diff --git a/tests/build.rs b/tests/build.rs
index 6ca86b6d267..75ca7dcee53 100644
--- a/tests/build.rs
+++ b/tests/build.rs
@@ -2511,16 +2511,7 @@ fn compiler_json_error_format() {
         "reason":"compiler-message",
         "package_id":"bar 0.5.0 ([..])",
         "target":{"kind":["lib"],"name":"bar","src_path":"[..]lib.rs"},
-        "message":{
-            "children":[],"code":null,"level":"warning","rendered":null,
-            "message":"function is never used: `dead`, #[warn(dead_code)] on by default",
-            "spans":[{
-                "byte_end":12,"byte_start":0,"column_end":13,"column_start":1,"expansion":null,
-                "file_name":"[..]","is_primary":true,"label":null,"line_end":1,"line_start":1,
-                "suggested_replacement":null,
-                "text":[{"highlight_end":13,"highlight_start":1,"text":"fn dead() {}"}]
-            }]
-        }
+        "message":"{...}"
     }
 
     {
@@ -2541,16 +2532,7 @@ fn compiler_json_error_format() {
         "reason":"compiler-message",
         "package_id":"foo 0.5.0 ([..])",
         "target":{"kind":["bin"],"name":"foo","src_path":"[..]main.rs"},
-        "message":{
-            "children":[],"code":null,"level":"warning","rendered":null,
-            "message":"unused variable: `unused`, #[warn(unused_variables)] on by default",
-            "spans":[{
-                "byte_end":22,"byte_start":16,"column_end":23,"column_start":17,"expansion":null,
-                "file_name":"[..]","is_primary":true,"label":null,"line_end":1,"line_start":1,
-                "suggested_replacement":null,
-                "text":[{"highlight_end":23,"highlight_start":17,"text":"[..]"}]
-            }]
-        }
+        "message":"{...}"
     }
 
     {
@@ -2599,20 +2581,7 @@ fn message_format_json_forward_stderr() {
         "reason":"compiler-message",
         "package_id":"foo 0.5.0 ([..])",
         "target":{"kind":["bin"],"name":"foo","src_path":"[..]"},
-        "message":{
-            "children":[],"code":null,"level":"warning","rendered":null,
-            "message":"unused variable: `unused`, #[warn(unused_variables)] on by default",
-            "spans":[{
-                "byte_end":22,"byte_start":16,"column_end":23,"column_start":17,"expansion":null,
-                "file_name":"[..]","is_primary":true,"label":null,"line_end":1,"line_start":1,
-                "suggested_replacement":null,
-                "text":[{
-                    "highlight_end":23,
-                    "highlight_start":17,
-                    "text":"fn main() { let unused = 0; }"
-                }]
-            }]
-        }
+        "message":"{...}"
     }
 
     {
diff --git a/tests/cargotest/support/mod.rs b/tests/cargotest/support/mod.rs
index de218b6fcc4..054d35cf73e 100644
--- a/tests/cargotest/support/mod.rs
+++ b/tests/cargotest/support/mod.rs
@@ -570,6 +570,8 @@ fn lines_match_works() {
 
 // Compares JSON object for approximate equality.
 // You can use `[..]` wildcard in strings (useful for OS dependent things such as paths).
+// You can use a `"{...}"` string literal as a wildcard for arbitrary nested JSON (useful
+// for parts of object emitted by other programs (e.g. rustc) rather than Cargo itself).
 // Arrays are sorted before comparison.
 fn find_mismatch<'a>(expected: &'a Json, actual: &'a Json) -> Option<(&'a Json, &'a Json)> {
     use rustc_serialize::json::Json::*;
@@ -586,8 +588,7 @@ fn find_mismatch<'a>(expected: &'a Json, actual: &'a Json) -> Option<(&'a Json,
 
             fn sorted(xs: &Vec<Json>) -> Vec<&Json> {
                 let mut result = xs.iter().collect::<Vec<_>>();
-                // `unwrap` should be safe because JSON spec does not allow NaNs
-                result.sort_by(|x, y| x.partial_cmp(y).unwrap());
+                result.sort_by(|x, y| x.partial_cmp(y).expect("JSON spec does not allow NaNs"));
                 result
             }
 
@@ -606,6 +607,8 @@ fn find_mismatch<'a>(expected: &'a Json, actual: &'a Json) -> Option<(&'a Json,
              .nth(0)
         }
         (&Null, &Null) => None,
+        // magic string literal "{...}" acts as wildcard for any sub-JSON
+        (&String(ref l), _) if l == "{...}" => None,
         _ => Some((expected, actual)),
     }