feat(tvix/derivation) Add fmt::Display implementation for Derivation

This adds the implementation of fmt::Display for Derivation so that we can
easily store the formatted content as a string. Internally, we use the
serialization function to generate the string.

Change-Id: I6caca0d6c1bea3ca44b6c535c5b1d5d66d8413b7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7741
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/derivation/src/tests/mod.rs

@@ -14,7 +14,8 @@ fn read_file(path: &str) -> String {
     return data;
 }
 
-fn assert_derivation_ok(path_to_drv_file: &str) {
+#[test_resources("src/tests/derivation_tests/*.drv")]
+fn check_serizaliation(path_to_drv_file: &str) {
     let data = read_file(&format!("{}.json", path_to_drv_file));
     let derivation: Derivation = serde_json::from_str(&data).expect("JSON was not well-formatted");
 
@@ -27,6 +28,11 @@ fn assert_derivation_ok(path_to_drv_file: &str) {
 }
 
 #[test_resources("src/tests/derivation_tests/*.drv")]
-fn derivation_files_ok(path: &str) {
-    assert_derivation_ok(path);
+fn check_to_string(path_to_drv_file: &str) {
+    let data = read_file(&format!("{}.json", path_to_drv_file));
+    let derivation: Derivation = serde_json::from_str(&data).expect("JSON was not well-formatted");
+
+    let expected = read_file(path_to_drv_file);
+
+    assert_eq!(expected, derivation.to_string());
 }