Use Parser::skip_whitespace in a few places to make Parser::try rewin…

…d less.

Gecko’s CSS parsing microbenchmarks before:

```
  43.437 ±  0.391 ms    Stylo.Servo_StyleSheet_FromUTF8Bytes_Bench
  29.244 ±  0.042 ms    Stylo.Gecko_nsCSSParser_ParseSheet_Bench
 281.884 ±  0.028 ms    Stylo.Servo_DeclarationBlock_SetPropertyById_Bench
 426.242 ±  0.008 ms    Stylo.Servo_DeclarationBlock_SetPropertyById_WithInitialSpace_Bench
```

After:

```
  29.779 ±  0.254 ms    Stylo.Servo_StyleSheet_FromUTF8Bytes_Bench
  28.841 ±  0.031 ms    Stylo.Gecko_nsCSSParser_ParseSheet_Bench
 296.240 ±  4.744 ms    Stylo.Servo_DeclarationBlock_SetPropertyById_Bench
 293.855 ±  4.304 ms    Stylo.Servo_DeclarationBlock_SetPropertyById_WithInitialSpace_Bench
```

components/selectors/Cargo.toml

@@ -25,7 +25,7 @@ unstable = []
 [dependencies]
 bitflags = "0.7"
 matches = "0.1"
-cssparser = "0.19"
+cssparser = "0.19.3"
 log = "0.3"
 fnv = "1.0"
 phf = "0.7.18"

components/selectors/parser.rs

@@ -1425,14 +1425,7 @@ fn parse_negation<'i, 't, P, E, Impl>(parser: &P,
     // We use a sequence because a type selector may be represented as two Components.
     let mut sequence = SmallVec::<[Component<Impl>; 2]>::new();
 
-    // Consume any leading whitespace.
-    loop {
-        let before_this_token = input.state();
-        if !matches!(input.next_including_whitespace(), Ok(&Token::WhiteSpace(_))) {
-            input.reset(&before_this_token);
-            break
-        }
-    }
+    input.skip_whitespace();
 
     // Get exactly one simple selector. The parse logic in the caller will verify
     // that there are no trailing tokens after we're done.
@@ -1468,14 +1461,8 @@ fn parse_compound_selector<'i, 't, P, E, Impl>(
     -> Result<bool, ParseError<'i, SelectorParseError<'i, E>>>
     where P: Parser<'i, Impl=Impl, Error=E>, Impl: SelectorImpl
 {
-    // Consume any leading whitespace.
-    loop {
-        let before_this_token = input.state();
-        if !matches!(input.next_including_whitespace(), Ok(&Token::WhiteSpace(_))) {
-            input.reset(&before_this_token);
-            break
-        }
-    }
+    input.skip_whitespace();
+
     let mut empty = true;
     if !parse_type_selector(parser, input, builder)? {
         if let Some(url) = parser.default_namespace() {

components/style/Cargo.toml

@@ -37,7 +37,7 @@ bitflags = "0.7"
 bit-vec = "0.4.3"
 byteorder = "1.0"
 cfg-if = "0.1.0"
-cssparser = "0.19"
+cssparser = "0.19.3"
 encoding = {version = "0.2", optional = true}
 euclid = "0.15"
 fnv = "1.0"

components/style/properties/properties.mako.rs

@@ -1552,6 +1552,9 @@ impl PropertyDeclaration {
         assert!(declarations.is_empty());
         match id {
             PropertyId::Custom(name) => {
+                // FIXME: fully implement https://github.com/w3c/csswg-drafts/issues/774
+                // before adding skip_whitespace here.
+                // This probably affects some test results.
                 let value = match input.try(|i| CSSWideKeyword::parse(i)) {
                     Ok(keyword) => DeclaredValueOwned::CSSWideKeyword(keyword),
                     Err(()) => match ::custom_properties::SpecifiedValue::parse(context, input) {
@@ -1564,6 +1567,7 @@ impl PropertyDeclaration {
                 Ok(())
             }
             PropertyId::Longhand(id) => {
+                input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
                 input.try(|i| CSSWideKeyword::parse(i)).map(|keyword| {
                     PropertyDeclaration::CSSWideKeyword(id, keyword)
                 }).or_else(|()| {
@@ -1595,6 +1599,7 @@ impl PropertyDeclaration {
                 })
             }
             PropertyId::Shorthand(id) => {
+                input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
                 if let Ok(keyword) = input.try(|i| CSSWideKeyword::parse(i)) {
                     if id == ShorthandId::All {
                         declarations.all_shorthand = AllShorthand::CSSWideKeyword(keyword)

components/style_traits/values.rs

@@ -246,11 +246,16 @@ impl Separator for Space {
     where
         F: for<'tt> FnMut(&mut Parser<'i, 'tt>) -> Result<T, ParseError<'i, E>>
     {
+        input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
         let mut results = vec![parse_one(input)?];
-        while let Ok(item) = input.try(&mut parse_one) {
-            results.push(item);
+        loop {
+            input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
+            if let Ok(item) = input.try(&mut parse_one) {
+                results.push(item);
+            } else {
+                return Ok(results)
+            }
         }
-        Ok(results)
     }
 }
 
@@ -266,9 +271,12 @@ impl Separator for CommaWithSpace {
     where
         F: for<'tt> FnMut(&mut Parser<'i, 'tt>) -> Result<T, ParseError<'i, E>>
     {
+        input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
         let mut results = vec![parse_one(input)?];
         loop {
+            input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
             let comma = input.try(|i| i.expect_comma()).is_ok();
+            input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
             if let Ok(item) = input.try(&mut parse_one) {
                 results.push(item);
             } else if comma {