Auto merge of #18395 - jdm:selector-smallvec, r=SimonSapin

Use a 1-element smallvec for selector lists.

Profiling shows this reducing parsing time by a few milliseconds on the tp6 facebook case. The gtest benchmark with the same concatenated stylesheets also shows a significant improvement.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] There are tests for these changes

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components/selectors/parser.rs

@@ -7,7 +7,7 @@ use attr::{ParsedCaseSensitivity, SELECTOR_WHITESPACE, NamespaceConstraint};
 use bloom::BLOOM_HASH_MASK;
 use builder::{SelectorBuilder, SpecificityAndFlags};
 use context::QuirksMode;
-use cssparser::{ParseError, BasicParseError, CowRcStr};
+use cssparser::{ParseError, BasicParseError, CowRcStr, Delimiter};
 use cssparser::{Token, Parser as CssParser, parse_nth, ToCss, serialize_identifier, CssStringWriter};
 use precomputed_hash::PrecomputedHash;
 use servo_arc::ThinArc;
@@ -176,7 +176,7 @@ pub trait Parser<'i> {
 }
 
 #[derive(Clone, Debug, Eq, PartialEq)]
-pub struct SelectorList<Impl: SelectorImpl>(pub Vec<Selector<Impl>>);
+pub struct SelectorList<Impl: SelectorImpl>(pub SmallVec<[Selector<Impl>; 1]>);
 
 impl<Impl: SelectorImpl> SelectorList<Impl> {
     /// Parse a comma-separated list of Selectors.
@@ -186,13 +186,20 @@ impl<Impl: SelectorImpl> SelectorList<Impl> {
     pub fn parse<'i, 't, P, E>(parser: &P, input: &mut CssParser<'i, 't>)
                                -> Result<Self, ParseError<'i, SelectorParseError<'i, E>>>
     where P: Parser<'i, Impl=Impl, Error=E> {
-        input.parse_comma_separated(|input| parse_selector(parser, input))
-             .map(SelectorList)
+        let mut values = SmallVec::new();
+        loop {
+            values.push(input.parse_until_before(Delimiter::Comma, |input| parse_selector(parser, input))?);
+            match input.next() {
+                Err(_) => return Ok(SelectorList(values)),
+                Ok(&Token::Comma) => continue,
+                Ok(_) => unreachable!(),
+            }
+        }
     }
 
     /// Creates a SelectorList from a Vec of selectors. Used in tests.
     pub fn from_vec(v: Vec<Selector<Impl>>) -> Self {
-        SelectorList(v)
+        SelectorList(SmallVec::from_vec(v))
     }
 }