Accept empty condition sets

src/designspace.rs

@@ -120,7 +120,8 @@ pub struct Rule {
     /// Name of the rule.
     #[serde(rename = "@name")]
     pub name: Option<String>,
-    /// Condition sets. If any condition is true, the rule is applied.
+    /// Condition sets. If any condition is true or the condition set is empty,
+    /// the rule is applied.
     #[serde(rename = "conditionset")]
     pub condition_sets: Vec<ConditionSet>,
     /// Subtitutions (in, out).
@@ -143,7 +144,7 @@ pub struct Substitution {
 #[derive(Clone, Debug, Default, PartialEq, Serialize, Deserialize)]
 pub struct ConditionSet {
     /// The conditions.
-    #[serde(rename = "condition")]
+    #[serde(rename = "condition", default)]
     pub conditions: Vec<Condition>,
 }
 
@@ -541,4 +542,36 @@ mod tests {
         );
         assert_eq!(ds_initial, ds_after);
     }
+
+    #[test]
+    fn accept_always_on_rules() {
+        // Given
+        let designspace = DesignSpaceDocument::load("testdata/MutatorSansAlwaysOnRules.designspace").unwrap();
+
+        // Then
+        assert_eq!(
+            &designspace.rules,
+            &Rules {
+                processing: RuleProcessing::Last,
+                rules: vec![
+                    Rule {
+                        name: Some("fold_I_serifs".into()),
+                        condition_sets: vec![ConditionSet { conditions: vec![] }],
+                        substitutions: vec![Substitution {
+                            name: "I".into(),
+                            with: "I.narrow".into()
+                        }],
+                    },
+                    Rule {
+                        name: Some("fold_S_terminals".into()),
+                        condition_sets: vec![ConditionSet { conditions: vec![] }],
+                        substitutions: vec![Substitution {
+                            name: "S".into(),
+                            with: "S.closed".into()
+                        }],
+                    },
+                ]
+            }
+        );
+    }
 }

testdata/MutatorSansAlwaysOnRules.designspace

@@ -0,0 +1,27 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<designspace format="4.1">
+  <axes>
+    <axis tag="wdth" name="width" minimum="0" maximum="1000" default="0"/>
+    <axis tag="wght" name="weight" minimum="0" maximum="1000" default="0"/>
+  </axes>
+  <rules processing="last">
+    <rule name="fold_I_serifs">
+      <conditionset>
+      </conditionset>
+      <sub name="I" with="I.narrow"/>
+    </rule>
+    <rule name="fold_S_terminals">
+      <conditionset>
+      </conditionset>
+      <sub name="S" with="S.closed"/>
+    </rule>
+  </rules>
+  <sources>
+    <source filename="MutatorSansLightCondensed.ufo" familyname="MutatorMathTest" stylename="LightCondensed">
+      <location>
+        <dimension name="width" xvalue="0"/>
+        <dimension name="weight" xvalue="0"/>
+      </location>
+    </source>
+  </sources>
+</designspace>