Macro static_cond_item 

macro_rules! static_cond_item {
    (if $lhs:tt == $rhs:tt $then:tt) => { ... };
    (if $lhs:tt != $rhs:tt $then:tt) => { ... };
    (if $lhs:tt == $rhs:tt $then:tt else $els:tt) => { ... };
    (if $lhs:tt != $rhs:tt $then:tt else $els:tt) => { ... };
}

Evaluates a conditional during macro expansion as item.

Currently limited to equality comparison. Can compare any two token trees. Can be nested.

This macro only works in item-position, see static_cond for a macro that works in expression-position.

Examples

static_cond_item!{
    if (+ 1 [2 3]) == (+ 1 [2 3]) {
        static_cond_item!{if black != white {
            fn foo() -> &'static str {"ok"}
        } else {
            the compiler will never even try to interpret this
        }}
    } else {
        blah blah blah blah blah unreachable
    }
}
assert_eq!(foo(), "ok");

The actual conditional and the code provided for the branches not followed is eliminated after macro expansion (check rustc --pretty=expanded).