Add test to check order of repr(int) enum fields

RFC #2195 specifies that a repr(int) enum such as:

    #[repr(u8)]
    enum MyEnum {
        B { x: u8, y: i16, z: u8 },
    }

has a layout that is equivalent to:

    #[repr(C)]
    enum MyEnumVariantB { tag: u8, x: u8, y: i16, z: u8 },

However this isn't actually implemented, with the actual layout being
roughly equivalent to:

    union MyEnumPayload {
        B { x: u8, y: i16, z: u8 },
    }

    #[repr(packed)]
    struct MyEnum {
        tag: u8,
        payload: MyEnumPayload,
    }

Thus the variant payload is *not* subject to repr(C) ordering rules, and
gets re-ordered as `{ x: u8, z: u8, z: i16 }`

The existing tests added in pull-req #45688 fail to catch this as the
repr(C) ordering just happens to match the current Rust ordering in this
case; adding a third field reveals the problem.

Author: petertodd

src/test/run-pass/structs-enums/enum-non-c-like-repr-c-and-int.rs

@@ -20,11 +20,12 @@ use std::mem;
 #[repr(C, u8)]
 #[derive(Copy, Clone, Eq, PartialEq, Debug)]
 enum MyEnum {
-    A(u32),                 // Single primitive value
-    B { x: u8, y: i16 },    // Composite, and the offset of `y` depends on tag being internal
-    C,                      // Empty
-    D(Option<u32>),         // Contains an enum
-    E(Duration),            // Contains a struct
+    A(u32),                     // Single primitive value
+    B { x: u8, y: i16, z: u8 }, // Composite, and the offsets of `y` and `z`
+                                // depend on tag being internal
+    C,                          // Empty
+    D(Option<u32>),             // Contains an enum
+    E(Duration),                // Contains a struct
 }
 
 #[repr(C)]
@@ -44,14 +45,14 @@ union MyEnumPayload {
 
 #[repr(u8)] #[derive(Copy, Clone)] enum MyEnumTag { A, B, C, D, E }
 #[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantA(u32);
-#[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantB {x: u8, y: i16 }
+#[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantB {x: u8, y: i16, z: u8 }
 #[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantD(Option<u32>);
 #[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantE(Duration);
 
 fn main() {
     let result: Vec<Result<MyEnum, ()>> = vec![
         Ok(MyEnum::A(17)),
-        Ok(MyEnum::B { x: 206, y: 1145 }),
+        Ok(MyEnum::B { x: 206, y: 1145, z: 78 }),
         Ok(MyEnum::C),
         Err(()),
         Ok(MyEnum::D(Some(407))),
@@ -63,7 +64,7 @@ fn main() {
     // Binary serialized version of the above (little-endian)
     let input: Vec<u8> = vec![
         0,  17, 0, 0, 0,
-        1,  206,  121, 4,
+        1,  206,  121, 4,  78,
         2,
         8,  /* invalid tag value */
         3,  0,  151, 1, 0, 0,
@@ -112,6 +113,7 @@ fn parse_my_enum<'a>(dest: &'a mut MyEnum, buf: &mut &[u8]) -> Result<(), ()> {
             MyEnumTag::B => {
                 dest.payload.B.x = read_u8(buf)?;
                 dest.payload.B.y = read_u16_le(buf)? as i16;
+                dest.payload.B.z = read_u8(buf)?;
             }
             MyEnumTag::C => {
                 /* do nothing */

src/test/run-pass/structs-enums/enum-non-c-like-repr-c.rs

@@ -20,11 +20,12 @@ use std::mem;
 #[repr(C)]
 #[derive(Copy, Clone, Eq, PartialEq, Debug)]
 enum MyEnum {
-    A(u32),                 // Single primitive value
-    B { x: u8, y: i16 },    // Composite, and the offset of `y` depends on tag being internal
-    C,                      // Empty
-    D(Option<u32>),         // Contains an enum
-    E(Duration),            // Contains a struct
+    A(u32),                     // Single primitive value
+    B { x: u8, y: i16, z: u8 }, // Composite, and the offset of `y` and `z`
+                                // depend on tag being internal
+    C,                          // Empty
+    D(Option<u32>),             // Contains an enum
+    E(Duration),                // Contains a struct
 }
 
