fix(add): Preserve formatting

Author: epage

src/cargo/util/toml_mut/manifest.rs

@@ -516,7 +516,9 @@ fn fix_feature_activations(
             } = parsed_value
             {
                 if dep_name == dep_key && weak {
-                    *value = format!("{dep_name}/{dep_feature}").into();
+                    let mut new_value = toml_edit::Value::from(format!("{dep_name}/{dep_feature}"));
+                    *new_value.decor_mut() = value.decor().clone();
+                    *value = new_value;
                 }
             }
         }

tests/testsuite/cargo_add/preserve_features_table/out/Cargo.toml

@@ -14,7 +14,8 @@ default = [
   "b",
   "c",
 ]
-a = ["your-face/nose",  # but not the mouth and nose
+a = [
+  "your-face/nose",  # but not the mouth and nose
 ]
 b = []
 c = []