encoding/json: modernize tests

There are no changes to what is being tested.
No test cases were removed or added.

Changes made:
* Use a local implementation of test case position marking. See #52751.
* Use consistent names for all test tables and variables.
* Generally speaking, follow modern Go style guide for tests.
* Move global tables local to the test function if possible.
* Make every table entry run in a distinct testing.T.Run.

The purpose of this change is to make it easier to perform
v1-to-v2 development where we want v2 to support close to
bug-for-bug compatibility when running in v1 mode.

Annotating each test case with the location of the test data
makes it easier to jump directly to the test data itself
and understand why this particular case is failing.

Having every test case run in its own t.Run makes it easier
to isolate a particular failing test and work on fixing the code
until that test case starts to pass again.

Unfortunately, many tests are annotated with an empty name.
An empty name is better than nothing, since the testing framework
auto assigns a numeric ID for duplicate names.
It is not worth the trouble to give descriptive names to each
of the thousands of test cases.

Change-Id: I43905f35249b3d77dfca234b9c7808d40e225de8
Reviewed-on: https://go-review.googlesource.com/c/go/+/522880
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

1 files changed, 210 insertions, 185 deletions

diff --git a/src/encoding/json/stream_test.go b/src/encoding/json/stream_test.go
index 97f9fbd6d8..32ede8cc7e 100644
--- a/src/encoding/json/stream_test.go
+++ b/src/encoding/json/stream_test.go
@@ -6,17 +6,44 @@ package json
 
 import (
 	"bytes"
+	"fmt"
 	"io"
 	"log"
 	"net"
 	"net/http"
 	"net/http/httptest"
+	"path"
 	"reflect"
+	"runtime"
 	"runtime/debug"
 	"strings"
 	"testing"
 )
 
+// TODO(https://go.dev/issue/52751): Replace with native testing support.
+
+// CaseName is a case name annotated with a file and line.
+type CaseName struct {
+	Name  string
+	Where CasePos
+}
+
+// Name annotates a case name with the file and line of the caller.
+func Name(s string) (c CaseName) {
+	c.Name = s
+	runtime.Callers(2, c.Where.pc[:])
+	return c
+}
+
+// CasePos represents a file and line number.
+type CasePos struct{ pc [1]uintptr }
+
+func (pos CasePos) String() string {
+	frames := runtime.CallersFrames(pos.pc[:])
+	frame, _ := frames.Next()
+	return fmt.Sprintf("%s:%d", path.Base(frame.File), frame.Line)
+}
+
 // Test values for the stream test.
 // One of each JSON kind.
 var streamTest = []any{
@@ -49,11 +76,11 @@ func TestEncoder(t *testing.T) {
 		enc.SetIndent("", "")
 		for j, v := range streamTest[0:i] {
 			if err := enc.Encode(v); err != nil {
-				t.Fatalf("encode #%d: %v", j, err)
+				t.Fatalf("#%d.%d Encode error: %v", i, j, err)
 			}
 		}
 		if have, want := buf.String(), nlines(streamEncoded, i); have != want {
-			t.Errorf("encoding %d items: mismatch", i)
+			t.Errorf("encoding %d items: mismatch:", i)
 			diff(t, []byte(have), []byte(want))
 			break
 		}
@@ -76,24 +103,24 @@ func TestEncoderErrorAndReuseEncodeState(t *testing.T) {
 	var buf bytes.Buffer
 	enc := NewEncoder(&buf)
 	if err := enc.Encode(dummy); err == nil {
-		t.Errorf("Encode(dummy) == nil; want error")
+		t.Errorf("Encode(dummy) error: got nil, want non-nil")
 	}
 
 	type Data struct {
 		A string
 		I int
 	}
-	data := Data{A: "a", I: 1}
-	if err := enc.Encode(data); err != nil {
-		t.Errorf("Marshal(%v) = %v", data, err)
+	want := Data{A: "a", I: 1}
+	if err := enc.Encode(want); err != nil {
+		t.Errorf("Marshal error: %v", err)
 	}
 
