| Invalid argument for the `offset_of!` macro. |
| |
| Erroneous code example: |
| |
| ```compile_fail,E0795 |
| #![feature(offset_of_enum)] |
| |
| 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)] |
| |
| 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. |
