Auto merge of #115334 - RalfJung:transparent-aligned-zst, r=compiler-errors

repr(transparent): it's fine if the one non-1-ZST field is a ZST

This code currently gets rejected:
```rust
#[repr(transparent)]
struct MyType([u16; 0])
```
That clearly seems like a bug to me: `repr(transparent)` [got defined ](#77841 (comment)) as having any number of 1-ZST fields plus optionally one more field; `MyType` clearly satisfies that definition.

This PR changes the `repr(transparent)` logic to actually match that definition.

Author: bors

compiler/rustc_error_codes/src/error_codes/E0691.md

@@ -1,9 +1,11 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 A struct, enum, or union with the `repr(transparent)` representation hint
 contains a zero-sized field that requires non-trivial alignment.
 
 Erroneous code example:
 
-```compile_fail,E0691
+```ignore (error is no longer emitted)
 #![feature(repr_align)]
 
 #[repr(align(32))]

compiler/rustc_hir_analysis/messages.ftl

@@ -304,13 +304,13 @@ hir_analysis_transparent_enum_variant = transparent enum needs exactly one varia
     .many_label = too many variants in `{$path}`
     .multi_label = variant here
 
-hir_analysis_transparent_non_zero_sized = transparent {$desc} needs at most one non-zero-sized field, but has {$field_count}
-    .label = needs at most one non-zero-sized field, but has {$field_count}
-    .labels = this field is non-zero-sized
+hir_analysis_transparent_non_zero_sized = transparent {$desc} needs at most one field with non-trivial size or alignment, but has {$field_count}
+    .label = needs at most one field with non-trivial size or alignment, but has {$field_count}
+    .labels = this field has non-zero size or requires alignment
 
-hir_analysis_transparent_non_zero_sized_enum = the variant of a transparent {$desc} needs at most one non-zero-sized field, but has {$field_count}
-    .label = needs at most one non-zero-sized field, but has {$field_count}
-    .labels = this field is non-zero-sized
+hir_analysis_transparent_non_zero_sized_enum = the variant of a transparent {$desc} needs at most one field with non-trivial size or alignment, but has {$field_count}
+    .label = needs at most one field with non-trivial size or alignment, but has {$field_count}
+    .labels = this field has non-zero size or requires alignment
 
 hir_analysis_typeof_reserved_keyword_used =
     `typeof` is a reserved keyword but unimplemented

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1138,19 +1138,19 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
         return;
     }
 
-    // For each field, figure out if it's known to be a ZST and align(1), with "known"
-    // respecting #[non_exhaustive] attributes.
+    // For each field, figure out if it's known to have "trivial" layout (i.e., is a 1-ZST), with
+    // "known" respecting #[non_exhaustive] attributes.
     let field_infos = adt.all_fields().map(|field| {
         let ty = field.ty(tcx, GenericArgs::identity_for_item(tcx, field.did));
         let param_env = tcx.param_env(field.did);
         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 zst = layout.is_ok_and(|layout| layout.is_zst());
-        let align = layout.ok().map(|layout| layout.align.abi.bytes());
-        if !zst {
-            return (span, zst, align, None);
+        let trivial = layout.is_ok_and(|layout| layout.is_1zst());
+        if !trivial {
+            return (span, trivial, None);
         }
+        // Even some 1-ZST fields are not allowed though, if they have `non_exhaustive`.
 
         fn check_non_exhaustive<'tcx>(
             tcx: TyCtxt<'tcx>,
@@ -1184,41 +1184,25 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
             }
         }
 
-        (span, zst, align, check_non_exhaustive(tcx, ty).break_value())
+        (span, trivial, check_non_exhaustive(tcx, ty).break_value())
     });
 
-    let non_zst_fields = field_infos
+    let non_trivial_fields = field_infos
         .clone()
-        .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()));
+        .filter_map(|(span, trivial, _non_exhaustive)| if !trivial { Some(span) } else { None });
+    let non_trivial_count = non_trivial_fields.clone().count();
+    if non_trivial_count >= 2 {
+        bad_non_zero_sized_fields(
+            tcx,
+            adt,
+            non_trivial_count,
+            non_trivial_fields,
+            tcx.def_span(adt.did()),
+        );
+        return;
     }
-    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, 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(),
-            );
-
-            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 {
+    for (span, _trivial, non_exhaustive) in field_infos {
+        if let Some((descr, def_id, args, non_exhaustive)) = non_exhaustive {
             tcx.struct_span_lint_hir(
                 REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS,
                 tcx.hir().local_def_id_to_hir_id(adt.did().expect_local()),

tests/ui/repr/repr-transparent.rs

@@ -23,23 +23,26 @@ struct ContainsMultipleZst(PhantomData<*const i32>, NoFields);
 struct ContainsZstAndNonZst((), [i32; 2]);
 
