What does it mean when “method exists but trait bounds not satisfied”?

Question

I\'m new to Rust and observed something that I couldn\'t reason against.

When I write

fn main() {
    (\'a\'..\'z\').all(|_| true);
}

Answer 1

What this means is that there is a trait in scope that has a function with that name, but that the object you are using does not implement such a trait.

In your particular case, the trait that contains the all method is std::iter::Iterator, but your object ('a'..'z') if of type Range that does not implement it.

Curiously enough, your second example compiles because (b'a'..b'z') is of type Range that does implement Iterator.

You are probably wondering why Range does not implement iterator. That is because there are invalid char values between valid ones, so these ranges just cannot be iterated. In particular, the only valid chars are those in the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF], IIRC.