Avoid retaining hidden simple deferred messages

reporter.go

@@ -249,6 +249,16 @@ func (r DeferredReporter) enqueue(category messageCategory, message string, deta
 	if state.closed {
 		return ErrReporterClosed
 	}
+	if !hasDetails(details) {
+		if message == "" {
+			return nil
+		}
+		if !state.visible(category) {
+			state.diagnostics.submitted = true
+			state.diagnostics.suppressed = true
+			return nil
+		}
+	}
 	state.deferred = append(state.deferred, deferredMessage{
 		category: category,
 		message:  message,
@@ -268,12 +278,16 @@ func (r *Reporter) finalize(outcome Outcome, completion func(*Reporter)) Outcome
 		state.mu.Unlock()
 		return normalizeOutcome(outcome)
 	}
-	deferred := append([]deferredMessage(nil), state.deferred...)
+	deferred := state.deferred
 	state.deferred = nil
 	state.mu.Unlock()
 
-	for _, message := range deferred {
-		_ = r.submitMessage(message.category, message.message, message.details)
+	for index := range deferred {
+		message := deferred[index]
+		deferred[index] = deferredMessage{}
+		state.mu.Lock()
+		_ = state.submitMessageLocked(message.category, message.message, message.details)
+		state.mu.Unlock()
 	}
 	if completion != nil {
 		completion(r)
@@ -339,10 +353,11 @@ func ensureFallbackExplanation(outcome Outcome) Outcome {
 }
 
 func hasExplanation(explanation Explanation) bool {
-	if explanation.Message != "" {
-		return true
-	}
-	for _, detail := range explanation.Details {
+	return explanation.Message != "" || hasDetails(explanation.Details)
+}
+
+func hasDetails(details []Detail) bool {
+	for _, detail := range details {
 		if detail != nil {
 			return true
 		}
@@ -405,16 +420,16 @@ func (r *Reporter) submitMessage(category messageCategory, message string, detai
 	if state.closed {
 		return ErrReporterClosed
 	}
+	return state.submitMessageLocked(category, message, append([]Detail(nil), details...))
+}
 
-	details = append([]Detail(nil), details...)
-	hasDetails := false
-	for _, detail := range details {
-		if detail != nil {
-			hasDetails = true
-			break
-		}
+// Queued detail lists have already been captured; delivery need not copy them.
+func (state *reportingState) submitMessageLocked(category messageCategory, message string, details []Detail) error {
+	if state.closed {
+		return ErrReporterClosed
 	}
-	if message == "" && !hasDetails {
+	rich := hasDetails(details)
+	if message == "" && !rich {
 		return nil
 	}
 
@@ -423,7 +438,7 @@ func (r *Reporter) submitMessage(category messageCategory, message string, detai
 	destination.submitted = true
 	if !visible {
 		destination.suppressed = true
-		if !hasDetails {
+		if !rich {
 			return nil
 		}
 	}