make build tests not depend on minutiæ of rustc output

This little patch arises from the maelstrom of my purity born of pain.

It's the pain of seeing rust-lang/rust#38103 in its perfect
crystalline beauty waste away on page four of
https://github.com/rust-lang/rust/pulls, waiting, ready, itching to
land, dying with anticipation to bring the light of clearer lint group
error messages to Rust users of all creeds and nations, only for its
promise to be cruelly blocked by the fateful, hateful hand of circular
dependency. For it is written in src/tools/cargotest/main.rs that the
Cargo tests must pass before the PR can receive Appveyor's blessing,
but the Cargo tests could not pass (because they depend on fine
details of the output that the PR is meant to change), and the Cargo
tests could not be changed (because updating the test expectation to
match the proposed new compiler output, would fail with the current
compiler).

The Gordian knot is cut in the bowels of cargotest's very notion of
comparison (of JSON objects) itself, by means of introducing a magic
string literal `"{...}"`, which can server as a wildcard for any JSON
sub-object.

And so it will be for the children, and the children's children, and
unto the 1.17.0 and 1.18.0 releases, that Cargo's build test
expectations will faithfully expect the exact JSON output by Cargo
itself, but the string literal `"{...}"` shall be a token upon the
JSON output by rustc, and when I see `"{...}"`, I will pass over you,
and the failure shall not be upon you.

And this day shall be unto you for a memorial.

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":"{...}"
     }
 
     {

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)),
     }