fix: preserve string context in + operator

nix-js/runtime-ts/src/operators.ts

@@ -7,7 +7,7 @@ import type { NixValue, NixList, NixAttrs, NixString, NixPath } from "./types";
 import { isNixPath } from "./types";
 import { force } from "./thunk";
 import { forceNumeric, forceList, forceAttrs, coerceNumeric } from "./type-assert";
-import { getStringValue, getStringContext, mergeContexts, mkStringWithContext } from "./string-context";
+import { type NixStringContext, getStringValue, getStringContext, mergeContexts, mkStringWithContext } from "./string-context";
 import { coerceToString, StringCoercionMode } from "./builtins/conversion";
 import { mkPath } from "./path";
 import { typeOf, isNixString } from "./builtins/type-check";
@@ -146,9 +146,14 @@ export const op = {
 
     // Auto-coerce to string if possible
     if (canCoerceToString(a) && canCoerceToString(b)) {
-      const strA = coerceToString(a, StringCoercionMode.Interpolation, false);
-      const strB = coerceToString(b, StringCoercionMode.Interpolation, false);
-      return strA + strB;
+      const context: NixStringContext = new Set();
+      const strA = coerceToString(a, StringCoercionMode.Interpolation, false, context);
+      const strB = coerceToString(b, StringCoercionMode.Interpolation, false, context);
+      const result = strA + strB;
+      if (context.size === 0) {
+        return result;
+      }
+      return mkStringWithContext(result, context);
     }
 
     // Numeric addition

nix-js/tests/derivation.rs

@@ -3,6 +3,42 @@ mod utils;
 use nix_js::value::Value;
 use utils::{eval_deep, eval_deep_result};
 
+#[test]
+fn add_operator_preserves_derivation_context() {
+    let result = eval_deep(
+        r#"
+        let
+          dep = derivation { name = "dep"; builder = "/bin/sh"; system = "x86_64-linux"; outputs = ["out" "dev"]; };
+          getOutput = output: pkg: pkg.${output} or pkg.out or pkg;
+          user = derivation {
+            name = "user";
+            builder = "/bin/sh";
+            system = "x86_64-linux";
+            libPath = (getOutput "lib" dep) + "/lib";
+            devPath = dep.dev + "/include";
+          };
+        in user.drvPath
+        "#,
+    );
+
+    let nix_result = eval_deep(
+        r#"
+        let
+          dep = derivation { name = "dep"; builder = "/bin/sh"; system = "x86_64-linux"; outputs = ["out" "dev"]; };
+          user = derivation {
+            name = "user";
+            builder = "/bin/sh";
+            system = "x86_64-linux";
+            libPath = "${dep.out}/lib";
+            devPath = "${dep.dev}/include";
+          };
+        in user.drvPath
+        "#,
+    );
+
+    assert_eq!(result, nix_result);
+}
+
 #[test]
 fn derivation_minimal() {
     let result = eval_deep(