-	var data2 Data
-	if err := Unmarshal(buf.Bytes(), &data2); err != nil {
-		t.Errorf("Unmarshal(%v) = %v", data2, err)
+	var got Data
+	if err := Unmarshal(buf.Bytes(), &got); err != nil {
+		t.Errorf("Unmarshal error: %v", err)
 	}
-	if data2 != data {
-		t.Errorf("expect: %v, but get: %v", data, data2)
+	if got != want {
+		t.Errorf("Marshal/Unmarshal roundtrip:\n\tgot:  %v\n\twant: %v", got, want)
 	}
 }
 
@@ -122,7 +149,7 @@ func TestEncoderIndent(t *testing.T) {
 		enc.Encode(v)
 	}
 	if have, want := buf.String(), streamEncodedIndent; have != want {
-		t.Error("indented encoding mismatch")
+		t.Error("Encode mismatch:")
 		diff(t, []byte(have), []byte(want))
 	}
 }
@@ -160,50 +187,51 @@ func TestEncoderSetEscapeHTML(t *testing.T) {
 		Bar string `json:"bar,string"`
 	}{`<html>foobar</html>`}
 
-	for _, tt := range []struct {
-		name       string
+	tests := []struct {
+		CaseName
 		v          any
 		wantEscape string
 		want       string
 	}{
-		{"c", c, `"\u003c\u0026\u003e"`, `"<&>"`},
-		{"ct", ct, `"\"\u003c\u0026\u003e\""`, `"\"<&>\""`},
-		{`"<&>"`, "<&>", `"\u003c\u0026\u003e"`, `"<&>"`},
+		{Name("c"), c, `"\u003c\u0026\u003e"`, `"<&>"`},
+		{Name("ct"), ct, `"\"\u003c\u0026\u003e\""`, `"\"<&>\""`},
+		{Name(`"<&>"`), "<&>", `"\u003c\u0026\u003e"`, `"<&>"`},
 		{
-			"tagStruct", tagStruct,
+			Name("tagStruct"), tagStruct,
 			`{"\u003c\u003e\u0026#! ":0,"Invalid":0}`,
 			`{"<>&#! ":0,"Invalid":0}`,
 		},
 		{
-			`"<str>"`, marshalerStruct,
+			Name(`"<str>"`), marshalerStruct,
 			`{"NonPtr":"\u003cstr\u003e","Ptr":"\u003cstr\u003e"}`,
 			`{"NonPtr":"<str>","Ptr":"<str>"}`,
 		},
 		{
-			"stringOption", stringOption,
+			Name("stringOption"), stringOption,
 			`{"bar":"\"\\u003chtml\\u003efoobar\\u003c/html\\u003e\""}`,
 			`{"bar":"\"<html>foobar</html>\""}`,
 		},
-	} {
-		var buf strings.Builder
-		enc := NewEncoder(&buf)
-		if err := enc.Encode(tt.v); err != nil {
-			t.Errorf("Encode(%s): %s", tt.name, err)
-			continue
-		}
-		if got := strings.TrimSpace(buf.String()); got != tt.wantEscape {
-			t.Errorf("Encode(%s) = %#q, want %#q", tt.name, got, tt.wantEscape)
-		}
-		buf.Reset()
-		enc.SetEscapeHTML(false)
-		if err := enc.Encode(tt.v); err != nil {
-			t.Errorf("SetEscapeHTML(false) Encode(%s): %s", tt.name, err)
-			continue
-		}
-		if got := strings.TrimSpace(buf.String()); got != tt.want {
-			t.Errorf("SetEscapeHTML(false) Encode(%s) = %#q, want %#q",
-				tt.name, got, tt.want)
-		}
+	}
+	for _, tt := range tests {
+		t.Run(tt.Name, func(t *testing.T) {
+			var buf strings.Builder
+			enc := NewEncoder(&buf)
+			if err := enc.Encode(tt.v); err != nil {
+				t.Fatalf("%s: Encode(%s) error: %s", tt.Where, tt.Name, err)
+			}
+			if got := strings.TrimSpace(buf.String()); got != tt.wantEscape {
+				t.Errorf("%s: Encode(%s):\n\tgot:  %s\n\twant: %s", tt.Where, tt.Name, got, tt.wantEscape)
+			}
+			buf.Reset()
+			enc.SetEscapeHTML(false)
+			if err := enc.Encode(tt.v); err != nil {
+				t.Fatalf("%s: SetEscapeHTML(false) Encode(%s) error: %s", tt.Where, tt.Name, err)
+			}
+			if got := strings.TrimSpace(buf.String()); got != tt.want {
+				t.Errorf("%s: SetEscapeHTML(false) Encode(%s):\n\tgot:  %s\n\twant: %s",
+					tt.Where, tt.Name, got, tt.want)
+			}
+		})
 	}
 }
 
