implement idempotency with regard to module aliasing

src/builtin/modules/mod.rs

@@ -28,8 +28,18 @@ pub(crate) struct Module(pub Scope);
 pub(crate) struct Modules(BTreeMap<Identifier, Module>);
 
 impl Modules {
-    pub fn insert(&mut self, name: Identifier, module: Module) {
+    pub fn insert(&mut self, name: Identifier, module: Module, span: Span) -> SassResult<()> {
+        if self.0.contains_key(&name) {
+            return Err((
+                format!("There's already a module with namespace \"{}\".", name),
+                span,
+            )
+                .into());
+        }
+
         self.0.insert(name, module);
+
+        Ok(())
     }
 
     pub fn get(&self, name: Identifier, span: Span) -> SassResult<&Module> {

src/parse/mod.rs

@@ -263,7 +263,7 @@ impl<'a> Parser<'a> {
                         },
                     };
 
-                    self.modules.insert(module_name.into(), module);
+                    self.modules.insert(module_name.into(), module, span)?;
                 }
                 Some(Token { kind: '/', .. }) => {
                     self.toks.next();

tests/use.rs

@@ -158,3 +158,36 @@ fn use_user_defined_function() {
         &grass::from_string(input.to_string(), &grass::Options::default()).expect(input)
     );
 }
+
+#[test]
+fn use_idempotent_no_alias() {
+    let input = "@use \"use_idempotent_no_alias\";\n@use \"use_idempotent_no_alias\";\n";
+    tempfile!("use_idempotent_no_alias.scss", "");
+
+    assert_err!(
+        "Error: There's already a module with namespace \"use-idempotent-no-alias\".",
+        input
+    );
+}
+
+#[test]
+fn use_idempotent_with_alias() {
+    let input = "@use \"use_idempotent_with_alias__a\" as foo;\n@use \"use_idempotent_with_alias__b\" as foo;\n";
+    tempfile!("use_idempotent_with_alias__a.scss", "");
+    tempfile!("use_idempotent_with_alias__b.scss", "");
+
+    assert_err!(
+        "Error: There's already a module with namespace \"foo\".",
+        input
+    );
+}
+
+#[test]
+fn use_idempotent_builtin() {
+    let input = "@use \"sass:math\";\n@use \"sass:math\";\n";
+
+    assert_err!(
+        "Error: There's already a module with namespace \"math\".",
+        input
+    );
+}