Supports parsing @font-face

Part of linebender/resvg#541

Author: yisibl

README.md

@@ -21,8 +21,8 @@ Since it's very simple we will start with limitations:
 
 ## Limitations
 
-- [At-rules](https://www.w3.org/TR/CSS21/syndata.html#at-rules) are not supported.
-  They will be skipped during parsing.
+- [Most at-rules](https://www.w3.org/TR/CSS21/syndata.html#at-rules) are not supported.
+  They will be skipped during parsing. The only supported at-rule is `@font-face`.
 - Property values are not parsed.
   In CSS like `* { width: 5px }` you will get a `width` property with a `5px` value as a string.
 - CDO/CDC comments are not supported.
@@ -34,6 +34,7 @@ Since it's very simple we will start with limitations:
 
 - Selector matching support.
 - The rules are sorted by specificity.
+- `@font-face` parsing support.
 - `!important` parsing support.
 - Has a high-level parsers and low-level, zero-allocation tokenizers.
 - No unsafe.

src/lib.rs

@@ -12,8 +12,8 @@ Since it's very simple we will start with limitations:
 
 ## Limitations
 
-- [At-rules](https://www.w3.org/TR/CSS21/syndata.html#at-rules) are not supported.
-  They will be skipped during parsing.
+- [Most at-rules](https://www.w3.org/TR/CSS21/syndata.html#at-rules) are not supported.
+  They will be skipped during parsing. The only supported at-rule is `@font-face`.
 - Property values are not parsed.
   In CSS like `* { width: 5px }` you will get a `width` property with a `5px` value as a string.
 - CDO/CDC comments are not supported.
@@ -24,6 +24,7 @@ Since it's very simple we will start with limitations:
 
 - Selector matching support.
 - The rules are sorted by specificity.
+- `@font-face` parsing support.
 - `!important` parsing support.
 - Has a high-level parsers and low-level, zero-allocation tokenizers.
 - No unsafe.
@@ -192,6 +193,13 @@ pub struct Declaration<'a> {
     pub important: bool,
 }
 
+/// A `@font-face` rule.
+#[derive(Clone, Debug)]
+pub struct FontFaceRule<'a> {
+    /// A list of declarations inside this `@font-face` rule.
+    pub declarations: Vec<Declaration<'a>>,
+}
+
 /// A rule.
 #[derive(Clone, Debug)]
 pub struct Rule<'a> {
@@ -206,17 +214,23 @@ pub struct Rule<'a> {
 pub struct StyleSheet<'a> {
     /// A list of rules.
     pub rules: Vec<Rule<'a>>,
+    /// A list of `@font-face` rules.
+    pub font_faces: Vec<FontFaceRule<'a>>,
 }
 
 impl<'a> StyleSheet<'a> {
     /// Creates an empty style sheet.
     pub fn new() -> Self {
-        StyleSheet { rules: Vec::new() }
+        StyleSheet {
+            rules: Vec::new(),
+            font_faces: Vec::new(),
+        }
     }
 
     /// Parses a style sheet from text.
     ///
-    /// At-rules are not supported and will be skipped.
+    /// Most at-rules are not supported and will be skipped, except `@font-face`
+    /// rules which are parsed into [`FontFaceRule`]s.
     ///
     /// # Errors
     ///
@@ -242,7 +256,7 @@ impl<'a> StyleSheet<'a> {
                 break;
             }
 
-            let _ = consume_statement(&mut s, &mut self.rules);
+            let _ = consume_statement(&mut s, &mut self.rules, &mut self.font_faces);
         }
 
         if !s.at_end() {
@@ -286,19 +300,50 @@ impl Default for StyleSheet<'_> {
     }
 }
 
-fn consume_statement<'a>(s: &mut Stream<'a>, rules: &mut Vec<Rule<'a>>) -> Result<(), Error> {
+fn consume_statement<'a>(
+    s: &mut Stream<'a>,
+    rules: &mut Vec<Rule<'a>>,
+    font_faces: &mut Vec<FontFaceRule<'a>>,
+) -> Result<(), Error> {
     if s.curr_byte() == Ok(b'@') {
         s.advance(1);
-        consume_at_rule(s)
+        consume_at_rule(s, font_faces)
     } else {
         consume_rule_set(s, rules)
     }
 }
 
