servo: Merge #14192 - implement support for the width query (from gterzian:support_equality_constraints); r=emilio

<!-- Please describe your changes on the following line: -->
implement support for the `width` media query

---
<!-- 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
- [x] These changes fix #https://github.com/servo/servo/issues/13670 (github issue number if applicable).

<!-- Either: -->
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Source-Repo: https://github.com/servo/servo
Source-Revision: bcc76f8a58f2ebe3886d16ff3eebbe68e3debf6c

servo/components/style/media_queries.rs

@@ -42,7 +42,7 @@ impl Default for MediaList {
 pub enum Range<T> {
     Min(T),
     Max(T),
-    //Eq(T),    // FIXME: Implement parsing support for equality then re-enable this.
+    Eq(T),
 }
 
 impl Range<specified::Length> {
@@ -54,7 +54,7 @@ impl Range<specified::Length> {
         match *self {
             Range::Min(ref width) => Range::Min(width.to_computed_value(&context)),
             Range::Max(ref width) => Range::Max(width.to_computed_value(&context)),
-            //Range::Eq(ref width) => Range::Eq(compute_width(width))
+            Range::Eq(ref width) => Range::Eq(width.to_computed_value(&context))
         }
     }
 }
@@ -64,7 +64,7 @@ impl<T: Ord> Range<T> {
         match *self {
             Range::Min(ref width) => { value >= *width },
             Range::Max(ref width) => { value <= *width },
-            //Range::Eq(ref width) => { value == *width },
+            Range::Eq(ref width) => { value == *width },
         }
     }
 }
@@ -127,10 +127,11 @@ impl ToCss for MediaQuery {
         for (i, &e) in self.expressions.iter().enumerate() {
             try!(write!(dest, "("));
             let (mm, l) = match e {
-                Expression::Width(Range::Min(ref l)) => ("min", l),
+                Expression::Width(Range::Min(ref l)) => ("min-", l),
-                Expression::Width(Range::Max(ref l)) => ("max", l),
+                Expression::Width(Range::Max(ref l)) => ("max-", l),
+                Expression::Width(Range::Eq(ref l)) => ("", l),
             };
-            try!(write!(dest, "{}-width: ", mm));
+            try!(write!(dest, "{}width: ", mm));
             try!(l.to_css(dest));
             if i == self.expressions.len() - 1 {
                 try!(write!(dest, ")"));
@@ -195,6 +196,9 @@ impl Expression {
                 "max-width" => {
                     Ok(Expression::Width(Range::Max(try!(specified::Length::parse_non_negative(input)))))
                 },
+                "width" => {
+                    Ok(Expression::Width(Range::Eq(try!(specified::Length::parse_non_negative(input)))))
+                },
                 _ => Err(())
             }
         })

servo/tests/unit/style/media_queries.rs

@@ -13,6 +13,7 @@ use style::media_queries::*;
 use style::parser::ParserContextExtraData;
 use style::stylesheets::{Stylesheet, Origin, CssRule};
 use style::values::specified;
+use style_traits::ToCss;
 
 pub struct CSSErrorReporterTest;
 
@@ -242,6 +243,18 @@ fn test_mq_expressions() {
         }
     });
 
+    test_media_rule("@media print and (width: 43px) { }", |list, css| {
+        assert!(list.media_queries.len() == 1, css.to_owned());
+        let q = &list.media_queries[0];
+        assert!(q.qualifier == None, css.to_owned());
+        assert!(q.media_type == MediaQueryType::MediaType(MediaType::Print), css.to_owned());
+        assert!(q.expressions.len() == 1, css.to_owned());
+        match q.expressions[0] {
+            Expression::Width(Range::Eq(w)) => assert!(w == specified::Length::Absolute(Au::from_px(43))),
+            _ => panic!("wrong expression type"),
+        }
+    });
+
     test_media_rule("@media fridge and (max-width: 52px) { }", |list, css| {
         assert!(list.media_queries.len() == 1, css.to_owned());
         let q = &list.media_queries[0];
@@ -255,6 +268,16 @@ fn test_mq_expressions() {
     });
 }
 
+#[test]
+fn test_to_css() {
+  test_media_rule("@media print and (width: 43px) { }", |list, _| {
+      let q = &list.media_queries[0];
+      let mut dest = String::new();
+      assert_eq!(Ok(()), q.to_css(&mut dest));
+      assert_eq!(dest, "print and (width: 43px)");
+  });
+}
+
 #[test]
 fn test_mq_multiple_expressions() {
     test_media_rule("@media (min-width: 100px) and (max-width: 200px) { }", |list, css| {