From 94becb4dcb8cfd896efe8a7812e855445652bcd3 Mon Sep 17 00:00:00 2001
From: Connor Skees <connor1skees@gmail.com>
Date: Thu, 6 Aug 2020 04:00:45 -0400
Subject: [PATCH] implement idempotency with regard to module aliasing

---
 src/builtin/modules/mod.rs | 12 +++++++++++-
 src/parse/mod.rs           |  2 +-
 tests/use.rs               | 33 +++++++++++++++++++++++++++++++++
 3 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/src/builtin/modules/mod.rs b/src/builtin/modules/mod.rs
index 555aeff..708bbbe 100644
--- a/src/builtin/modules/mod.rs
+++ b/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> {
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index 1d5bab7..45b67ca 100644
--- a/src/parse/mod.rs
+++ b/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();
diff --git a/tests/use.rs b/tests/use.rs
index f5e1b3f..d30af39 100644
--- a/tests/use.rs
+++ b/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
+    );
+}