Invalid argument for the offset_of!
macro.
Erroneous code example:
#![feature(offset_of_enum, offset_of_nested)] let x = std::mem::offset_of!(Option<u8>, Some);
The offset_of!
macro gives the offset of a field within a type. It can navigate through enum variants, but the final component of its second argument must be a field and not a variant.
The offset of the contained u8
in the Option<u8>
can be found by specifying the field name 0
:
#![feature(offset_of_enum, offset_of_nested)] let x: usize = std::mem::offset_of!(Option<u8>, Some.0);
The discriminant of an enumeration may be read with core::mem::discriminant
, but this is not always a value physically present within the enum.
Further information about enum layout may be found at https://rust-lang.github.io/unsafe-code-guidelines/layout/enums.html.