Bug 1374628 - Improve static_asserts in TelemetryHistogram.cpp. r=gfritzsche

With bug 1374419 solved, static asserts in TelemetryHistogram.cpp can be improved.

Specifically, JS::gcreason::NUM_TELEMETRY_REASONS can now be compared against GC_MINOR_REASON, GC_MINOR_REASON_LONG, and GC_REASON_2 in the now constexpr gHistograms. Likewise, mozilla::StartupTimeline::MAX_EVENT_ID can be compared against STARTUP_MEASUREMENT_ERRORS.

This commit modifies the static asserts to perform these comparisons in place of ones against hardcoded values and modifies the accompanying comment to remove the TODO section.

toolkit/components/telemetry/TelemetryHistogram.cpp

@@ -1883,22 +1883,27 @@ void TelemetryHistogram::InitializeGlobalState(bool canRecordBase,
     }
   }
 
-  // Some Telemetry histograms depend on the value of C++ constants and hardcode
-  // their values in Histograms.json.
-  // We add static asserts here for those values to match so that future changes
-  // don't go unnoticed.
-  // TODO: Compare explicitly with gHistograms[<histogram id>].bucketCount here
-  // once we can make gHistograms constexpr (requires VS2015).
-  static_assert((JS::gcreason::NUM_TELEMETRY_REASONS == 100),
-      "NUM_TELEMETRY_REASONS is assumed to be a fixed value in Histograms.json."
-      " If this was an intentional change, update this assert with its value "
-      "and update the n_values for the following in Histograms.json: "
-      "GC_MINOR_REASON, GC_MINOR_REASON_LONG, GC_REASON_2");
-  static_assert((mozilla::StartupTimeline::MAX_EVENT_ID == 16),
-      "MAX_EVENT_ID is assumed to be a fixed value in Histograms.json.  If this"
-      " was an intentional change, update this assert with its value and update"
-      " the n_values for the following in Histograms.json:"
-      " STARTUP_MEASUREMENT_ERRORS");
+    // Some Telemetry histograms depend on the value of C++ constants and hardcode
+    // their values in Histograms.json.
+    // We add static asserts here for those values to match so that future changes
+    // don't go unnoticed.
+    static_assert((JS::gcreason::NUM_TELEMETRY_REASONS + 1) ==
+                        gHistograms[mozilla::Telemetry::GC_MINOR_REASON].bucketCount &&
+                  (JS::gcreason::NUM_TELEMETRY_REASONS + 1) ==
+                        gHistograms[mozilla::Telemetry::GC_MINOR_REASON_LONG].bucketCount &&
+                  (JS::gcreason::NUM_TELEMETRY_REASONS + 1) ==
+                        gHistograms[mozilla::Telemetry::GC_REASON_2].bucketCount,
+                  "NUM_TELEMETRY_REASONS is assumed to be a fixed value in Histograms.json."
+                  " If this was an intentional change, update the n_values for the "
+                  "following in Histograms.json: GC_MINOR_REASON, GC_MINOR_REASON_LONG, "
+                  "GC_REASON_2");
+
+    static_assert((mozilla::StartupTimeline::MAX_EVENT_ID + 1) ==
+                        gHistograms[mozilla::Telemetry::STARTUP_MEASUREMENT_ERRORS].bucketCount,
+                  "MAX_EVENT_ID is assumed to be a fixed value in Histograms.json.  If this"
+                  " was an intentional change, update the n_values for the following in "
+                  "Histograms.json: STARTUP_MEASUREMENT_ERRORS");
+
 
   gInitDone = true;
 }