fix(theming): fix broken sass expressions nested in theme classes

* Fixes certain theme selectors being broken due to uses of the `&` operator at the end of the selector.
* Adds a custom Stylelint rule to catch future improper uses of the ampersand inside themes.

Relates to angular#3928.
Fixes angular#4077.

src/lib/input/_input-theme.scss

@@ -22,18 +22,19 @@
 
   .mat-input-placeholder {
     color: $input-placeholder-color;
+  }
 
-    // :focus is applied to the input, but we apply mat-focused to the other elements
-    // that need to listen to it.
-    .mat-focused & {
-      color: $input-floating-placeholder-color;
+  // :focus is applied to the input, but we apply mat-focused to the other elements
+  // that need to listen to it.
+  .mat-focused .mat-input-placeholder {
+    color: $input-floating-placeholder-color;
 
-      &.mat-accent {
-        color: $input-underline-color-accent;
-      }
-      &.mat-warn {
-        color: $input-underline-color-warn;
-      }
+    &.mat-accent {
+      color: $input-underline-color-accent;
+    }
+
+    &.mat-warn {
+      color: $input-underline-color-warn;
     }
   }
 

src/lib/radio/_radio-theme.scss

@@ -11,28 +11,26 @@
 
   .mat-radio-outer-circle {
     border-color: mat-color($foreground, secondary-text);
+  }
 
-    .mat-radio-checked & {
-      border-color: mat-color($accent);
-    }
+  .mat-radio-checked .mat-radio-outer-circle {
+    border-color: mat-color($accent);
+  }
 
-    .mat-radio-disabled & {
-      border-color: mat-color($foreground, disabled);
-    }
+  .mat-radio-disabled .mat-radio-outer-circle {
+    border-color: mat-color($foreground, disabled);
   }
 
   .mat-radio-inner-circle {
     background-color: mat-color($accent);
-
-    .mat-radio-disabled & {
-      background-color: mat-color($foreground, disabled);
-    }
   }
 
   .mat-radio-ripple .mat-ripple-element {
     background-color: mat-color($accent, 0.26);
+  }
 
-    .mat-radio-disabled & {
+  .mat-radio-disabled {
+    .mat-radio-ripple .mat-ripple-element, .mat-radio-inner-circle {
       background-color: mat-color($foreground, disabled);
     }
   }

src/lib/select/_select-theme.scss

@@ -20,6 +20,11 @@
   $accent: map-get($theme, accent);
   $warn: map-get($theme, warn);
 
+  .mat-select-trigger,
+  .mat-select-arrow {
+    color: mat-color($foreground, hint-text);
+  }
+
   .mat-select-underline {
     background-color: mat-color($foreground, divider);
   }

src/lib/slider/_slider-theme.scss

@@ -1,6 +1,17 @@
 @import '../core/theming/palette';
 @import '../core/theming/theming';
 
+@mixin _mat-slider-inner-content-theme($palette) {
+  .mat-slider-track-fill,
+  .mat-slider-thumb,
+  .mat-slider-thumb-label {
+    background-color: mat-color($palette);
+  }
+
+  .mat-slider-thumb-label-text {
+    color: mat-color($palette, default-contrast);
+  }
+}
 
 @mixin mat-slider-theme($theme) {
   $primary: map-get($theme, primary);
@@ -19,40 +30,22 @@
     background-color: $mat-slider-off-color;
   }
 
-  .mat-slider-track-fill,
-  .mat-slider-thumb,
-  .mat-slider-thumb-label {
-    .mat-primary & {
-      background-color: mat-color($primary);
-    }
+  .mat-primary {
+    @include _mat-slider-inner-content-theme($primary);
+  }
 
-    .mat-accent & {
-      background-color: mat-color($accent);
-    }
+  .mat-accent {
+    @include _mat-slider-inner-content-theme($accent);
+  }
 
-    .mat-warn & {
-      background-color: mat-color($warn);
-    }
+  .mat-warn {
+    @include _mat-slider-inner-content-theme($warn);
   }
 
   .mat-slider-focus-ring {
     background-color: $mat-slider-focus-ring-color;
   }
 
-  .mat-slider-thumb-label-text {
-    .mat-primary & {
-      color: mat-color($primary, default-contrast);
-    }
-
-    .mat-accent & {
-      color: mat-color($accent, default-contrast);
-    }
-
-    .mat-warn & {
-      color: mat-color($warn, default-contrast);
-    }
-  }
-
   .mat-slider:hover,
   .cdk-focused {
     .mat-slider-track-background {

src/lib/tabs/_tabs-theme.scss

@@ -12,8 +12,11 @@
   .mat-tab-nav-bar,
   .mat-tab-header {
     border-bottom: $header-border;
+  }
 
-    .mat-tab-group-inverted-header & {
+  .mat-tab-group-inverted-header {
+    .mat-tab-nav-bar,
+    .mat-tab-header {
       border-top: $header-border;
       border-bottom: none;
     }

stylelint-config.json

@@ -1,9 +1,15 @@
 {
   "plugins": [
-    "./tools/stylelint/no-prefixes/no-prefixes.js"
+    "./tools/stylelint/no-prefixes/no-prefixes.js",
+    "./tools/stylelint/selector-nested-pattern-scoped/index.js"
   ],
   "rules": {
     "material/no-prefixes": [["last 2 versions", "not ie <= 10", "not ie_mob <= 10"]],
+    "material/selector-nested-pattern-scoped": [".*[^&]$", {
+      "message": "The & operator is not allowed at the end of theme selectors.",
+      "filePattern": "-theme\\.scss$"
+    }],
+
     "color-hex-case": "lower",
     "color-no-invalid-hex": true,
 

tools/stylelint/selector-nested-pattern-scoped/index.js

@@ -0,0 +1,46 @@
+const stylelint = require('stylelint');
+const path = require('path');
+const isStandardSyntaxRule = require('stylelint/lib/utils/isStandardSyntaxRule');
+const isStandardSyntaxSelector = require('stylelint/lib/utils/isStandardSyntaxSelector');
+
+const ruleName = 'material/selector-nested-pattern-scoped';
+const messages = stylelint.utils.ruleMessages(ruleName, {
+  expected: selector => `Expected nested selector '${selector}' to match specified pattern`,
+});
+
+/**
+ * Re-implementation of the `selector-nested-pattern` Stylelint rule, allowing us
+ * to scope it to a particular set of files via the custom `filePattern` option. The
+ * primary use-case is to be able to apply the rule only to theme files.
+ *
+ * Reference: https://stylelint.io/user-guide/rules/selector-nested-pattern/
+ * Source: https://github.com/stylelint/stylelint/blob/master/lib/rules/selector-nested-pattern/
+ */
+const plugin = stylelint.createPlugin(ruleName, (pattern, options) => {
+  return (root, result) => {
+    const selectorPattern = new RegExp(pattern);
+    const filePattern = new RegExp(options.filePattern);
+    const fileName = path.basename(root.source.input.file);
+
+    if (!filePattern.test(fileName)) return;
+
+    root.walkRules(rule => {
+      if (rule.parent.type === 'rule' &&
+          isStandardSyntaxRule(rule) &&
+          isStandardSyntaxSelector(rule.selector) &&
+          !selectorPattern.test(rule.selector)) {
+
+        stylelint.utils.report({
+          result,
+          ruleName,
+          message: messages.expected(rule.selector),
+          node: rule
+        });
+      }
+    });
+  };
+});
+
+plugin.ruleName = ruleName;
+plugin.messages = messages;
+module.exports = plugin;