@@ -224,14 +252,14 @@ func TestDecoder(t *testing.T) {
 		dec := NewDecoder(&buf)
 		for j := range out {
 			if err := dec.Decode(&out[j]); err != nil {
-				t.Fatalf("decode #%d/%d: %v", j, i, err)
+				t.Fatalf("decode #%d/%d error: %v", j, i, err)
 			}
 		}
 		if !reflect.DeepEqual(out, streamTest[0:i]) {
-			t.Errorf("decoding %d items: mismatch", i)
+			t.Errorf("decoding %d items: mismatch:", i)
 			for j := range out {
 				if !reflect.DeepEqual(out[j], streamTest[j]) {
-					t.Errorf("#%d: have %v want %v", j, out[j], streamTest[j])
+					t.Errorf("#%d:\n\tgot:  %v\n\twant: %v", j, out[j], streamTest[j])
 				}
 			}
 			break
@@ -250,14 +278,14 @@ func TestDecoderBuffered(t *testing.T) {
 		t.Fatal(err)
 	}
 	if m.Name != "Gopher" {
-		t.Errorf("Name = %q; want Gopher", m.Name)
+		t.Errorf("Name = %s, want Gopher", m.Name)
 	}
 	rest, err := io.ReadAll(d.Buffered())
 	if err != nil {
 		t.Fatal(err)
 	}
-	if g, w := string(rest), " extra "; g != w {
-		t.Errorf("Remaining = %q; want %q", g, w)
+	if got, want := string(rest), " extra "; got != want {
+		t.Errorf("Remaining = %s, want %s", got, want)
 	}
 }
 
@@ -282,20 +310,20 @@ func TestRawMessage(t *testing.T) {
 		Y  float32
 	}
 	const raw = `["\u0056",null]`
-	const msg = `{"X":0.1,"Id":["\u0056",null],"Y":0.2}`
-	err := Unmarshal([]byte(msg), &data)
+	const want = `{"X":0.1,"Id":["\u0056",null],"Y":0.2}`
+	err := Unmarshal([]byte(want), &data)
 	if err != nil {
-		t.Fatalf("Unmarshal: %v", err)
+		t.Fatalf("Unmarshal error: %v", err)
 	}
 	if string([]byte(data.Id)) != raw {
-		t.Fatalf("Raw mismatch: have %#q want %#q", []byte(data.Id), raw)
+		t.Fatalf("Unmarshal:\n\tgot:  %s\n\twant: %s", []byte(data.Id), raw)
 	}
-	b, err := Marshal(&data)
+	got, err := Marshal(&data)
 	if err != nil {
-		t.Fatalf("Marshal: %v", err)
+		t.Fatalf("Marshal error: %v", err)
 	}
-	if string(b) != msg {
-		t.Fatalf("Marshal: have %#q want %#q", b, msg)
+	if string(got) != want {
+		t.Fatalf("Marshal:\n\tgot:  %s\n\twant: %s", got, want)
 	}
 }
 
@@ -306,159 +334,156 @@ func TestNullRawMessage(t *testing.T) {
 		IdPtr *RawMessage
 		Y     float32
 	}
-	const msg = `{"X":0.1,"Id":null,"IdPtr":null,"Y":0.2}`
-	err := Unmarshal([]byte(msg), &data)
+	const want = `{"X":0.1,"Id":null,"IdPtr":null,"Y":0.2}`
+	err := Unmarshal([]byte(want), &data)
 	if err != nil {
-		t.Fatalf("Unmarshal: %v", err)
+		t.Fatalf("Unmarshal error: %v", err)
 	}
 	if want, got := "null", string(data.Id); want != got {
-		t.Fatalf("Raw mismatch: have %q, want %q", got, want)
+		t.Fatalf("Unmarshal:\n\tgot:  %s\n\twant: %s", got, want)
 	}
 	if data.IdPtr != nil {
-		t.Fatalf("Raw pointer mismatch: have non-nil, want nil")
+		t.Fatalf("pointer mismatch: got non-nil, want nil")
 	}
-	b, err := Marshal(&data)
+	got, err := Marshal(&data)
 	if err != nil {
-		t.Fatalf("Marshal: %v", err)
+		t.Fatalf("Marshal error: %v", err)
 	}
-	if string(b) != msg {
-		t.Fatalf("Marshal: have %#q want %#q", b, msg)
+	if string(got) != want {
+		t.Fatalf("Marshal:\n\tgot:  %s\n\twant: %s", got, want)
 	}
 }
 
