Add Stringers field constructor for slices of Stringer-compatible obj…

…ects (#1155)

Add a new Zap field constructor that encodes a slice of zero or more
objects which implement fmt.Stringer interface into a Zap array.

Usage:

```go
//	type Request struct{ ... }
//	func (a Request) String() string
//
//	var requests []Request = ...
//	logger.Info("sending requests", zap.Stringers("requests", requests))
```

This API uses a type parameter so that users don't have to
build a `[]fmt.Stringer` out of a `[]Request`,
and can pass the `[]Request` directly to the API.

We may need a StringerValues constructor to mirror ObjectValues in the future.

Credits: @zmanji, @abhinav for the suggestions

array_go118.go

@@ -23,7 +23,11 @@
 
 package zap
 
-import "go.uber.org/zap/zapcore"
+import (
+	"fmt"
+
+	"go.uber.org/zap/zapcore"
+)
 
 // Objects constructs a field with the given key, holding a list of the
 // provided objects that can be marshaled by Zap.
@@ -122,3 +126,31 @@ func (os objectValues[T, P]) MarshalLogArray(arr zapcore.ArrayEncoder) error {
 	}
 	return nil
 }
+
+// Stringers constructs a field with the given key, holding a list of the
+// output provided by the value's String method
+//
+// Given an object that implements String on the value receiver, you
+// can log a slice of those objects with Objects like so:
+//
+//	type Request struct{ ... }
+//	func (a Request) String() string
+//
+//	var requests []Request = ...
+//	logger.Info("sending requests", zap.Stringers("requests", requests))
+//
+// Note that these objects must implement fmt.Stringer directly.
+// That is, if you're trying to marshal a []Request, the String method
+// must be declared on the Request type, not its pointer (*Request).
+func Stringers[T fmt.Stringer](key string, values []T) Field {
+	return Array(key, stringers[T](values))
+}
+
+type stringers[T fmt.Stringer] []T
+
+func (os stringers[T]) MarshalLogArray(arr zapcore.ArrayEncoder) error {
+	for _, o := range os {
+		arr.AppendString(o.String())
+	}
+	return nil
+}

array_go118_test.go

@@ -25,6 +25,7 @@ package zap
 
 import (
 	"errors"
+	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -179,3 +180,61 @@ func TestObjectsAndObjectValues_marshalError(t *testing.T) {
 		})
 	}
 }
+
+type stringerObject struct {
+	value string
+}
+
+func (s stringerObject) String() string {
+	return s.value
+}
+
+func TestStringers(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		desc string
+		give Field
+		want []any
+	}{
+		{
+			desc: "Stringers",
+			give: Stringers("", []stringerObject{
+				{value: "foo"},
+				{value: "bar"},
+				{value: "baz"},
+			}),
+			want: []any{
+				"foo",
+				"bar",
+				"baz",
+			},
+		},
+		{
+			desc: "Stringers with []fmt.Stringer",
+			give: Stringers("", []fmt.Stringer{
+				stringerObject{value: "foo"},
+				stringerObject{value: "bar"},
+				stringerObject{value: "baz"},
+			}),
+			want: []any{
+				"foo",
+				"bar",
+				"baz",
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.desc, func(t *testing.T) {
+			t.Parallel()
+
+			tt.give.Key = "k"
+
+			enc := zapcore.NewMapObjectEncoder()
+			tt.give.AddTo(enc)
+			assert.Equal(t, tt.want, enc.Fields["k"])
+		})
+	}
+}