usage: add string exception for attrpaths

Fixes Github issue #91

src/dead_code_tests.rs

@@ -461,3 +461,24 @@ fn let_partially_shadowed_by_attrset() {
     assert_eq!(1, results.len());
     assert_eq!(results[0].binding.name.to_string(), "dead");
 }
+
+#[test]
+fn let_args_splice() {
+    let results = run("
+      { config, ... }:
+
+      \"${config.bar}\"
+    ");
+    assert_eq!(0, results.len());
+}
+
+#[test]
+fn let_args_string_splice() {
+    let results = run("
+      { config, ... }: {
+
+        \"${config.bar}\" = 42;
+      }
+    ");
+    assert_eq!(0, results.len());
+}

src/usage.rs

@@ -27,9 +27,13 @@ pub fn find(name: &Ident, node: &SyntaxNode<NixLanguage>) -> bool {
         ident.syntax().text() == name.syntax().text()
     } else if node.kind() == SyntaxKind::NODE_ATTRPATH {
         // Don't search for idents in keys, they introduce new scopes
-        // anyway. Except for `${...}`
+        // anyway. Except for `${...}` and `"..."` which do not
+        // introduce new scopes in attrsets that are not declared
+        // `rec`.
         node.children().any(|node|
-            node.kind() == SyntaxKind::NODE_DYNAMIC &&
+            (node.kind() == SyntaxKind::NODE_DYNAMIC ||
+             node.kind() == SyntaxKind::NODE_STRING
+            ) &&
             find(name, &node)
         )
     } else {