-var blockingTests = []string{
-	`{"x": 1}`,
-	`[1, 2, 3]`,
-}
-
 func TestBlocking(t *testing.T) {
-	for _, enc := range blockingTests {
-		r, w := net.Pipe()
-		go w.Write([]byte(enc))
-		var val any
-
-		// If Decode reads beyond what w.Write writes above,
-		// it will block, and the test will deadlock.
-		if err := NewDecoder(r).Decode(&val); err != nil {
-			t.Errorf("decoding %s: %v", enc, err)
-		}
-		r.Close()
-		w.Close()
+	tests := []struct {
+		CaseName
+		in string
+	}{
+		{Name(""), `{"x": 1}`},
+		{Name(""), `[1, 2, 3]`},
 	}
-}
+	for _, tt := range tests {
+		t.Run(tt.Name, func(t *testing.T) {
+			r, w := net.Pipe()
+			go w.Write([]byte(tt.in))
+			var val any
 
-type tokenStreamCase struct {
-	json      string
-	expTokens []any
+			// If Decode reads beyond what w.Write writes above,
+			// it will block, and the test will deadlock.
+			if err := NewDecoder(r).Decode(&val); err != nil {
+				t.Errorf("%s: NewDecoder(%s).Decode error: %v", tt.Where, tt.in, err)
+			}
+			r.Close()
+			w.Close()
+		})
+	}
 }
 
