Port to the new cssparser.

[SimonSapin]

servo/rust-cssparser#68

r? @larsbergstrom

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[pcwalton]

@larsbergstrom How would you like to split this up?

[larsbergstrom]

@pcwalton I was just going to review it since you are on PTO today and am happy to either let you review it or divide it up however you would like.

[SimonSapin]

I just realized I forgot to write any docs. Sorry!

I’ll write more down tomorrow. In the meantime, the main important point is the convention for parsing functions:

• Take (at least) a input: &mut cssparser::Parser parameter
• Return Result<_, ()>
• When returning Ok(_), the function must have consume exactly the amount of input that represents the parsed value.
• When returning Err(()), any amount of input may have been consumed.

As a consequence, when calling another parsing function, either:

• Any Err(()) return value must be propagated. This happens by definition for tail calls, and can otherwise be done with the try! macro.
• Or the call must be wrapped in a cssparser::Parser::try call. try takes a closure that takes a Parser and returns a Result, calls it once, and returns itself that same result. If the result is Err, it restores the position inside the input to the one saved before calling the closure.

Examples:

// 'none' | <image>
fn parse_background_image(context: &ParserContext, input: &mut Parser) 
                                    -> Result<Option<Image>, ()> {
    if input.try(|input| input.expect_ident_matching("none")).is_ok() {
        Ok(None)
    } else {
        Image::parse(context, input).map(Some)  // tail call
    }
}

// [ <length> | <percentage> ] [ <length> | <percentage> ]?
fn parse_border_spacing(_context: &ParserContext, input: &mut Parser) 
                          -> Result<(LengthOrPercentage, LengthOrPercentage), ()> {
    let first = try!(LengthOrPercentage::parse);
    let second = input.try(LengthOrPercentage::parse).unwrap_or(first);
    (first, second)
}

[SimonSapin]

r=larsbergstrom

[SimonSapin]

r=larsbergstrom