Auto merge of #12783 - Wafflespeanut:word_spacing, r=Manishearth

Prefer length and percentage for word spacing

<!-- Please describe your changes on the following line: -->
The goal is to make use of `LengthOrPercentage` for word spacing in `ShapingOptions`, but since it makes use of `f32` which doesn't implement `Hash`, we're going for `NotNan<f32>` from [ordered-float](https://github.com/reem/rust-ordered-float/), which supports hashing. Instead of implementing `Hash` for `LengthOrPercentage` and thereby the inner types like `CSSFloat`, `CalcLengthOrPercentage`, etc., we convert it to `(Au, NotNan<f32>)`.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [ ] There are tests for these changes

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components/gfx/Cargo.toml

@@ -28,6 +28,7 @@ log = "0.3.5"
 mime = "0.2"
 msg = {path = "../msg"}
 net_traits = {path = "../net_traits"}
+ordered-float = "0.2.2"
 plugins = {path = "../plugins"}
 profile_traits = {path = "../profile_traits"}
 rand = "0.3"

components/gfx/font.rs

@@ -5,6 +5,7 @@
 use app_units::Au;
 use euclid::{Point2D, Rect, Size2D};
 use font_template::FontTemplateDescriptor;
+use ordered_float::NotNaN;
 use platform::font::{FontHandle, FontTable};
 use platform::font_context::FontContextHandle;
 use platform::font_template::FontTemplateData;
@@ -157,7 +158,7 @@ pub struct ShapingOptions {
     /// NB: You will probably want to set the `IGNORE_LIGATURES_SHAPING_FLAG` if this is non-null.
     pub letter_spacing: Option<Au>,
     /// Spacing to add between each word. Corresponds to the CSS 2.1 `word-spacing` property.
-    pub word_spacing: Au,
+    pub word_spacing: (Au, NotNaN<f32>),
     /// The Unicode script property of the characters in this run.
     pub script: Script,
     /// Various flags.
@@ -225,7 +226,9 @@ impl Font {
 
             let mut advance = Au::from_f64_px(self.glyph_h_advance(glyph_id));
             if character == ' ' {
-                advance += options.word_spacing;
+                // https://drafts.csswg.org/css-text-3/#word-spacing-property
+                let (length, percent) = options.word_spacing;
+                advance = (advance + length) + Au((advance.0 as f32 * percent.into_inner()) as i32);
             }
             if let Some(letter_spacing) = options.letter_spacing {
                 advance += letter_spacing;

components/gfx/lib.rs

@@ -60,6 +60,7 @@ extern crate log;
 extern crate mime;
 extern crate msg;
 extern crate net_traits;
+extern crate ordered_float;
 #[macro_use]
 extern crate profile_traits;
 extern crate rand;

components/gfx/text/shaping/harfbuzz.rs

@@ -403,7 +403,9 @@ impl Shaper {
         // applied. The effect of the property on other word-separator characters is undefined."
         // We elect to only space the two required code points.
         if character == ' ' || character == '\u{a0}' {
-            advance = advance + options.word_spacing
+            // https://drafts.csswg.org/css-text-3/#word-spacing-property
+            let (length, percent) = options.word_spacing;
+            advance = (advance + length) + Au((advance.0 as f32 * percent.into_inner()) as i32);
         }
 
         advance