 #[repr(transparent)]
-struct MultipleNonZst(u8, u8); //~ ERROR needs at most one non-zero-sized field
+struct MultipleNonZst(u8, u8); //~ ERROR needs at most one field with non-trivial size or alignment
 
 trait Mirror { type It: ?Sized; }
 impl<T: ?Sized> Mirror for T { type It = Self; }
 
 #[repr(transparent)]
 pub struct StructWithProjection(f32, <f32 as Mirror>::It);
-//~^ ERROR needs at most one non-zero-sized field
+//~^ ERROR needs at most one field with non-trivial size or alignment
 
 #[repr(transparent)]
-struct NontrivialAlignZst(u32, [u16; 0]); //~ ERROR alignment larger than 1
+struct NontrivialAlignZst(u32, [u16; 0]); //~ ERROR needs at most one field with non-trivial size or alignment
 
 #[repr(align(32))]
 struct ZstAlign32<T>(PhantomData<T>);
 
 #[repr(transparent)]
-struct GenericAlign<T>(ZstAlign32<T>, u32); //~ ERROR alignment larger than 1
+struct GenericAlign<T>(ZstAlign32<T>, u32); //~ ERROR needs at most one field with non-trivial size or alignment
+
+#[repr(transparent)]
+struct WrapsZstWithAlignment([i32; 0]);
 
 #[repr(transparent)] //~ ERROR unsupported representation for zero-variant enum
 enum Void {} //~ ERROR transparent enum needs exactly one variant, but has 0
@@ -58,7 +61,7 @@ enum UnitFieldEnum {
 enum TooManyFieldsEnum {
     Foo(u32, String),
 }
-//~^^^ ERROR transparent enum needs at most one non-zero-sized field, but has 2
+//~^^^ ERROR transparent enum needs at most one field with non-trivial size or alignment, but has 2
 
 #[repr(transparent)]
 enum MultipleVariants { //~ ERROR transparent enum needs exactly one variant, but has 2
@@ -67,13 +70,13 @@ enum MultipleVariants { //~ ERROR transparent enum needs exactly one variant, bu
 }
 
 #[repr(transparent)]
-enum NontrivialAlignZstEnum {
-    Foo(u32, [u16; 0]), //~ ERROR alignment larger than 1
+enum NontrivialAlignZstEnum { //~ ERROR needs at most one field with non-trivial size or alignment
+    Foo(u32, [u16; 0]),
 }
 
 #[repr(transparent)]
-enum GenericAlignEnum<T> {
-    Foo { bar: ZstAlign32<T>, baz: u32 } //~ ERROR alignment larger than 1
+enum GenericAlignEnum<T> { //~ ERROR needs at most one field with non-trivial size or alignment
+    Foo { bar: ZstAlign32<T>, baz: u32 }
 }
 
 #[repr(transparent)]
@@ -82,7 +85,7 @@ union UnitUnion {
 }
 
 #[repr(transparent)]
-union TooManyFields { //~ ERROR transparent union needs at most one non-zero-sized field, but has 2
+union TooManyFields { //~ ERROR transparent union needs at most one field with non-trivial size or alignment, but has 2
     u: u32,
     s: i32
 }

tests/ui/repr/repr-transparent.stderr

@@ -1,59 +1,65 @@
-error[E0690]: transparent struct needs at most one non-zero-sized field, but has 2
+error[E0690]: transparent struct needs at most one field with non-trivial size or alignment, but has 2
   --> $DIR/repr-transparent.rs:26:1
    |
 LL | struct MultipleNonZst(u8, u8);
-   | ^^^^^^^^^^^^^^^^^^^^^ --  -- this field is non-zero-sized
+   | ^^^^^^^^^^^^^^^^^^^^^ --  -- this field has non-zero size or requires alignment
    | |                     |
-   | |                     this field is non-zero-sized
-   | needs at most one non-zero-sized field, but has 2
+   | |                     this field has non-zero size or requires alignment
+   | needs at most one field with non-trivial size or alignment, but has 2
 
-error[E0690]: transparent struct needs at most one non-zero-sized field, but has 2
+error[E0690]: transparent struct needs at most one field with non-trivial size or alignment, but has 2
   --> $DIR/repr-transparent.rs:32:1
    |
 LL | pub struct StructWithProjection(f32, <f32 as Mirror>::It);
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ---  ------------------- this field is non-zero-sized
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ---  ------------------- this field has non-zero size or requires alignment
    | |                               |
-   | |                               this field is non-zero-sized
-   | needs at most one non-zero-sized field, but has 2
+   | |                               this field has non-zero size or requires alignment
+   | needs at most one field with non-trivial size or alignment, but has 2
 