 type decodeThis struct {
 	v any
 }
 
-var tokenStreamCases = []tokenStreamCase{
-	// streaming token cases
-	{json: `10`, expTokens: []any{float64(10)}},
-	{json: ` [10] `, expTokens: []any{
-		Delim('['), float64(10), Delim(']')}},
-	{json: ` [false,10,"b"] `, expTokens: []any{
-		Delim('['), false, float64(10), "b", Delim(']')}},
-	{json: `{ "a": 1 }`, expTokens: []any{
-		Delim('{'), "a", float64(1), Delim('}')}},
-	{json: `{"a": 1, "b":"3"}`, expTokens: []any{
-		Delim('{'), "a", float64(1), "b", "3", Delim('}')}},
-	{json: ` [{"a": 1},{"a": 2}] `, expTokens: []any{
-		Delim('['),
-		Delim('{'), "a", float64(1), Delim('}'),
-		Delim('{'), "a", float64(2), Delim('}'),
-		Delim(']')}},
-	{json: `{"obj": {"a": 1}}`, expTokens: []any{
-		Delim('{'), "obj", Delim('{'), "a", float64(1), Delim('}'),
-		Delim('}')}},
-	{json: `{"obj": [{"a": 1}]}`, expTokens: []any{
-		Delim('{'), "obj", Delim('['),
-		Delim('{'), "a", float64(1), Delim('}'),
-		Delim(']'), Delim('}')}},
-
-	// streaming tokens with intermittent Decode()
-	{json: `{ "a": 1 }`, expTokens: []any{
-		Delim('{'), "a",
-		decodeThis{float64(1)},
-		Delim('}')}},
-	{json: ` [ { "a" : 1 } ] `, expTokens: []any{
-		Delim('['),
-		decodeThis{map[string]any{"a": float64(1)}},
-		Delim(']')}},
-	{json: ` [{"a": 1},{"a": 2}] `, expTokens: []any{
-		Delim('['),
-		decodeThis{map[string]any{"a": float64(1)}},
-		decodeThis{map[string]any{"a": float64(2)}},
-		Delim(']')}},
-	{json: `{ "obj" : [ { "a" : 1 } ] }`, expTokens: []any{
-		Delim('{'), "obj", Delim('['),
-		decodeThis{map[string]any{"a": float64(1)}},
-		Delim(']'), Delim('}')}},
-
-	{json: `{"obj": {"a": 1}}`, expTokens: []any{
-		Delim('{'), "obj",
-		decodeThis{map[string]any{"a": float64(1)}},
-		Delim('}')}},
-	{json: `{"obj": [{"a": 1}]}`, expTokens: []any{
-		Delim('{'), "obj",
-		decodeThis{[]any{
-			map[string]any{"a": float64(1)},
-		}},
-		Delim('}')}},
-	{json: ` [{"a": 1} {"a": 2}] `, expTokens: []any{
-		Delim('['),
-		decodeThis{map[string]any{"a": float64(1)}},
-		decodeThis{&SyntaxError{"expected comma after array element", 11}},
-	}},
-	{json: `{ "` + strings.Repeat("a", 513) + `" 1 }`, expTokens: []any{
-		Delim('{'), strings.Repeat("a", 513),
-		decodeThis{&SyntaxError{"expected colon after object key", 518}},
-	}},
-	{json: `{ "\a" }`, expTokens: []any{
-		Delim('{'),
-		&SyntaxError{"invalid character 'a' in string escape code", 3},
-	}},
-	{json: ` \a`, expTokens: []any{
-		&SyntaxError{"invalid character '\\\\' looking for beginning of value", 1},
-	}},
-}
-
 func TestDecodeInStream(t *testing.T) {
-	for ci, tcase := range tokenStreamCases {
+	tests := []struct {
+		CaseName
+		json      string
+		expTokens []any
+	}{
+		// streaming token cases
+		{CaseName: Name(""), json: `10`, expTokens: []any{float64(10)}},
+		{CaseName: Name(""), json: ` [10] `, expTokens: []any{
+			Delim('['), float64(10), Delim(']')}},
+		{CaseName: Name(""), json: ` [false,10,"b"] `, expTokens: []any{
+			Delim('['), false, float64(10), "b", Delim(']')}},
+		{CaseName: Name(""), json: `{ "a": 1 }`, expTokens: []any{
+			Delim('{'), "a", float64(1), Delim('}')}},
+		{CaseName: Name(""), json: `{"a": 1, "b":"3"}`, expTokens: []any{
+			Delim('{'), "a", float64(1), "b", "3", Delim('}')}},
+		{CaseName: Name(""), json: ` [{"a": 1},{"a": 2}] `, expTokens: []any{
+			Delim('['),
+			Delim('{'), "a", float64(1), Delim('}'),
+			Delim('{'), "a", float64(2), Delim('}'),
+			Delim(']')}},
+		{CaseName: Name(""), json: `{"obj": {"a": 1}}`, expTokens: []any{
+			Delim('{'), "obj", Delim('{'), "a", float64(1), Delim('}'),
+			Delim('}')}},
+		{CaseName: Name(""), json: `{"obj": [{"a": 1}]}`, expTokens: []any{
+			Delim('{'), "obj", Delim('['),
+			Delim('{'), "a", float64(1), Delim('}'),
+			Delim(']'), Delim('}')}},
 
-		dec := NewDecoder(strings.NewReader(tcase.json))
-		for i, etk := range tcase.expTokens {
+		// streaming tokens with intermittent Decode()
+		{CaseName: Name(""), json: `{ "a": 1 }`, expTokens: []any{
+			Delim('{'), "a",
+			decodeThis{float64(1)},
+			Delim('}')}},
+		{CaseName: Name(""), json: ` [ { "a" : 1 } ] `, expTokens: []any{
+			Delim('['),
+			decodeThis{map[string]any{"a": float64(1)}},
+			Delim(']')}},
+		{CaseName: Name(""), json: ` [{"a": 1},{"a": 2}] `, expTokens: []any{
+			Delim('['),
+			decodeThis{map[string]any{"a": float64(1)}},
+			decodeThis{map[string]any{"a": float64(2)}},
+			Delim(']')}},
+		{CaseName: Name(""), json: `{ "obj" : [ { "a" : 1 } ] }`, expTokens: []any{
+			Delim('{'), "obj", Delim('['),
+			decodeThis{map[string]any{"a": float64(1)}},
+			Delim(']'), Delim('}')}},
 
