build: add stylelint rule to avoid top-level ampersands in themes

src/lib/radio/_radio-theme.scss

@@ -2,7 +2,7 @@
 @import '../core/theming/theming';
 @import '../core/typography/typography-utils';
 
-@mixin mat-radio-color($palette) {
+@mixin _mat-radio-color($palette) {
   &.mat-radio-checked .mat-radio-outer-circle {
     border-color: mat-color($palette);
   }
@@ -43,15 +43,15 @@
 
   .mat-radio-button {
     &.mat-primary {
-      @include mat-radio-color($primary);
+      @include _mat-radio-color($primary);
     }
 
     &.mat-accent {
-      @include mat-radio-color($accent);
+      @include _mat-radio-color($accent);
     }
 
     &.mat-warn {
-      @include mat-radio-color($warn);
+      @include _mat-radio-color($warn);
     }
   }
 }

stylelint-config.json

@@ -4,7 +4,8 @@
     "./tools/stylelint/selector-nested-pattern-scoped/index.js",
     "./tools/stylelint/selector-no-deep/index.js",
     "./tools/stylelint/no-nested-mixin/index.js",
-    "./tools/stylelint/no-concrete-rules/index.js"
+    "./tools/stylelint/no-concrete-rules/index.js",
+    "./tools/stylelint/no-top-level-ampersand-in-mixin/index.js"
   ],
   "rules": {
     "material/no-prefixes": [["last 2 versions", "not ie <= 10", "not ie_mob <= 10"]],
@@ -17,6 +18,9 @@
     "material/no-concrete-rules": [true, {
       "filePattern": "^_.*\\.scss$"
     }],
+    "material/no-top-level-ampersand-in-mixin": [true, {
+      "filePattern": "-theme\\.scss$"
+    }],
 
     "color-hex-case": "lower",
     "color-no-invalid-hex": true,

tools/stylelint/no-top-level-ampersand-in-mixin/index.js

@@ -0,0 +1,44 @@
+const stylelint = require('stylelint');
+const path = require('path');
+const ruleName = 'material/no-top-level-ampersand-in-mixin';
+const messages = stylelint.utils.ruleMessages(ruleName, {
+  expected: () => `Selectors starting with an ampersand ` +
+                  `are not allowed inside top-level mixin rules`
+});
+
+/**
+ * Stylelint rule that doesn't allow for the top-level rules inside a
+ * mixin to start with an ampersand. Does not apply to internal mixins.
+ */
+const plugin = stylelint.createPlugin(ruleName, (isEnabled, options) => {
+  return (root, result) => {
+    if (!isEnabled) return;
+
+    const filePattern = new RegExp(options.filePattern);
+    const fileName = path.basename(root.source.input.file);
+
+    if (!filePattern.test(fileName)) return;
+
+    root.walkAtRules(node => {
+      // Skip non-mixin atrules and internal mixins.
+      if (node.name !== 'mixin' || node.params.indexOf('_') === 0) {
+        return;
+      }
+
+      node.nodes.forEach(childNode => {
+        if (childNode.type === 'rule' && childNode.selector.indexOf('&') === 0) {
+          stylelint.utils.report({
+            result,
+            ruleName,
+            message: messages.expected(),
+            node: childNode
+          });
+        }
+      });
+    });
+  };
+});
+
+plugin.ruleName = ruleName;
+plugin.messages = messages;
+module.exports = plugin;