Update based on wesleywiser review

compiler/rustc_mir_transform/src/gvn.rs

@@ -467,9 +467,9 @@ fn eval_to_const(&mut self, value: VnIndex) -> Option<OpTy<'tcx>> {
                 let val = match null_op {
                     NullOp::SizeOf => layout.size.bytes(),
                     NullOp::AlignOf => layout.align.abi.bytes(),
-                    NullOp::OffsetOf(fields) => layout
-                        .offset_of_subfield(&self.ecx, fields.iter().map(|f| f.index()))
-                        .bytes(),
+                    NullOp::OffsetOf(fields) => {
+                        layout.offset_of_subfield(&self.ecx, fields.iter()).bytes()
+                    }
                 };
                 let usize_layout = self.ecx.layout_of(self.tcx.types.usize).unwrap();
                 let imm = ImmTy::try_from_uint(val, usize_layout)?;

library/core/src/mem/mod.rs

@@ -1299,8 +1299,8 @@ impl<T> SizedTypeProperties for T {}
 /// Enum variants may be traversed as if they were fields. Variants themselves do
 /// not have an offset.
 ///
-/// Note that type layout is, in general, [platform-specific, and subject to
-/// change](https://doc.rust-lang.org/reference/type-layout.html).
+/// Note that type layout is, in general, [subject to change and
+/// platform-specific](https://doc.rust-lang.org/reference/type-layout.html).
 ///
 /// # Examples
 ///

tests/ui/offset-of/offset-of-enum.rs

@@ -14,4 +14,5 @@ fn main() {
     offset_of!(Alpha, Two.1); //~ ERROR no field named `1` on enum variant `Alpha::Two`
     offset_of!(Alpha, Two.foo); //~ ERROR no field named `foo` on enum variant `Alpha::Two`
     offset_of!(Alpha, NonExistent); //~ ERROR no variant named `NonExistent` found for enum `Alpha`
+    offset_of!(Beta, One); //~ ERROR cannot find type `Beta` in this scope
 }

tests/ui/offset-of/offset-of-enum.stderr

@@ -7,6 +7,12 @@ LL |     offset_of!(Alpha::One, 0);
    |                not a type
    |                help: try using the variant's enum: `Alpha`
 
+error[E0412]: cannot find type `Beta` in this scope
+  --> $DIR/offset-of-enum.rs:17:16
+   |
+LL |     offset_of!(Beta, One);
+   |                ^^^^ not found in this scope
+
 error[E0795]: `One` is an enum variant; expected field at end of `offset_of`
   --> $DIR/offset-of-enum.rs:12:23
    |
@@ -35,7 +41,7 @@ error[E0599]: no variant named `NonExistent` found for enum `Alpha`
 LL |     offset_of!(Alpha, NonExistent);
    |                       ^^^^^^^^^^^ variant not found
 
-error: aborting due to 5 previous errors
+error: aborting due to 6 previous errors
 
-Some errors have detailed explanations: E0573, E0599, E0609, E0795.
-For more information about an error, try `rustc --explain E0573`.
+Some errors have detailed explanations: E0412, E0573, E0599, E0609, E0795.
+For more information about an error, try `rustc --explain E0412`.