tests.nixpkgs-check-by-name: Introduce result_map

Convenience function to run another validation over a successful validation result.

This will be usable in more locations in future commits, making the code
nicer.

pkgs/test/nixpkgs-check-by-name/src/main.rs

@@ -86,14 +86,9 @@ pub fn check_nixpkgs<W: io::Write>(
         );
         Success(())
     } else {
-        match check_structure(&nixpkgs_path)? {
-            Failure(errors) => Failure(errors),
-            Success(package_names) =>
+        check_structure(&nixpkgs_path)?.result_map(|package_names|
             // Only if we could successfully parse the structure, we do the evaluation checks
-            {
-                eval::check_values(version, &nixpkgs_path, package_names, eval_accessible_paths)?
-            }
-        }
+            eval::check_values(version, &nixpkgs_path, package_names, eval_accessible_paths))?
     };
 
     match check_result {

pkgs/test/nixpkgs-check-by-name/src/validation.rs

@@ -58,6 +58,15 @@ impl<A> Validation<A> {
             Success(value) => Success(f(value)),
         }
     }
+
+    /// Map a `Validation<A>` to a `Result<B>` by applying a function `A -> Result<B>`
+    /// only if there is a `Success` value
+    pub fn result_map<B>(self, f: impl FnOnce(A) -> Result<B>) -> Result<B> {
+        match self {
+            Failure(err) => Ok(Failure(err)),
+            Success(value) => f(value),
+        }
+    }
 }
 
 impl Validation<()> {