Auto merge of #17414 - jyc:space-separator, r=nox

style: Have OneOrMoreSeparated replace OneOrMoreCommaSeparated.

**NOTE** The alternative for me is just to duplicate the ToCss code, which is not bad and is less code changed! Don't know what others would think, though. Looking for feedback!

A future patch series has some values that should be separated by spaces. This
allows us to re-use the code for serialization, but the types do get a little
clunky. The separator is now indicated with an associated type.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because they shoudl be tested by existing serialization code

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17414)
<!-- Reviewable:end -->

components/style/counter_style/mod.rs

@@ -19,7 +19,7 @@ use std::ascii::AsciiExt;
 use std::borrow::Cow;
 use std::fmt;
 use std::ops::Range;
-use style_traits::{ToCss, OneOrMoreCommaSeparated, ParseError, StyleParseError};
+use style_traits::{ToCss, OneOrMoreSeparated, CommaSeparator, ParseError, StyleParseError};
 use values::CustomIdent;
 
 /// Parse the prelude of an @counter-style rule
@@ -552,7 +552,9 @@ pub struct AdditiveTuple {
     pub symbol: Symbol,
 }
 
-impl OneOrMoreCommaSeparated for AdditiveTuple {}
+impl OneOrMoreSeparated for AdditiveTuple {
+    type S = CommaSeparator;
+}
 
 impl Parse for AdditiveTuple {
     fn parse<'i, 't>(context: &ParserContext, input: &mut Parser<'i, 't>) -> Result<Self, ParseError<'i>> {

components/style/font_face.rs

@@ -20,7 +20,7 @@ use parser::{ParserContext, log_css_error, Parse};
 use selectors::parser::SelectorParseError;
 use shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
 use std::fmt;
-use style_traits::{ToCss, OneOrMoreCommaSeparated, ParseError, StyleParseError};
+use style_traits::{ToCss, OneOrMoreSeparated, CommaSeparator, ParseError, StyleParseError};
 use values::specified::url::SpecifiedUrl;
 
 /// A source for a font-face rule.
@@ -34,7 +34,9 @@ pub enum Source {
     Local(FamilyName),
 }
 
-impl OneOrMoreCommaSeparated for Source {}
+impl OneOrMoreSeparated for Source {
+    type S = CommaSeparator;
+}
 
 /// A `UrlSource` represents a font-face source that has been specified with a
 /// `url()` function.

components/style/parser.rs

@@ -7,7 +7,7 @@
 use context::QuirksMode;
 use cssparser::{Parser, SourcePosition, UnicodeRange};
 use error_reporting::{ParseErrorReporter, ContextualParseError};
-use style_traits::{OneOrMoreCommaSeparated, ParseError, ParsingMode};
+use style_traits::{OneOrMoreSeparated, IsCommaSeparator, ParseError, ParsingMode};
 #[cfg(feature = "gecko")]
 use style_traits::{PARSING_MODE_DEFAULT, PARSING_MODE_ALLOW_UNITLESS_LENGTH, PARSING_MODE_ALLOW_ALL_NUMERIC_VALUES};
 use stylesheets::{CssRuleType, Origin, UrlExtraData, Namespaces};
@@ -161,7 +161,9 @@ pub trait Parse : Sized {
                      -> Result<Self, ParseError<'i>>;
 }
 
-impl<T> Parse for Vec<T> where T: Parse + OneOrMoreCommaSeparated {
+impl<T> Parse for Vec<T> where T: Parse + OneOrMoreSeparated,
+                               <T as OneOrMoreSeparated>::S: IsCommaSeparator
+{
     fn parse<'i, 't>(context: &ParserContext, input: &mut Parser<'i, 't>)
                      -> Result<Self, ParseError<'i>> {
         input.parse_comma_separated(|input| T::parse(context, input))

components/style/values/generics/mod.rs

@@ -9,7 +9,7 @@ use counter_style::{Symbols, parse_counter_style_name};
 use cssparser::Parser;
 use parser::{Parse, ParserContext};
 use std::fmt;
-use style_traits::{OneOrMoreCommaSeparated, ToCss, ParseError, StyleParseError};
+use style_traits::{OneOrMoreSeparated, CommaSeparator, ToCss, ParseError, StyleParseError};
 use super::CustomIdent;
 use values::specified::url::SpecifiedUrl;
 
@@ -123,7 +123,9 @@ pub struct FontSettingTag<T> {
     pub value: T,
 }
 
-impl<T> OneOrMoreCommaSeparated for FontSettingTag<T> {}
+impl<T> OneOrMoreSeparated for FontSettingTag<T> {
+    type S = CommaSeparator;
+}
 
 impl<T: ToCss> ToCss for FontSettingTag<T> {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {

components/style_traits/lib.rs

@@ -71,7 +71,7 @@ pub mod values;
 #[macro_use]
 pub mod viewport;
 
-pub use values::{ToCss, OneOrMoreCommaSeparated};
+pub use values::{ToCss, OneOrMoreSeparated, CommaSeparator, SpaceSeparator, IsCommaSeparator};
 pub use viewport::HasViewportPercentage;
 
 /// The error type for all CSS parsing routines.

components/style_traits/values.rs

@@ -55,17 +55,59 @@ impl ToCss for String {
     }
 }
 
-/// Marker trait to automatically implement ToCss for Vec<T>.
-pub trait OneOrMoreCommaSeparated {}
+/// Type used as the associated type in the `OneOrMoreSeparated` trait on a
+/// type to indicate that a serialized list of elements of this type is
+/// separated by commas.
+pub struct CommaSeparator;
+
+/// Type used as the associated type in the `OneOrMoreSeparated` trait on a
+/// type to indicate that a serialized list of elements of this type is
+/// separated by spaces.
+pub struct SpaceSeparator;
+
+/// A trait satisfied by the types corresponding to separators.
+pub trait Separator {
+    /// The separator string that the satisfying separator type corresponds to.
+    fn separator() -> &'static str;
+}
+
+impl Separator for CommaSeparator {
+    fn separator() -> &'static str {
+        ", "
+    }
+}
+
+impl Separator for SpaceSeparator {
+    fn separator() -> &'static str {
+        " "
+    }
+}
+
+/// Trait that indicates that satisfying separator types are comma separators.
+/// This seems kind of redundant, but we aren't able to express type equality
+/// constraints yet.
+/// https://github.com/rust-lang/rust/issues/20041
+pub trait IsCommaSeparator {}
+
+impl IsCommaSeparator for CommaSeparator {}
 
-impl OneOrMoreCommaSeparated for UnicodeRange {}
+/// Marker trait on T to automatically implement ToCss for Vec<T> when T's are
+/// separated by some delimiter `delim`.
+pub trait OneOrMoreSeparated {
+    /// Associated type indicating which separator is used.
+    type S: Separator;
+}
+
+impl OneOrMoreSeparated for UnicodeRange {
+    type S = CommaSeparator;
+}
 
-impl<T> ToCss for Vec<T> where T: ToCss + OneOrMoreCommaSeparated {
+impl<T> ToCss for Vec<T> where T: ToCss + OneOrMoreSeparated {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
         let mut iter = self.iter();
         iter.next().unwrap().to_css(dest)?;
         for item in iter {
-            dest.write_str(", ")?;
+            dest.write_str(<T as OneOrMoreSeparated>::S::separator())?;
             item.to_css(dest)?;
         }
         Ok(())