internal/godoc: add unit test for TestDecodeBasicLit

Test: go generate ./internal/godoc/... && go test -v -run
TestDecodeBasicLit

- This test ensures that the encoding and decoding of fields remains
  consistent.
- A golden byte slice represesnting an ast.BasicLit should still be able
  to be decoded if the codec changes (i.e. a new field is added).

Fixes golang/go#76350

Change-Id: I9747b1961247f2250c0377da5d1c4684fa810e92
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/727201
Auto-Submit: Ethan Lee <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
kokoro-CI: kokoro <[email protected]>

Author: ethanalee-work

internal/godoc/godoc_test.go

@@ -6,12 +6,14 @@ package godoc
 
 import (
 	"bytes"
+	"go/ast"
 	"go/format"
 	"go/parser"
 	"go/token"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
+	"golang.org/x/pkgsite/internal/godoc/codec"
 )
 
 func TestRemoveUnusedASTNodes(t *testing.T) {
@@ -110,3 +112,32 @@ func (t) U()
 		t.Errorf("mismatch (-want, +got):\n%s", diff)
 	}
 }
+
+func TestDecodeBasicLit(t *testing.T) {
+	// byte slice representing an encoded ast.BasicLit from Go 1.25.
+	golden := []byte{
+		0xf6, 0x1, 0xf7, 0xd, 0x2a, 0x61, 0x73, 0x74, 0x2e, 0x42, 0x61, 0x73, 0x69,
+		0x63, 0x4c, 0x69, 0x74, 0xf6, 0x2, 0x0, 0xf4, 0x1, 0xa, 0x2, 0xf7, 0x3,
+		0x31, 0x32, 0x33, 0xf3,
+	}
+
+	d := codec.NewDecoder(golden)
+	val, err := d.Decode()
+	if err != nil {
+		t.Fatalf("Decode() failed: %v", err)
+	}
+
+	got, ok := val.(*ast.BasicLit)
+	if !ok {
+		t.Fatalf("decoded value is not an ast.BasicLit, got %T", val)
+	}
+
+	want := ast.BasicLit{
+		Kind:  token.INT,
+		Value: "123",
+	}
+
+	if got.Kind != want.Kind || got.Value != want.Value {
+		t.Errorf("Decode(...) = %+v, want %+v", got, want)
+	}
+}