Add long diagnostics for E0046 and E0054

[nham]

cc #24407

[rust-highfive]

r? @nikomatsakis

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[alexcrichton]

Perhaps this could clarify "It is not allowed to cast a numeric type to a bool"?

[nham]

From my (admittedly shallow) reading of the code in librustc_typeck/check/cast.rs, it seemed like any casting to bool is disallowed. For example, you can try to cast a bare function to a bool and get E0054.

fn main() {
    fn foo() -> u8 { 5 }
    let x = foo as bool; // error: cannot cast as `bool`, compare with zero instead [E0054]
}

[alexcrichton]

Aha! Never mind then!

[alexcrichton]

@bors: r+ 11a7bad rollup