Fix unused module for discarded alias collision

Author: jtdowney

compiler-core/src/reference.rs

@@ -748,9 +748,10 @@ impl ReferenceTracker {
     /// that remembers which module it came from.
     ///
     fn register_module_reference_by_module_name(&mut self, name: EcoString) {
-        let target = match self.module_name_to_node.get(&name) {
-            Some(target) => *target,
-            None => self.get_or_create_node(name, EntityLayer::Module),
+        // If the module has no node then it was imported with a discarded
+        // alias, meaning there's no import for the reference to point at.
+        let Some(target) = self.module_name_to_node.get(&name).copied() else {
+            return;
         };
         _ = self.graph.add_edge(self.current_node, target, ());
     }

compiler-core/src/type_/tests/snapshots/gleam_core__type___tests__warnings__unused_module_not_marked_as_used_by_discard_aliased_import_with_same_name.snap

@@ -0,0 +1,26 @@
+---
+source: compiler-core/src/type_/tests/warnings.rs
+expression: "\n            import wibble/wobble\n            import wobble.{two} as _wobble\n            pub fn main() { two }\n        "
+---
+----- SOURCE CODE
+-- wibble/wobble.gleam
+pub const one = 1
+
+-- wobble.gleam
+pub const two = 2
+
+-- main.gleam
+
+            import wibble/wobble
+            import wobble.{two} as _wobble
+            pub fn main() { two }
+        
+
+----- WARNING
+warning: Unused imported module
+  ┌─ /src/warning/wrn.gleam:2:13
+  │
+2 │             import wibble/wobble
+  │             ^^^^^^^^^^^^^^^^^^^^ This imported module is never used
+
+Hint: You can safely remove it.

compiler-core/src/type_/tests/warnings.rs

@@ -1189,6 +1189,19 @@ fn unused_module_alias_not_confused_with_another_module_name() {
     );
 }
 
+#[test]
+fn unused_module_not_marked_as_used_by_discard_aliased_import_with_same_name() {
+    assert_warning!(
+        ("thepackage", "wibble/wobble", "pub const one = 1"),
+        ("thepackage", "wobble", "pub const two = 2"),
+        r#"
+            import wibble/wobble
+            import wobble.{two} as _wobble
+            pub fn main() { two }
+        "#
+    );
+}
+
 #[test]
 fn result_in_case_discarded() {
     assert_warning!(