-			var tk any
-			var err error
+		{CaseName: Name(""), json: `{"obj": {"a": 1}}`, expTokens: []any{
+			Delim('{'), "obj",
+			decodeThis{map[string]any{"a": float64(1)}},
+			Delim('}')}},
+		{CaseName: Name(""), json: `{"obj": [{"a": 1}]}`, expTokens: []any{
+			Delim('{'), "obj",
+			decodeThis{[]any{
+				map[string]any{"a": float64(1)},
+			}},
+			Delim('}')}},
+		{CaseName: Name(""), json: ` [{"a": 1} {"a": 2}] `, expTokens: []any{
+			Delim('['),
+			decodeThis{map[string]any{"a": float64(1)}},
+			decodeThis{&SyntaxError{"expected comma after array element", 11}},
+		}},
+		{CaseName: Name(""), json: `{ "` + strings.Repeat("a", 513) + `" 1 }`, expTokens: []any{
+			Delim('{'), strings.Repeat("a", 513),
+			decodeThis{&SyntaxError{"expected colon after object key", 518}},
+		}},
+		{CaseName: Name(""), json: `{ "\a" }`, expTokens: []any{
+			Delim('{'),
+			&SyntaxError{"invalid character 'a' in string escape code", 3},
+		}},
+		{CaseName: Name(""), json: ` \a`, expTokens: []any{
+			&SyntaxError{"invalid character '\\\\' looking for beginning of value", 1},
+		}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.Name, func(t *testing.T) {
+			dec := NewDecoder(strings.NewReader(tt.json))
+			for i, want := range tt.expTokens {
+				var got any
+				var err error
 
-			if dt, ok := etk.(decodeThis); ok {
-				etk = dt.v
-				err = dec.Decode(&tk)
-			} else {
-				tk, err = dec.Token()
-			}
-			if experr, ok := etk.(error); ok {
-				if err == nil || !reflect.DeepEqual(err, experr) {
-					t.Errorf("case %v: Expected error %#v in %q, but was %#v", ci, experr, tcase.json, err)
+				if dt, ok := want.(decodeThis); ok {
+					want = dt.v
+					err = dec.Decode(&got)
+				} else {
+					got, err = dec.Token()
+				}
+				if errWant, ok := want.(error); ok {
+					if err == nil || !reflect.DeepEqual(err, errWant) {
+						t.Fatalf("%s:\n\tinput: %s\n\tgot error:  %v\n\twant error: %v", tt.Where, tt.json, err, errWant)
+					}
+					break
+				} else if err != nil {
+					t.Fatalf("%s:\n\tinput: %s\n\tgot error:  %v\n\twant error: nil", tt.Where, tt.json, err)
+				}
+				if !reflect.DeepEqual(got, want) {
+					t.Fatalf("%s: token %d:\n\tinput: %s\n\tgot:  %T(%v)\n\twant: %T(%v)", tt.Where, i, tt.json, got, got, want, want)
 				}
-				break
-			} else if err == io.EOF {
-				t.Errorf("case %v: Unexpected EOF in %q", ci, tcase.json)
-				break
-			} else if err != nil {
-				t.Errorf("case %v: Unexpected error '%#v' in %q", ci, err, tcase.json)
-				break
-			}
-			if !reflect.DeepEqual(tk, etk) {
-				t.Errorf(`case %v: %q @ %v expected %T(%v) was %T(%v)`, ci, tcase.json, i, etk, etk, tk, tk)
-				break
 			}
-		}
+		})
 	}
 }
 
@@ -472,7 +497,7 @@ func TestHTTPDecoding(t *testing.T) {
 	defer ts.Close()
 	res, err := http.Get(ts.URL)
 	if err != nil {
-		log.Fatalf("GET failed: %v", err)
+		log.Fatalf("http.Get error: %v", err)
 	}
 	defer res.Body.Close()
 
@@ -483,15 +508,15 @@ func TestHTTPDecoding(t *testing.T) {
 	d := NewDecoder(res.Body)
 	err = d.Decode(&foo)
 	if err != nil {
-		t.Fatalf("Decode: %v", err)
+		t.Fatalf("Decode error: %v", err)
 	}
 	if foo.Foo != "bar" {
-		t.Errorf("decoded %q; want \"bar\"", foo.Foo)
+		t.Errorf(`Decode: got %q, want "bar"`, foo.Foo)
 	}
 
 	// make sure we get the EOF the second time
 	err = d.Decode(&foo)
 	if err != io.EOF {
-		t.Errorf("err = %v; want io.EOF", err)
+		t.Errorf("Decode error:\n\tgot:  %v\n\twant: io.EOF", err)
 	}
 }