deny mixins in functions, control flow, and mixins

src/parse/mixin.rs

@@ -18,6 +18,19 @@ impl<'a> Parser<'a> {
     pub(super) fn parse_mixin(&mut self) -> SassResult<()> {
         self.whitespace();
         let Spanned { node: name, span } = self.parse_identifier()?;
+
+        if self.flags.in_mixin() {
+            return Err(("Mixins may not contain mixin declarations.", span).into());
+        }
+
+        if self.flags.in_function() {
+            return Err(("This at-rule is not allowed here.", span).into());
+        }
+
+        if self.flags.in_control_flow() {
+            return Err(("Mixins may not be declared in control directives.", span).into());
+        }
+
         self.whitespace();
         let args = match self.toks.next() {
             Some(Token { kind: '(', .. }) => self.parse_func_args()?,

tests/mixins.rs

@@ -449,3 +449,32 @@ test!(
     }",
     "a {\n  color: foo;\n  color: foo;\n}\n"
 );
+error!(
+    mixin_in_function,
+    "@function foo() {
+        @mixin bar {}
+    }
+    a {
+        color: foo();
+    }
+    ",
+    "Error: This at-rule is not allowed here."
+);
+error!(
+    mixin_in_mixin,
+    "@mixin foo {
+        @mixin bar {}
+    }
+    a {
+        @include foo;
+    }
+    ",
+    "Error: Mixins may not contain mixin declarations."
+);
+error!(
+    mixin_in_control_directives,
+    "@if true {
+        @mixin bar {}
+    }",
+    "Error: Mixins may not be declared in control directives."
+);