log/slog: export Source method in Record for custom handler support

Currently, the `source` method in `slog.Record` is not accessible to custom handlers, requiring developers to re-implement logic for retrieving source location information. This commit exports the `source` method as `Source`, enabling consistent access for custom logging handlers and reducing code redundancy.

Fixes #70280

src/log/slog/handler.go

@@ -294,7 +294,7 @@ func (h *commonHandler) handle(r Record) error {
 	}
 	// source
 	if h.opts.AddSource {
-		state.appendAttr(Any(SourceKey, r.source()))
+		state.appendAttr(Any(SourceKey, r.Source()))
 	}
 	key = MessageKey
 	msg := r.Message

src/log/slog/handler_test.go

@@ -530,7 +530,7 @@ func TestJSONAndTextHandlers(t *testing.T) {
 		},
 	} {
 		r := NewRecord(testTime, LevelInfo, "message", callerPC(2))
-		line := strconv.Itoa(r.source().Line)
+		line := strconv.Itoa(r.Source().Line)
 		r.AddAttrs(test.attrs...)
 		var buf bytes.Buffer
 		opts := HandlerOptions{ReplaceAttr: test.replace, AddSource: test.addSource}

src/log/slog/logger_test.go

@@ -185,7 +185,7 @@ func TestCallDepth(t *testing.T) {
 		const wantFunc = "log/slog.TestCallDepth"
 		const wantFile = "logger_test.go"
 		wantLine := startLine + count*2
-		got := h.r.source()
+		got := h.r.Source()
 		gotFile := filepath.Base(got.File)
 		if got.Function != wantFunc || gotFile != wantFile || got.Line != wantLine {
 			t.Errorf("got (%s, %s, %d), want (%s, %s, %d)",

src/log/slog/record.go

@@ -215,7 +215,7 @@ func (s *Source) group() Value {
 // If the Record was created without the necessary information,
 // or if the location is unavailable, it returns a non-nil *Source
 // with zero fields.
-func (r Record) source() *Source {
+func (r Record) Source() *Source {
 	fs := runtime.CallersFrames([]uintptr{r.PC})
 	f, _ := fs.Next()
 	return &Source{

src/log/slog/record_test.go

@@ -56,7 +56,7 @@ func TestRecordSource(t *testing.T) {
 			pc = callerPC(test.depth + 1)
 		}
 		r := NewRecord(time.Time{}, 0, "", pc)
-		got := r.source()
+		got := r.Source()
 		if i := strings.LastIndexByte(got.File, '/'); i >= 0 {
 			got.File = got.File[i+1:]
 		}