Auto merge of #86279 - JohnTitor:transparent-zero-size-fields, r=nikomatsakis

Permit zero non-zero-field on transparent types

Fixes #77841

This makes the transparent fields meet the below:
> * A `repr(transparent)` type `T` must meet the following rules:
>   * It may have any number of 1-ZST fields
>   * In addition, it may have at most one other field of type U

r? `@nikomatsakis`

2 files changed, 42 insertions, 72 deletions

diff --git a/src/test/ui/repr/repr-transparent.rs b/src/test/ui/repr/repr-transparent.rs
index 8fbdb4cc80b..8c9d1639c0a 100644
--- a/src/test/ui/repr/repr-transparent.rs
+++ b/src/test/ui/repr/repr-transparent.rs
@@ -8,27 +8,29 @@
 use std::marker::PhantomData;
 
 #[repr(transparent)]
-struct NoFields; //~ ERROR needs exactly one non-zero-sized field
+struct NoFields;
 
 #[repr(transparent)]
-struct ContainsOnlyZst(()); //~ ERROR needs exactly one non-zero-sized field
+struct ContainsOnlyZst(());
 
 #[repr(transparent)]
-struct ContainsOnlyZstArray([bool; 0]); //~ ERROR needs exactly one non-zero-sized field
+struct ContainsOnlyZstArray([bool; 0]);
 
 #[repr(transparent)]
 struct ContainsMultipleZst(PhantomData<*const i32>, NoFields);
-//~^ ERROR needs exactly one non-zero-sized field
 
 #[repr(transparent)]
-struct MultipleNonZst(u8, u8); //~ ERROR needs exactly one non-zero-sized field
+struct ContainsZstAndNonZst((), [i32; 2]);
+
+#[repr(transparent)]
+struct MultipleNonZst(u8, u8); //~ ERROR needs at most one non-zero-sized field
 
 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 exactly one non-zero-sized field
+//~^ ERROR needs at most one non-zero-sized field
 
 #[repr(transparent)]
 struct NontrivialAlignZst(u32, [u16; 0]); //~ ERROR alignment larger than 1
@@ -40,22 +42,26 @@ struct ZstAlign32<T>(PhantomData<T>);
 struct GenericAlign<T>(ZstAlign32<T>, 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
+enum Void {} //~ ERROR transparent enum needs exactly one variant, but has 0
 
 #[repr(transparent)]
-enum FieldlessEnum { //~ ERROR transparent enum needs exactly one non-zero-sized field, but has 0
+enum FieldlessEnum {
     Foo,
 }
 
 #[repr(transparent)]
+enum UnitFieldEnum {
+    Foo(()),
+}
+
+#[repr(transparent)]
 enum TooManyFieldsEnum {
     Foo(u32, String),
 }
-//~^^^ ERROR transparent enum needs exactly one non-zero-sized field, but has 2
+//~^^^ ERROR transparent enum needs at most one non-zero-sized field, but has 2
 
 #[repr(transparent)]
-enum TooManyVariants { //~ ERROR transparent enum needs exactly one variant, but has 2
+enum MultipleVariants { //~ ERROR transparent enum needs exactly one variant, but has 2
     Foo(String),
     Bar,
 }
@@ -71,12 +77,12 @@ enum GenericAlignEnum<T> {
 }
 
 #[repr(transparent)]
-union UnitUnion { //~ ERROR transparent union needs exactly one non-zero-sized field, but has 0
+union UnitUnion {
     u: (),
 }
 
 #[repr(transparent)]
-union TooManyFields { //~ ERROR transparent union needs exactly one non-zero-sized field, but has 2
+union TooManyFields { //~ ERROR transparent union needs at most one non-zero-sized field, but has 2
     u: u32,
     s: i32
 }
diff --git a/src/test/ui/repr/repr-transparent.stderr b/src/test/ui/repr/repr-transparent.stderr
index cbc74fbb6a2..001a181881f 100644
--- a/src/test/ui/repr/repr-transparent.stderr
+++ b/src/test/ui/repr/repr-transparent.stderr
@@ -1,61 +1,37 @@
-error[E0690]: transparent struct needs exactly one non-zero-sized field, but has 0
-  --> $DIR/repr-transparent.rs:11:1
-   |
-LL | struct NoFields;
-   | ^^^^^^^^^^^^^^^^ needs exactly one non-zero-sized field, but has 0
-
-error[E0690]: transparent struct needs exactly one non-zero-sized field, but has 0
-  --> $DIR/repr-transparent.rs:14:1
-   |
-LL | struct ContainsOnlyZst(());
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ needs exactly one non-zero-sized field, but has 0
-
-error[E0690]: transparent struct needs exactly one non-zero-sized field, but has 0
-  --> $DIR/repr-transparent.rs:17:1
-   |
-LL | struct ContainsOnlyZstArray([bool; 0]);
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ needs exactly one non-zero-sized field, but has 0
-
-error[E0690]: transparent struct needs exactly one non-zero-sized field, but has 0
-  --> $DIR/repr-transparent.rs:20:1
-   |
-LL | struct ContainsMultipleZst(PhantomData<*const i32>, NoFields);
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ needs exactly one non-zero-sized field, but has 0
-
-error[E0690]: transparent struct needs exactly one non-zero-sized field, but has 2
-  --> $DIR/repr-transparent.rs:24:1
+error[E0690]: transparent struct needs at most one non-zero-sized field, 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 exactly one non-zero-sized field, but has 2
+   | needs at most one non-zero-sized field, but has 2
 
