Separate hasValuers for prefix and suffix

conveys intent much more clearly, and
binds suffix values only when needed.

log/log.go

@@ -36,9 +36,10 @@ func With(logger Logger, keyvals ...interface{}) Logger {
 		// backing array is created if the slice must grow in Log or With.
 		// Using the extra capacity without copying risks a data race that
 		// would violate the Logger interface contract.
-		keyvals:   kvs[:len(kvs):len(kvs)],
-		sKeyvals:  l.sKeyvals,
-		hasValuer: l.hasValuer || containsValuer(keyvals),
+		keyvals:    kvs[:len(kvs):len(kvs)],
+		hasValuer:  l.hasValuer || containsValuer(keyvals),
+		sKeyvals:   l.sKeyvals,
+		sHasValuer: l.sHasValuer,
 	}
 }
 
@@ -68,10 +69,11 @@ func WithPrefix(logger Logger, keyvals ...interface{}) Logger {
 	}
 	kvs = append(kvs, l.keyvals...)
 	return &context{
-		logger:    l.logger,
-		keyvals:   kvs,
-		sKeyvals:  l.sKeyvals,
-		hasValuer: l.hasValuer || containsValuer(keyvals),
+		logger:     l.logger,
+		keyvals:    kvs,
+		hasValuer:  l.hasValuer || containsValuer(keyvals),
+		sKeyvals:   l.sKeyvals,
+		sHasValuer: l.sHasValuer,
 	}
 }
 
@@ -101,10 +103,11 @@ func WithSuffix(logger Logger, keyvals ...interface{}) Logger {
 	}
 	kvs = append(l.sKeyvals, kvs...)
 	return &context{
-		logger:    l.logger,
-		keyvals:   l.keyvals,
-		sKeyvals:  kvs,
-		hasValuer: l.hasValuer || containsValuer(keyvals),
+		logger:     l.logger,
+		keyvals:    l.keyvals,
+		hasValuer:  l.hasValuer,
+		sKeyvals:   kvs,
+		sHasValuer: l.sHasValuer || containsValuer(keyvals),
 	}
 }
 
@@ -128,10 +131,12 @@ func WithSuffix(logger Logger, keyvals ...interface{}) Logger {
 //       returning a newly constructed context with a merged keyvals rather
 //       than simply wrapping the existing context.
 type context struct {
-	logger    Logger
-	keyvals   []interface{}
-	sKeyvals  []interface{}
-	hasValuer bool
+	logger     Logger
+	keyvals    []interface{}
+	hasValuer  bool
+	// suffixes
+	sKeyvals   []interface{}
+	sHasValuer bool
 }
 
 func newContext(logger Logger) *context {
@@ -149,15 +154,16 @@ func (l *context) Log(keyvals ...interface{}) error {
 	if len(kvs)%2 != 0 {
 		kvs = append(kvs, ErrMissingValue)
 	}
-	kvs = append(kvs, l.sKeyvals...)
 	if l.hasValuer {
-		// If no keyvals were appended above then we must copy l.keyvals
-		// and l.sKeyvals so that future log events will reevaluate
-		// the stored Valuers.
+		// If no keyvals were appended above then we must copy l.keyvals so
+		// that future log events will reevaluate the stored Valuers.
 		if len(keyvals) == 0 {
-			kvs = append([]interface{}{}, append(l.keyvals, l.sKeyvals...)...)
+			kvs = append([]interface{}{}, l.keyvals...)
 		}
 		bindValues(kvs[:(len(l.keyvals))])
+	}
+	kvs = append(kvs, l.sKeyvals...)
+	if l.sHasValuer {
 		bindValues(kvs[len(kvs) - len(l.sKeyvals):])
 	}
 	return l.logger.Log(kvs...)