compiler/hir/check: clarify error message on transparent ADT

Replace the phrase "layout of non-zero size or alignment larger than 1"
with "non-trivial layout", and then elaborate that a layout is trivial
if, and only if, it is zero-sized with an alignment of 1.

Additionally, add a comment to the check-function explaining which
information is gathered to check transparent ADTs, and elaborate why it
is needed.

Author: dvdhrm

compiler/rustc_error_codes/src/error_codes/E0690.md

@@ -5,8 +5,8 @@ Erroneous code example:
 
 ```compile_fail,E0690
 #[repr(transparent)]
-struct LengthWithUnit<U> { // error: transparent struct needs at most one
-    value: f32,            //        non-zero-sized field, but has 2
+struct LengthWithUnit<U> { // error: transparent struct needs at most one field
+    value: f32,            //        with non-trivial layout, but has 2
     unit: U,
 }
 ```

compiler/rustc_hir_analysis/messages.ftl

@@ -270,15 +270,17 @@ 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_layout = transparent {$desc} needs at most one field of non-zero size or alignment larger than 1, but has {$field_count}
-    .label = needs at most one field of non-zero size or alignment larger than 1, but has {$field_count}
+hir_analysis_transparent_layout = transparent {$desc} needs at most one field with non-trivial layout, but has {$field_count}
+    .label = needs at most one field with non-trivial layout, but has {$field_count}
     .non_layout_labels = this field may have an alignment larger than 1
     .non_zst_labels = this field is non-zero-sized
+    .note = a layout is trivial if, any only if, it is zero-sized with an alignment of 1
 
-hir_analysis_transparent_layout_enum = the variant of a transparent {$desc} needs at most one field of non-zero size or alignment larger than 1, but has {$field_count}
-    .label = needs at most one field of non-zero size or alignment larger than 1, but has {$field_count}
+hir_analysis_transparent_layout_enum = the variant of a transparent {$desc} needs at most one field with non-trivial layout, but has {$field_count}
+    .label = needs at most one field with non-trivial layout, but has {$field_count}
     .non_layout_labels = this field may have an alignment larger than 1
     .non_zst_labels = this field is non-zero-sized
+    .note = a layout is trivial if, any only if, it is zero-sized with an alignment of 1
 
 hir_analysis_typeof_reserved_keyword_used =
     `typeof` is a reserved keyword but unimplemented

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1130,8 +1130,8 @@ 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 with layout, with "known"
-    // respecting #[non_exhaustive] attributes.
+    // For each field, collect information on its layout and whether it has a #[non_exhaustive]
+    // annotation.
     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);
@@ -1178,6 +1178,13 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
         (span, zst, layout, check_non_exhaustive(tcx, ty).break_value())
     });
 
+    // For a transparent ADT we must know which member field controls its layout. Hence, we only
+    // allow at most one field with non-trivial or unknown layout. A layout is trivial if, and only
+    // if, it is zero-sized with an alignment of 1. A layout is unknown if, for instance, it
+    // contains generic type parameters or has a #[non_exhaustive] annotation in a non-local type.
+    // We will reject any transparent ADT with more than one field with non-trivial or unknown
+    // layout, but we try our best to point out the violating properties.
+
     let non_layout_fields = field_infos
         .clone()
         .filter_map(|(span, _, layout, _)| if layout.is_err() { Some(span) } else { None });

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -516,7 +516,7 @@ fn bad_variant_count<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>, sp: Span, d
     });
 }
 
-/// Emit an error when encountering two or more fields without layout or non-zero size
+/// Emit an error when encountering two or more fields with non-trivial layout
 /// in a transparent Adt.
 fn bad_transparent_layout<'tcx>(
     tcx: TyCtxt<'tcx>,
@@ -533,6 +533,7 @@ fn bad_transparent_layout<'tcx>(
             non_zst_spans: non_zst_spans.collect(),
             field_count,
             desc: adt.descr(),
+            note: (),
         });
     } else {
         tcx.sess.emit_err(errors::TransparentLayout {
@@ -541,6 +542,7 @@ fn bad_transparent_layout<'tcx>(
             non_zst_spans: non_zst_spans.collect(),
             field_count,
             desc: adt.descr(),
+            note: (),
         });
     }
 }

compiler/rustc_hir_analysis/src/errors.rs

@@ -788,6 +788,8 @@ pub(crate) struct TransparentLayoutEnum<'a> {
     pub non_zst_spans: Vec<Span>,
     pub field_count: usize,
     pub desc: &'a str,
+    #[note]
+    pub note: (),
 }
 
 #[derive(Diagnostic)]
@@ -802,6 +804,8 @@ pub(crate) struct TransparentLayout<'a> {
     pub non_zst_spans: Vec<Span>,
     pub field_count: usize,
     pub desc: &'a str,
+    #[note]
+    pub note: (),
 }
 
 #[derive(Diagnostic)]

tests/ui/repr/repr-transparent.rs

@@ -24,14 +24,14 @@ struct ContainsZstAndNonZst((), [i32; 2]);
 
