Refine debug print-out.

src/engine.rs

@@ -52,7 +52,7 @@ pub type Precedence = NonZeroU8;
 // We cannot use Cow<str> here because `eval` may load a [module][Module] and
 // the module name will live beyond the AST of the eval script text.
 // The best we can do is a shared reference.
-#[derive(Debug, Clone, Default)]
+#[derive(Clone, Default)]
 pub struct Imports(StaticVec<Identifier>, StaticVec<Shared<Module>>);
 
 impl Imports {
@@ -147,6 +147,20 @@ impl Imports {
     }
 }
 
+impl fmt::Debug for Imports {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("Imports")?;
+
+        if self.is_empty() {
+            f.debug_map().finish()
+        } else {
+            f.debug_map()
+                .entries(self.0.iter().zip(self.1.iter()))
+                .finish()
+        }
+    }
+}
+
 #[cfg(not(feature = "unchecked"))]
 #[cfg(debug_assertions)]
 #[cfg(not(feature = "no_function"))]