Rollup merge of rust-lang#56223 - Mark-Simulacrum:self-profile-json, r=wesleywiser

Make JSON output from -Zprofile-json valid

r? @wesleywiser

cc rust-lang/rustc-perf#299

Author: kennytm

Diff for: src/librustc/util/profiling.rs

@@ -102,7 +102,7 @@ macro_rules! define_categories {
                         };
 
                     json.push_str(&format!(
-                        "{{ \"category\": {}, \"time_ms\": {},
+                        "{{ \"category\": \"{}\", \"time_ms\": {},\
                             \"query_count\": {}, \"query_hits\": {} }},",
                         stringify!($name),
                         self.times.$name / 1_000_000,