cue: add large Value.MarshalJSON benchmark

Adapted from a smaller benchmark that Roger provided in
https://cuelang.org/issue/2470 to also include nesting of structs
as well as a long string, a long list, and a long struct.

As can be seen by the current results, we allocate nearly 50MiB,
which is very wasteful for the current benchmark size of 2000.

    cpu: AMD Ryzen 7 PRO 5850U with Radeon Graphics
                            │     old     │
                            │   sec/op    │
    LargeValueMarshalJSON-8   106.2m ± 1%

                            │     old      │
                            │     B/op     │
    LargeValueMarshalJSON-8   48.24Mi ± 8%

                            │     old     │
                            │  allocs/op  │
    LargeValueMarshalJSON-8   106.2k ± 2%

For #2470.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I437cc8fb644dd33dd0327860ca04ff66b7ab9275
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201803
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cue/bench_test.go

@@ -15,11 +15,14 @@
 package cue_test
 
 import (
+	"bytes"
+	"fmt"
 	"io/fs"
 	"os"
 	"path/filepath"
 	"testing"
 
+	"cuelang.org/go/cue/cuecontext"
 	"cuelang.org/go/internal/core/eval"
 	"cuelang.org/go/internal/core/runtime"
 	"cuelang.org/go/internal/cuetdtest"
@@ -107,3 +110,71 @@ func Benchmark(b *testing.B) {
 		b.Fatal(err)
 	}
 }
+
+// TODO(mvdan): move this benchmark to internal/encoding
+// and cover other encodings too.
+// We should also cover both encoding and decoding performance.
+func BenchmarkLargeValueMarshalJSON(b *testing.B) {
+	b.ReportAllocs()
+	size := 2000
+
+	var buf bytes.Buffer
+
+	fmt.Fprintf(&buf, "longString: \"")
+	for range size {
+		fmt.Fprintf(&buf, "x")
+	}
+	fmt.Fprintf(&buf, "\"\n")
+
+	fmt.Fprintf(&buf, "nestedList: ")
+	for range size {
+		fmt.Fprintf(&buf, "[")
+	}
+	fmt.Fprintf(&buf, "0")
+	for range size {
+		fmt.Fprintf(&buf, "]")
+	}
+	fmt.Fprintf(&buf, "\n")
+
+	fmt.Fprintf(&buf, "longList: [")
+	for i := range size {
+		if i > 0 {
+			fmt.Fprintf(&buf, ",")
+		}
+		fmt.Fprintf(&buf, "0")
+	}
+	fmt.Fprintf(&buf, "]\n")
+
+	fmt.Fprintf(&buf, "nestedStruct: ")
+	for range size {
+		fmt.Fprintf(&buf, "{k:")
+	}
+	fmt.Fprintf(&buf, "0")
+	for range size {
+		fmt.Fprintf(&buf, "}")
+	}
+	fmt.Fprintf(&buf, "\n")
+
+	fmt.Fprintf(&buf, "longStruct: {")
+	for i := range size {
+		if i > 0 {
+			fmt.Fprintf(&buf, ",")
+		}
+		fmt.Fprintf(&buf, "k%d: 0", i)
+	}
+	fmt.Fprintf(&buf, "}\n")
+
+	ctx := cuecontext.New()
+	val := ctx.CompileBytes(buf.Bytes())
+	if err := val.Err(); err != nil {
+		b.Fatal(err)
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		data, err := val.MarshalJSON()
+		if err != nil {
+			b.Fatal(err)
+		}
+		_ = data
+	}
+}