Add at-mixin-no-risky-nesting-selector rule.

src/rules/at-mixin-no-risky-nesting-selector/README.md

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+# at-mixin-no-risky-nesting-selector
+
+Disallow risky nesting selectors within a mixin.
+
+If a mixin contains a parent selector within another style rule, and is used in a nested context, 
+the output selector may include the outermost parent selector in an unexpected way.
+
+In this example:
+```scss
+@mixin foo {
+  .a {
+    color: blue;
+    .b & {
+      color: red;
+    }
+  }
+}
+
+.c {
+  @include foo;
+}
+```
+
+The user may expect `.c` to go outside all selectors in `foo`:
+`.c .b .a {...}`
+
+But this outputs:
+```scss
+.c .a {
+  color: blue;
+}
+.b .c .a {
+  color: red;
+}
+```
+
+However, if we pull the parent selector into the child and make the child style rule a sibling:
+```scss
+@mixin foo {
+  .a {
+    color: blue;
+  }
+  .b .a {
+    color: red;
+  }
+}
+
+.c {
+  @include foo;
+}
+```
+
+It outputs:
+```scss
+.c .a {
+  color: blue;
+}
+.c .b .a {
+  color: red;
+}
+```
+
+This only occurs when a parent selector meets all of the following conditions:
+- Is within a `@mixin` rule.
+- Is nested within another style rule.
+- Is not positioned at the beginning of a complex selector.
+
+## Options
+
+### `true`
+
+The following patterns are considered warnings:
+
+```scss
+@mixin foo {
+  .bar {
+    color: blue;
+    .baz & {
+      color: red;
+    }
+  }
+}
+```
+
+```scss
+@mixin foo {
+  .bar {
+    color: blue;
+    .qux, .baz & .quux{
+      color: red;
+    }
+  }
+}
+```
+
+The following patterns are _not_ considered warnings:
+
+```scss
+.foo {
+  .bar {
+    color: blue;
+    .baz & {
+      color: red;
+    }
+  }
+}
+```
+
+```scss
+@mixin foo {
+  .bar {
+    color: blue;
+    & .baz {
+      color: red;
+    }
+  }
+}
+```
+
+```scss
+.bar {
+  color: blue;
+  .baz & {
+    color: red;
+  }
+}
+```
+
+```scss
+.foo {
+  color: blue;
+  & .bar, .baz & .qux {
+    color: red;
+  }
+}
+```
+
+```scss
+@mixin foo {
+  .baz & {
+    color: red;
+  }
+}
+```

src/rules/at-mixin-no-risky-nesting-selector/__tests__/index.js

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+"use strict";
+
+const { ruleName } = require("..");
+
+testRule({
+  ruleName,
+  config: [true],
+  customSyntax: "postcss-scss",
+
+  accept: [
+    {
+      code: `
+      .parent {
+        color: blue;
+    
+        & .b {
+          color: red;
+        }
+      }
+    `,
+      description: "Nested parent selector"
+    },
+    {
+      code: `
+      .parent {
+        color: blue;
+    
+        & .b, &.context {
+          color: red;
+        }
+      }
+    `,
+      description: "Nested parent selector in complex selector"
+    },
+    {
+      code: `
+      .bar {
+        & .parent {
+          color: blue;
+      
+          .context {
+            color: red;
+          }
+        }
+      }
+    `,
+      description: "Parent selector nested in another style rule"
+    },
+    {
+      code: `
+      @mixin foo {
+        &.context {
+          color: red;
+        }
+      }
+    `,
+      description: "Parent selector in mixin"
+    },
+    {
+      code: `
+      .bar {
+        .parent {
+          color: blue;
+      
+          & .context {
+            color: red;
+          }
+        }
+      }
+    `,
+      description: "Parent selector nested in more than one style rule"
+    },
+    {
+      code: `
+      .parent {
+        color: blue;
+    
+        .context & {
+          color: red;
+        }
+      }
+    `,
+      description: "Selector ending in parent selector"
+    },
+    {
+      code: `
+      .parent {
+        color: blue;
+    
+        .context & .b {
+          color: red;
+        }
+      }
+    `,
+      description: "Parent selector in the middle of complex selector"
+    },
+    {
+      code: `
+      .parent {
+        color: blue;
+    
+        & .b, .context & {
+          color: red;
+        }
+      }
+    `,
+      description: "Complex selector, one ending in parent selector"
+    },
+    {
+      code: `
+      @mixin foo {
+        .parent {
+          color: blue;
+      
+          & .context {
+            color: red;
+          }
+        }
+      }
+    `,
+      description: "Parent selector in mixin, nested, at the beginning"
+    },
+    {
+      code: `
+      @mixin foo {
+        .bar & {
+          color: red;
+        }
+      }
+    `,
+      description: "Parent selector in mixin, at the end"
+    },
+    {
+      code: `
+      @mixin foo {
+        .parent {
+          .bar, & {
+            color: red;
+          }
+        }
+      }
+    `,
+      description: "Complex selector in mixin, nested, no following class."
+    }
+  ],
+
+  reject: [
+    {
+      code: `
+      @mixin foo {
+        .bar {
+          color: blue;
+          .baz & {
+            color: red;
+          }
+        }
+      }
+    `,
+      description: "Parent selector nested in selector within a mixin",
+      message:
+        "Unexpected nested parent selector in @mixin rule. (scss/at-mixin-no-risky-nesting-selector)",
+      column: 11,
+      endColumn: 12,
+      endLine: 7,
+      line: 5
+    },
+    {
+      code: `
+      @mixin foo {
+        .bar {
+          color: blue;
+          & .qux, .baz & .quux{
+            color: red;
+          }
+        }
+      }
+    `,
+      description: "Parent selector nested in complex selector within mixin",
+      message:
+        "Unexpected nested parent selector in @mixin rule. (scss/at-mixin-no-risky-nesting-selector)",
+      column: 11,
+      endColumn: 12,
+      endLine: 7,
+      line: 5
+    },
+    {
+      code: `
+      @mixin foo {
+        .bar {
+          color: blue;
+          & .qux & {
+            color: red;
+          }
+        }
+      }
+    `,
+      description: "Parent selector nested in complex selector within mixin",
+      message:
+        "Unexpected nested parent selector in @mixin rule. (scss/at-mixin-no-risky-nesting-selector)",
+      column: 11,
+      endColumn: 12,
+      endLine: 7,
+      line: 5
+    }
+  ]
+});