^ has alignment larger than 1
+ | ^^^^^^^^^^^^^^^^^^ has alignment of 32, which is larger than 1
+
+error[E0691]: zero-sized field in transparent enum has alignment larger than 1
+ --> $DIR/repr-transparent.rs:84:11
+ |
+LL | #[repr(transparent)]
+ | ----------- help: Try using `repr(C)` instead
+LL | enum GenericAlignEnumZeroArray<T> {
+LL | Foo { bar: [T; 0], baz: u32 }
+ | ^^^^^^^^^^^ may have alignment larger than 1
 
 error[E0690]: transparent union needs at most one non-zero-sized field, but has 2
- --> $DIR/repr-transparent.rs:85:1
+ --> $DIR/repr-transparent.rs:93:1
  |
 LL | union TooManyFields {
  | ^^^^^^^^^^^^^^^^^^^ needs at most one non-zero-sized field, but has 2
@@ -86,7 +113,7 @@ LL | u: u32,
 LL | s: i32
  | ------ this field is non-zero-sized
 
-error: aborting due to 11 previous errors
+error: aborting due to 13 previous errors
 
 Some errors have detailed explanations: E0084, E0690, E0691, E0731.
 For more information about an error, try `rustc --explain E0084`.