encoding/json v1.26.5: Prove that json.Encoder.Encode always writes a trailing newline to its output.

search_known_solution returned a REFERENCE_ONLY match for sampleId sha256:bc28421aeb3987f9f08eb71bdb73b5ee10e20ed8f2faa88b5c6f2d6b650d055c, which proves `omitempty` edge cases only.
This sample targets a different trap: `json.Encoder.Encode` does add a trailing newline, so believing it behaves exactly like `json.Marshal` fails silently (valid build, mismatched bytes).

{"case":{"believed":"A caller can treat json.Encoder.Encode as json.Marshal followed by Write, so the output bytes are exactly the JSON value.","caseId":"case:sha256:62b1480e93a4081b1bd27c3b782d9cb6f4f0a7b4e9c694c6f26c5f367d2dd20d","contract":["assert that json.NewEncoder(\u0026buf).Encode(v) appends a trailing \\n byte to the encoded output","assert that the output from Encode differs from json.Marshal(v) exactly by that newline"],"goal":"Prove that json.Encoder.Encode always writes a trailing newline to its output.","kind":"HOW","packages":["pkg:golang/encoding/json@1.26.5"],"schemaVersion":1,"symbols":["encoding/json.Encoder.Encode"]},"contractCommand":["go","test","./..."],"environment":{"arch":"x64","ecosystem":"golang","executionContext":"go","language":"go","os":"linux","packageManager":"go","runtime":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/encoding/json@1.26.5"],"schemaVersion":1,"symbols":["encoding/json.Encoder.Encode"],"verifierAdapter":"golang@1"}

package sample

import (
	"bytes"
	"encoding/json"
	"testing"
)

type message struct {
	Message string `json:"message"`
}

func TestEncoderAddsTrailingNewline(t *testing.T) {
	var buf bytes.Buffer
	enc := json.NewEncoder(&buf)

	v := message{Message: "hello"}
	if err := enc.Encode(v); err != nil {
		t.Fatalf("Encode failed: %v", err)
	}

	got := buf.String()
	want := `{"message":"hello"}
`

	if got != want {
		t.Fatalf("unexpected encoded output: %q", got)
	}

	marshal, err := json.Marshal(v)
	if err != nil {
		t.Fatalf("json.Marshal failed: %v", err)
	}
	if got == string(marshal) {
		t.Fatalf("encoder output should differ from Marshal by a trailing newline")
	}
}

module example.com/codesample

go 1.26