disallow interpolation in the name of mixin declarations

src/parse/mixin.rs

@@ -17,7 +17,7 @@ use super::{common::ContextFlags, Parser, Stmt};
 impl<'a> Parser<'a> {
     pub(super) fn parse_mixin(&mut self) -> SassResult<()> {
         self.whitespace();
-        let Spanned { node: name, span } = self.parse_identifier()?;
+        let Spanned { node: name, span } = self.parse_identifier_no_interpolation(false)?;
 
         if self.flags.in_mixin() {
             return Err(("Mixins may not contain mixin declarations.", span).into());

tests/mixins.rs

@@ -478,3 +478,14 @@ error!(
     }",
     "Error: Mixins may not be declared in control directives."
 );
+error!(
+    does_not_allow_interpolation_in_name_of_declaration,
+    "@mixin n#{a}me {
+        color: red;
+    }
+
+    a {
+        @include name;
+    }",
+    "Error: expected \"{\"."
+);