Consider supporting concat! macro in attributes

[dtolnay]

For example a macro_rules macro might expand to:

#[derive(Serialize)]
struct $struct {
    $(
        #[serde(rename = concat!($prefix, stringify!($field)))]
        $field: $ty,
    )*
}

[davidhewitt]

(Repeating what I said in #1895)

Related: #450 (which is asking for #[serde(rename = SOME_CONST)], which I think has similar technical constraints).

[carlocorradini]

Any update? Thanks 😥