Macro static_name 

macro_rules! static_name {
    ($(#[$meta:meta])* $vis:vis $name:ident, $value:expr) => { ... };
    ($(#[$meta:meta])* $name:ident, $value:expr) => { ... };
}

Declare a StaticName marker type.

static_name!(XRequestId, "x-request-id");