fix compiler error in 'std.fmt.Parser.until'

[saurabh-mish]

Fixes _ = &std.fmt.Parser.until; in #20505

[Vexu]

until should not be using ++, a better solution would be to call nextCodepoint like the other functions next to it and return a slice like it did before #18533

[saurabh-mish]

until should not be using ++, a better solution would be to call nextCodepoint like the other functions next to it and return a slice like it did before #18533

Would it involve converting u21 to u8 ?

[Vexu]

No, it would look something like:

pub fn until(self: *@This(), ch: u21) []const u8 {
    const start = self.iter.i;
    while (self.peek(0)) |code_point| {
        if (code_point == ch)
            break;
        _ = self.iter.nextCodepoint();
    }
    return self.iter.bytes[start..self.iter.i];
}

[andrewrk]

This kind of easily tested change should come with unit test coverage.

[Vexu]

Fair enough, none of the previous fixes in #20505 added any specific tests so I figured the existing usage as part of format strings would be sufficient.

[saurabh-mish]

I've merged this PR with #22275