-fn consume_at_rule(s: &mut Stream<'_>) -> Result<(), Error> {
+fn consume_at_rule<'a>(
+    s: &mut Stream<'a>,
+    font_faces: &mut Vec<FontFaceRule<'a>>,
+) -> Result<(), Error> {
     let ident = s.consume_ident()?;
-    warn!("The @{} rule is not supported. Skipped.", ident);
 
+    if ident == "font-face" {
+        s.skip_spaces_and_comments()?;
+
+        if s.curr_byte() == Ok(b'{') {
+            s.advance(1);
+
+            let declarations = consume_declarations(s)?;
+            s.try_consume_byte(b'}');
+
+            if !declarations.is_empty() {
+                font_faces.push(FontFaceRule { declarations });
+            }
+        } else {
+            // Malformed `@font-face`; fall back to skipping it as an unknown at-rule.
+            skip_at_rule_body(s)?;
+        }
+    } else {
+        warn!("The @{} rule is not supported. Skipped.", ident);
+        skip_at_rule_body(s)?;
+    }
+
+    Ok(())
+}
+
+fn skip_at_rule_body(s: &mut Stream<'_>) -> Result<(), Error> {
     s.skip_bytes(|c| c != b';' && c != b'{');
 
     match s.curr_byte()? {

tests/stylesheet.rs

@@ -145,3 +145,77 @@ fn style_21() {
     let style = StyleSheet::parse(":le>*");
     assert_eq!(style.to_string(), "");
 }
+
+#[test]
+fn font_face_01() {
+    let style = StyleSheet::parse(
+        "@font-face { font-family: 'Noto Serif'; src: url(NotoSerif.woff2) format('woff2'); }",
+    );
+
+    assert_eq!(style.rules.len(), 0);
+    assert_eq!(style.font_faces.len(), 1);
+
+    let ff = &style.font_faces[0];
+    assert_eq!(
+        ff.declarations,
+        vec![
+            Declaration {
+                name: "font-family",
+                value: "'Noto Serif'",
+                important: false,
+            },
+            Declaration {
+                name: "src",
+                value: "url(NotoSerif.woff2) format('woff2')",
+                important: false,
+            },
+        ]
+    );
+}
+
+#[test]
+fn font_face_02_mixed_with_rules() {
+    let style = StyleSheet::parse(
+        "@font-face { font-family: 'MyFont'; src: url(https://foo.com/my.woff2); font-weight: normal; } div { color: red; }",
+    );
+
+    assert_eq!(style.rules.len(), 1);
+    assert_eq!(style.font_faces.len(), 1);
+
+    assert_eq!(style.rules[0].selector.to_string(), "div");
+    assert_eq!(style.rules[0].declarations.len(), 1);
+    assert_eq!(style.rules[0].declarations[0].name, "color");
+    assert_eq!(style.rules[0].declarations[0].value, "red");
+
+    assert_eq!(style.font_faces[0].declarations[0].name, "font-family");
+    assert_eq!(style.font_faces[0].declarations[0].value, "'MyFont'");
+    assert_eq!(style.font_faces[0].declarations[1].name, "src");
+    assert_eq!(
+        style.font_faces[0].declarations[1].value,
+        "url(https://foo.com/my.woff2)"
+    );
+    assert_eq!(style.font_faces[0].declarations[2].name, "font-weight");
+    assert_eq!(style.font_faces[0].declarations[2].value, "normal");
+}
+
+#[test]
+fn font_face_03_mixed_with_rules() {
+    let style = StyleSheet::parse(
+        "@font-palette-values --identifier { font-family: Bixa; override-colors: 0 green, 1 #999; } div { color: red; } @font-face { font-family: 'MyFont'; src: local('Airal'); font-weight: 200 800; }",
+    );
+
+    assert_eq!(style.rules.len(), 1);
+    assert_eq!(style.font_faces.len(), 1);
+
+    assert_eq!(style.rules[0].selector.to_string(), "div");
+    assert_eq!(style.rules[0].declarations.len(), 1);
+    assert_eq!(style.rules[0].declarations[0].name, "color");
+    assert_eq!(style.rules[0].declarations[0].value, "red");
+
+    assert_eq!(style.font_faces[0].declarations[0].name, "font-family");
+    assert_eq!(style.font_faces[0].declarations[0].value, "'MyFont'");
+    assert_eq!(style.font_faces[0].declarations[1].name, "src");
+    assert_eq!(style.font_faces[0].declarations[1].value, "local('Airal')");
+    assert_eq!(style.font_faces[0].declarations[2].name, "font-weight");
+    assert_eq!(style.font_faces[0].declarations[2].value, "200 800");
+}