-error[E0691]: zero-sized field in transparent struct has alignment larger than 1
-  --> $DIR/repr-transparent.rs:36:32
+error[E0690]: transparent struct needs at most one field with non-trivial size or alignment, but has 2
+  --> $DIR/repr-transparent.rs:36:1
    |
 LL | struct NontrivialAlignZst(u32, [u16; 0]);
-   |                                ^^^^^^^^ has alignment of 2, which is larger than 1
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ ---  -------- this field has non-zero size or requires alignment
+   | |                         |
+   | |                         this field has non-zero size or requires alignment
+   | needs at most one field with non-trivial size or alignment, but has 2
 
-error[E0691]: zero-sized field in transparent struct has alignment larger than 1
-  --> $DIR/repr-transparent.rs:42:24
+error[E0690]: transparent struct needs at most one field with non-trivial size or alignment, but has 2
+  --> $DIR/repr-transparent.rs:42:1
    |
 LL | struct GenericAlign<T>(ZstAlign32<T>, u32);
-   |                        ^^^^^^^^^^^^^ has alignment of 32, which is larger than 1
+   | ^^^^^^^^^^^^^^^^^^^^^^ -------------  --- this field has non-zero size or requires alignment
+   | |                      |
+   | |                      this field has non-zero size or requires alignment
+   | needs at most one field with non-trivial size or alignment, but has 2
 
 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
+error[E0690]: the variant of a transparent enum needs at most one field with non-trivial size or alignment, but has 2
+  --> $DIR/repr-transparent.rs:61:1
    |
 LL | enum TooManyFieldsEnum {
-   | ^^^^^^^^^^^^^^^^^^^^^^ needs at most one non-zero-sized field, but has 2
+   | ^^^^^^^^^^^^^^^^^^^^^^ needs at most one field with non-trivial size or alignment, but has 2
 LL |     Foo(u32, String),
-   |         ---  ------ this field is non-zero-sized
+   |         ---  ------ this field has non-zero size or requires alignment
    |         |
-   |         this field is non-zero-sized
+   |         this field has non-zero size or requires alignment
 
 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
@@ -62,29 +68,37 @@ LL |     Foo(String),
 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
+error[E0690]: the variant of a transparent enum needs at most one field with non-trivial size or alignment, but has 2
+  --> $DIR/repr-transparent.rs:73:1
    |
+LL | enum NontrivialAlignZstEnum {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ needs at most one field with non-trivial size or alignment, but has 2
 LL |     Foo(u32, [u16; 0]),
-   |              ^^^^^^^^ has alignment of 2, which is larger than 1
+   |         ---  -------- this field has non-zero size or requires alignment
+   |         |
+   |         this field has non-zero size or requires alignment
 
-error[E0691]: zero-sized field in transparent enum has alignment larger than 1
-  --> $DIR/repr-transparent.rs:76:11
+error[E0690]: the variant of a transparent enum needs at most one field with non-trivial size or alignment, but has 2
+  --> $DIR/repr-transparent.rs:78:1
    |
+LL | enum GenericAlignEnum<T> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^ needs at most one field with non-trivial size or alignment, but has 2
 LL |     Foo { bar: ZstAlign32<T>, baz: u32 }
-   |           ^^^^^^^^^^^^^^^^^^ has alignment of 32, which is larger than 1
+   |           ------------------  -------- this field has non-zero size or requires alignment
+   |           |
+   |           this field has non-zero size or requires alignment
 
-error[E0690]: transparent union needs at most one non-zero-sized field, but has 2
-  --> $DIR/repr-transparent.rs:85:1
+error[E0690]: transparent union needs at most one field with non-trivial size or alignment, but has 2
+  --> $DIR/repr-transparent.rs:88:1
    |
 LL | union TooManyFields {
-   | ^^^^^^^^^^^^^^^^^^^ needs at most one non-zero-sized field, but has 2
+   | ^^^^^^^^^^^^^^^^^^^ needs at most one field with non-trivial size or alignment, but has 2
 LL |     u: u32,
-   |     ------ this field is non-zero-sized
+   |     ------ this field has non-zero size or requires alignment
 LL |     s: i32
-   |     ------ this field is non-zero-sized
+   |     ------ this field has non-zero size or requires alignment
 
 error: aborting due to 11 previous errors
 
-Some errors have detailed explanations: E0084, E0690, E0691, E0731.
+Some errors have detailed explanations: E0084, E0690, E0731.
 For more information about an error, try `rustc --explain E0084`.