 #[repr(transparent)]
 struct MultipleNonZst(u8, u8);
-//~^ ERROR needs at most one field of non-zero size or alignment larger than 1
+//~^ ERROR needs at most one field with non-trivial layout
 
 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 field of non-zero size or alignment larger than 1
+//~^ ERROR needs at most one field with non-trivial layout
 
 #[repr(transparent)]
 struct NontrivialAlignZst(u32, [u16; 0]); //~ ERROR alignment larger than 1
@@ -59,7 +59,7 @@ enum UnitFieldEnum {
 enum TooManyFieldsEnum {
     Foo(u32, String),
 }
-//~^^^ ERROR needs at most one field of non-zero size or alignment larger than 1, but has 2
+//~^^^ ERROR needs at most one field with non-trivial layout, but has 2
 
 #[repr(transparent)]
 enum MultipleVariants { //~ ERROR transparent enum needs exactly one variant, but has 2
@@ -87,6 +87,6 @@ union TooManyFields {
     u: u32,
     s: i32
 }
-//~^^^^ ERROR transparent union needs at most one field of non-zero size or alignment larger than 1, but has 2
+//~^^^^ ERROR transparent union needs at most one field with non-trivial layout, but has 2
 
 fn main() {}

tests/ui/repr/repr-transparent.stderr

@@ -1,20 +1,24 @@
-error[E0690]: transparent struct needs at most one field of non-zero size or alignment larger than 1, but has 2
+error[E0690]: transparent struct needs at most one field with non-trivial layout, but has 2
   --> $DIR/repr-transparent.rs:26:1
    |
 LL | struct MultipleNonZst(u8, u8);
    | ^^^^^^^^^^^^^^^^^^^^^ --  -- this field is non-zero-sized
    | |                     |
    | |                     this field is non-zero-sized
-   | needs at most one field of non-zero size or alignment larger than 1, but has 2
+   | needs at most one field with non-trivial layout, but has 2
+   |
+   = note: a layout is trivial if, any only if, it is zero-sized with an alignment of 1
 
-error[E0690]: transparent struct needs at most one field of non-zero size or alignment larger than 1, but has 2
+error[E0690]: transparent struct needs at most one field with non-trivial layout, but has 2
   --> $DIR/repr-transparent.rs:33:1
    |
 LL | pub struct StructWithProjection(f32, <f32 as Mirror>::It);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ---  ------------------- this field is non-zero-sized
    | |                               |
    | |                               this field is non-zero-sized
-   | needs at most one field of non-zero size or alignment larger than 1, but has 2
+   | needs at most one field with non-trivial layout, but has 2
+   |
+   = note: a layout is trivial if, any only if, it is zero-sized with an alignment of 1
 
 error[E0691]: zero-sized field in transparent struct has alignment larger than 1
   --> $DIR/repr-transparent.rs:37:32
@@ -42,15 +46,17 @@ error[E0731]: transparent enum needs exactly one variant, but has 0
 LL | enum Void {}
    | ^^^^^^^^^ needs exactly one variant, but has 0
 
-error[E0690]: the variant of a transparent enum needs at most one field of non-zero size or alignment larger than 1, but has 2
+error[E0690]: the variant of a transparent enum needs at most one field with non-trivial layout, but has 2
   --> $DIR/repr-transparent.rs:59:1
    |
 LL | enum TooManyFieldsEnum {
-   | ^^^^^^^^^^^^^^^^^^^^^^ needs at most one field of non-zero size or alignment larger than 1, but has 2
+   | ^^^^^^^^^^^^^^^^^^^^^^ needs at most one field with non-trivial layout, but has 2
 LL |     Foo(u32, String),
    |         ---  ------ this field is non-zero-sized
    |         |
    |         this field is non-zero-sized
+   |
+   = note: a layout is trivial if, any only if, it is zero-sized with an alignment of 1
 
 error[E0731]: transparent enum needs exactly one variant, but has 2
   --> $DIR/repr-transparent.rs:65:1
@@ -74,15 +80,17 @@ error[E0691]: zero-sized field in transparent enum has alignment larger than 1
 LL |     Foo { bar: ZstAlign32<T>, baz: u32 }
    |           ^^^^^^^^^^^^^^^^^^ has alignment of 32, which is larger than 1
 
-error[E0690]: transparent union needs at most one field of non-zero size or alignment larger than 1, but has 2
+error[E0690]: transparent union needs at most one field with non-trivial layout, but has 2
   --> $DIR/repr-transparent.rs:86:1
    |
 LL | union TooManyFields {
-   | ^^^^^^^^^^^^^^^^^^^ needs at most one field of non-zero size or alignment larger than 1, but has 2
+   | ^^^^^^^^^^^^^^^^^^^ needs at most one field with non-trivial layout, but has 2
 LL |     u: u32,
    |     ------ this field is non-zero-sized
 LL |     s: i32
    |     ------ this field is non-zero-sized
+   |
+   = note: a layout is trivial if, any only if, it is zero-sized with an alignment of 1
 
 error: aborting due to 11 previous errors