 #[repr(C)]
@@ -44,14 +45,14 @@ union MyEnumPayload {
 
 #[repr(C)] #[derive(Copy, Clone)] enum MyEnumTag { A, B, C, D, E }
 #[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantA(u32);
-#[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantB {x: u8, y: i16 }
+#[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantB {x: u8, y: i16, z: u8 }
 #[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantD(Option<u32>);
 #[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantE(Duration);
 
 fn main() {
     let result: Vec<Result<MyEnum, ()>> = vec![
         Ok(MyEnum::A(17)),
-        Ok(MyEnum::B { x: 206, y: 1145 }),
+        Ok(MyEnum::B { x: 206, y: 1145, z: 78 }),
         Ok(MyEnum::C),
         Err(()),
         Ok(MyEnum::D(Some(407))),
@@ -63,7 +64,7 @@ fn main() {
     // Binary serialized version of the above (little-endian)
     let input: Vec<u8> = vec![
         0,  17, 0, 0, 0,
-        1,  206,  121, 4,
+        1,  206,  121, 4,  78,
         2,
         8,  /* invalid tag value */
         3,  0,  151, 1, 0, 0,
@@ -112,6 +113,7 @@ fn parse_my_enum<'a>(dest: &'a mut MyEnum, buf: &mut &[u8]) -> Result<(), ()> {
             MyEnumTag::B => {
                 dest.payload.B.x = read_u8(buf)?;
                 dest.payload.B.y = read_u16_le(buf)? as i16;
+                dest.payload.B.z = read_u8(buf)?;
             }
             MyEnumTag::C => {
                 /* do nothing */

src/test/run-pass/structs-enums/enum-non-c-like-repr-int.rs

@@ -20,11 +20,12 @@ use std::mem;
 #[repr(u8)]
 #[derive(Copy, Clone, Eq, PartialEq, Debug)]
 enum MyEnum {
-    A(u32),                 // Single primitive value
-    B { x: u8, y: i16 },    // Composite, and the offset of `y` depends on tag being internal
-    C,                      // Empty
-    D(Option<u32>),         // Contains an enum
-    E(Duration),            // Contains a struct
+    A(u32),                     // Single primitive value
+    B { x: u8, y: i16, z: u8 }, // Composite, and the offset of `y` and `z`
+                                // depend on tag being internal
+    C,                          // Empty
+    D(Option<u32>),             // Contains an enum
+    E(Duration),                // Contains a struct
 }
 
 #[allow(non_snake_case)]
@@ -39,15 +40,15 @@ union MyEnumRepr {
 
 #[repr(u8)] #[derive(Copy, Clone)] enum MyEnumTag { A, B, C, D, E }
 #[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantA(MyEnumTag, u32);
-#[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantB { tag: MyEnumTag, x: u8, y: i16 }
+#[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantB { tag: MyEnumTag, x: u8, y: i16, z: u8 }
 #[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantC(MyEnumTag);
 #[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantD(MyEnumTag, Option<u32>);
 #[repr(C)] #[derive(Copy, Clone)] struct MyEnumVariantE(MyEnumTag, Duration);
 
 fn main() {
     let result: Vec<Result<MyEnum, ()>> = vec![
         Ok(MyEnum::A(17)),
-        Ok(MyEnum::B { x: 206, y: 1145 }),
+        Ok(MyEnum::B { x: 206, y: 1145, z: 78 }),
         Ok(MyEnum::C),
         Err(()),
         Ok(MyEnum::D(Some(407))),
@@ -59,7 +60,7 @@ fn main() {
     // Binary serialized version of the above (little-endian)
     let input: Vec<u8> = vec![
         0,  17, 0, 0, 0,
-        1,  206,  121, 4,
+        1,  206,  121, 4,  78,
         2,
         8,  /* invalid tag value */
         3,  0,  151, 1, 0, 0,
@@ -108,6 +109,7 @@ fn parse_my_enum<'a>(dest: &'a mut MyEnum, buf: &mut &[u8]) -> Result<(), ()> {
             MyEnumTag::B => {
                 dest.B.x = read_u8(buf)?;
                 dest.B.y = read_u16_le(buf)? as i16;
+                dest.B.z = read_u8(buf)?;
             }
             MyEnumTag::C => {
                 /* do nothing */