Support building feature variations using the "condition" statement.

Lib/fontTools/feaLib/ast.py

@@ -2084,6 +2084,8 @@ def build(self, builder):
         for key, value in builder.features_.items():
             items = builder.feature_variations_.setdefault(key,{}).setdefault(self.conditionset,[])
             items.extend(value)
+            if key not in features:
+                features[key] = []  # Ensure we make a feature record
         builder.features_ = features
         builder.end_feature()
 

Lib/fontTools/feaLib/builder.py

@@ -34,6 +34,7 @@
 from fontTools.otlLib.error import OpenTypeLibError
 from fontTools.varLib.varStore import OnlineVarStoreBuilder
 from fontTools.varLib.builder import buildVarDevTable
+from fontTools.varLib.featureVars import addFeatureVariationsRaw
 from fontTools.varLib.models import normalizeValue
 from collections import defaultdict
 import itertools
@@ -211,6 +212,8 @@ def build(self, tables=None, debug=False):
             if tag not in tables:
                 continue
             table = self.makeTable(tag)
+            if self.feature_variations_:
+                self.makeFeatureVariations(table, tag)
             if (
                 table.ScriptList.ScriptCount > 0
                 or table.FeatureList.FeatureCount > 0
@@ -874,7 +877,8 @@ def makeTable(self, tag):
             )
 
             size_feature = tag == "GPOS" and feature_tag == "size"
-            if len(lookup_indices) == 0 and not size_feature:
+            force_feature = self.any_feature_variations(feature_tag, tag)
+            if len(lookup_indices) == 0 and not size_feature and not force_feature:
                 continue
 
             for ix in lookup_indices:
@@ -940,6 +944,42 @@ def makeTable(self, tag):
         table.LookupList.LookupCount = len(table.LookupList.Lookup)
         return table
 
+    def makeFeatureVariations(self, table, table_tag):
+        feature_vars = {}
+        has_any_variations = False
+        # Sort out which lookups to build, gather their indices
+        for (
+            script_,
+            language,
+            feature_tag,
+        ), variations in self.feature_variations_.items():
+            feature_vars[feature_tag] = []
+            for conditionset, builders in variations.items():
+                raw_conditionset = self.conditionsets_[conditionset]
+                indices = []
+                for b in builders:
+                    if b.table != table_tag:
+                        continue
+                    assert b.lookup_index is not None
+                    indices.append(b.lookup_index)
+                    has_any_variations = True
+                feature_vars[feature_tag].append((raw_conditionset, indices))
+
+        if has_any_variations:
+            for feature_tag, conditions_and_lookups in feature_vars.items():
+                addFeatureVariationsRaw(
+                    self.font, table, conditions_and_lookups, feature_tag
+                )
+
+    def any_feature_variations(self, feature_tag, table_tag):
+        for (_, _, feature), variations in self.feature_variations_.items():
+            if feature != feature_tag:
+                continue
+            for conditionset, builders in variations.items():
+                if any(b.table == table_tag for b in builders):
+                    return True
+        return False
+
     def get_lookup_name_(self, lookup):
         rev = {v: k for k, v in self.named_lookups_.items()}
         if lookup in rev:
@@ -1480,7 +1520,7 @@ def add_conditionset(self, key, value):
         # Normalize
         axisMap = {
             axis.axisTag: (axis.minValue, axis.defaultValue, axis.maxValue)
-            for axis in self.font["fvar"].axes
+            for axis in self.axes
         }
 
         value = {

Tests/feaLib/builder_test.py

@@ -76,7 +76,7 @@ class BuilderTest(unittest.TestCase):
         SingleSubstSubtable aalt_chain_contextual_subst AlternateChained 
         MultipleLookupsPerGlyph MultipleLookupsPerGlyph2 GSUB_6_formats
         GSUB_5_formats delete_glyph STAT_test STAT_test_elidedFallbackNameID
-        variable_scalar_valuerecord variable_scalar_anchor
+        variable_scalar_valuerecord variable_scalar_anchor variable_conditionset
     """.split()
 
     VARFONT_AXES = [

Tests/feaLib/data/variable_conditionset.fea

@@ -0,0 +1,13 @@
+languagesystem DFLT dflt;
+
+lookup symbols_heavy {
+	sub a by b;
+} symbols_heavy;
+
+conditionset heavy {
+	wght 700 900;
+} heavy;
+
+variation rvrn heavy {
+     lookup symbols_heavy;
+} rvrn;

Tests/feaLib/data/variable_conditionset.ttx

@@ -0,0 +1,63 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ttFont>
+
+  <GSUB>
+    <Version value="0x00010001"/>
+    <ScriptList>
+      <!-- ScriptCount=1 -->
+      <ScriptRecord index="0">
+        <ScriptTag value="DFLT"/>
+        <Script>
+          <DefaultLangSys>
+            <ReqFeatureIndex value="65535"/>
+            <!-- FeatureCount=1 -->
+            <FeatureIndex index="0" value="0"/>
+          </DefaultLangSys>
+          <!-- LangSysCount=0 -->
+        </Script>
+      </ScriptRecord>
+    </ScriptList>
+    <FeatureList>
+      <!-- FeatureCount=1 -->
+      <FeatureRecord index="0">
+        <FeatureTag value="rvrn"/>
+        <Feature>
+          <!-- LookupCount=0 -->
+        </Feature>
+      </FeatureRecord>
+    </FeatureList>
+    <LookupList>
+      <!-- LookupCount=1 -->
+      <Lookup index="0">
+        <LookupType value="1"/>
+        <LookupFlag value="0"/>
+        <!-- SubTableCount=1 -->
+        <SingleSubst index="0">
+          <Substitution in="a" out="b"/>
+        </SingleSubst>
+      </Lookup>
+    </LookupList>
+    <FeatureVariations>
+      <Version value="0x00010000"/>
+      <FeatureVariationRecord index="0">
+        <ConditionSet>
+          <ConditionTable index="0" Format="1">
+            <AxisIndex value="0"/>
+            <FilterRangeMinValue value="0.625"/>
+            <FilterRangeMaxValue value="0.875"/>
+          </ConditionTable>
+        </ConditionSet>
+        <FeatureTableSubstitution>
+          <Version value="0x00010000"/>
+          <SubstitutionRecord index="0">
+            <FeatureIndex value="0"/>
+            <Feature>
+              <LookupListIndex index="0" value="0"/>
+            </Feature>
+          </SubstitutionRecord>
+        </FeatureTableSubstitution>
+      </FeatureVariationRecord>
+    </FeatureVariations>
+  </GSUB>
+
+</ttFont>