-error[E0690]: transparent struct needs exactly one non-zero-sized field, but has 2
-  --> $DIR/repr-transparent.rs:30:1
+error[E0690]: transparent struct needs at most one non-zero-sized field, 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 is non-zero-sized
-   | needs exactly one non-zero-sized field, but has 2
+   | needs at most one non-zero-sized field, but has 2
 
 error[E0691]: zero-sized field in transparent struct has alignment larger than 1
-  --> $DIR/repr-transparent.rs:34:32
+  --> $DIR/repr-transparent.rs:36:32
    |
 LL | struct NontrivialAlignZst(u32, [u16; 0]);
    |                                ^^^^^^^^ has alignment larger than 1
 
 error[E0691]: zero-sized field in transparent struct has alignment larger than 1
-  --> $DIR/repr-transparent.rs:40:24
+  --> $DIR/repr-transparent.rs:42:24
    |
 LL | struct GenericAlign<T>(ZstAlign32<T>, u32);
    |                        ^^^^^^^^^^^^^ has alignment larger than 1
 
 error[E0084]: unsupported representation for zero-variant enum
-  --> $DIR/repr-transparent.rs:42:1
+  --> $DIR/repr-transparent.rs:44:1
    |
 LL | #[repr(transparent)]
    | ^^^^^^^^^^^^^^^^^^^^
@@ -63,66 +39,54 @@ LL | enum Void {}
    | ------------ zero-variant enum
 
 error[E0731]: transparent enum needs exactly one variant, but has 0
-  --> $DIR/repr-transparent.rs:43:1
+  --> $DIR/repr-transparent.rs:45:1
    |
 LL | enum Void {}
    | ^^^^^^^^^ needs exactly one variant, but has 0
 
-error[E0690]: the variant of a transparent enum needs exactly one non-zero-sized field, but has 0
-  --> $DIR/repr-transparent.rs:47:1
-   |
-LL | enum FieldlessEnum {
-   | ^^^^^^^^^^^^^^^^^^ needs exactly one non-zero-sized field, but has 0
-
-error[E0690]: the variant of a transparent enum needs exactly one non-zero-sized field, but has 2
-  --> $DIR/repr-transparent.rs:52:1
+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
    |
 LL | enum TooManyFieldsEnum {
-   | ^^^^^^^^^^^^^^^^^^^^^^ needs exactly one non-zero-sized field, but has 2
+   | ^^^^^^^^^^^^^^^^^^^^^^ needs at most one non-zero-sized field, but has 2
 LL |     Foo(u32, String),
    |         ---  ------ this field is non-zero-sized
    |         |
    |         this field is non-zero-sized
 
 error[E0731]: transparent enum needs exactly one variant, but has 2
-  --> $DIR/repr-transparent.rs:58:1
+  --> $DIR/repr-transparent.rs:64:1
    |
-LL | enum TooManyVariants {
-   | ^^^^^^^^^^^^^^^^^^^^ needs exactly one variant, but has 2
+LL | enum MultipleVariants {
+   | ^^^^^^^^^^^^^^^^^^^^^ needs exactly one variant, but has 2
 LL |     Foo(String),
    |     -----------
 LL |     Bar,
-   |     --- too many variants in `TooManyVariants`
+   |     --- too many variants in `MultipleVariants`
 
 error[E0691]: zero-sized field in transparent enum has alignment larger than 1
-  --> $DIR/repr-transparent.rs:65:14
+  --> $DIR/repr-transparent.rs:71:14
    |
 LL |     Foo(u32, [u16; 0]),
    |              ^^^^^^^^ has alignment larger than 1
 
 error[E0691]: zero-sized field in transparent enum has alignment larger than 1
-  --> $DIR/repr-transparent.rs:70:11
+  --> $DIR/repr-transparent.rs:76:11
    |
 LL |     Foo { bar: ZstAlign32<T>, baz: u32 }
    |           ^^^^^^^^^^^^^^^^^^ has alignment larger than 1
 
-error[E0690]: transparent union needs exactly one non-zero-sized field, but has 0
-  --> $DIR/repr-transparent.rs:74:1
-   |
-LL | union UnitUnion {
-   | ^^^^^^^^^^^^^^^ needs exactly one non-zero-sized field, but has 0
-
-error[E0690]: transparent union needs exactly one non-zero-sized field, but has 2
-  --> $DIR/repr-transparent.rs:79:1
+error[E0690]: transparent union needs at most one non-zero-sized field, but has 2
+  --> $DIR/repr-transparent.rs:85:1
    |
 LL | union TooManyFields {
-   | ^^^^^^^^^^^^^^^^^^^ needs exactly one non-zero-sized field, but has 2
+   | ^^^^^^^^^^^^^^^^^^^ needs at most one non-zero-sized field, but has 2
 LL |     u: u32,
    |     ------ this field is non-zero-sized
 LL |     s: i32
    |     ------ this field is non-zero-sized
 
-error: aborting due to 17 previous errors
+error: aborting due to 11 previous errors
 
 Some errors have detailed explanations: E0084, E0690, E0691, E0731.
 For more information about an error, try `rustc --explain E0084`.