Macro static_cond 

macro_rules! static_cond {
    (@expr $lhs:tt $rhs:tt $arm1:tt $arm2:tt) => { ... };
    (@item $lhs:tt $rhs:tt $arm1:tt $arm2:tt) => { ... };
    (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 expression.

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

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

Examples

let x = static_cond!(if (+ 1 [2 3]) == (+ 1 [2 3]) {
    static_cond!(if black != white {
        "ok"
    } else {
        the compiler will never even try to interpret this
    })
} else {
    blah blah blah blah blah unreachable
});
assert_eq!(x, "ok");

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