needs_separator_when_before omits CDC, so serialized tokens re-parse wrong

Start in src/serializer.rs around line 519 and read needs_separator_when_before, especially the rows for Number, DelimHash, DelimAt, and DelimMinus. Verify the four CDC combinations against the issue's reproducers and existing serializer coverage. Done means each combination requests a separator and serialized tokens re-parse to the original token sequence.

TokenSerializationType::needs_separator_when_before returns false when a Number, DelimHash, DelimAt, or DelimMinus is followed by CDC. Concatenating the two serializations produces text that tokenizes into different tokens than the input.

Reproducer

cssparser 0.37.0, default features.

use cssparser::{Parser, ParserInput, ToCss};

fn main() {
    let mut pi = ParserInput::new("5 -->");
    let mut p = Parser::new(&mut pi);
    let a = p.next().unwrap().clone(); // Number { value: 5.0, int_value: Some(5) }
    let b = p.next().unwrap().clone(); // CDC

    // no separator requested
    assert!(!a
        .serialization_type()
        .needs_separator_when_before(b.serialization_type()));

    let mut s = String::new();
    a.to_css(&mut s).unwrap();
    b.to_css(&mut s).unwrap();
    assert_eq!(s, "5-->");
}

Observed vs expected

Observed: the call returns false and serialization yields 5-->. Re-parsing 5--> gives Dimension { value: 5.0, unit: "--" } followed by Delim('>').

Expected: true, which signals that an empty comment is needed so the output round-trips. The doc comment on needs_separator_when_before says it returns true if "an empty comment /**/ needs to be inserted between them so that they are not re-parsed as a single token". CSS Syntax Level 3 §Serialization requires the serialized form to round-trip.

Root cause

src/serializer.rs:519. The Ident and AtKeywordOrHash | Dimension rows list CDC. The rows covering Number, DelimHash, DelimAt, and DelimMinus do not.

Scope

Four pairs return false: Number then CDC gives 5-->, re-parsing as Dimension{5,"--"} + Delim('>'). DelimHash then CDC gives #-->, re-parsing as IDHash("--") + Delim('>'). DelimAt then CDC gives @-->, re-parsing as AtKeyword("--") + Delim('>'). DelimMinus then CDC gives --->, re-parsing as Ident("---") + Delim('>').

Any consumer using this API to re-emit CSS is affected. Stylo calls it for custom-property and var() substitution serialization. Present on main as of the 2026-07-21 commit.