Can't use "enum as u8" in constexpr

[kmcallister]

#[repr(u8)]
enum Tag {
    Dynamic,
    Inline,
    Static,
}

pub const DYNAMIC_TAG: u8 = Dynamic as u8;
pub const INLINE_TAG: u8 = Inline as u8;
pub const STATIC_TAG: u8 = Static as u8;

produces

/home/keegan/string-cache/shared/repr.rs:23:29: 23:42 error: non-constant path in constant expr  
/home/keegan/string-cache/shared/repr.rs:23 pub const DYNAMIC_TAG: u8 = Dynamic as u8;  

[mahkoh]

This is part of rust-lang/rfcs#259. Note that this works if you explicitly specify the values.

[huonw]

Dupe of #5873.