context: don't depend on fmt

So the net package doesn't indirectly depend on unicode tables.

But we're still not quite there, because a new test added in this CL
reveals that we still have a path to unicode via:

deps_test.go:570:
  TODO(issue 30440): policy violation: net => sort => reflect => unicode

Updates #30440

Change-Id: I710c2061dfbaa8e866c92e6c824bd8df35784165
Reviewed-on: https://go-review.googlesource.com/c/go/+/169080
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 32 insertions, 5 deletions

diff --git a/src/context/context.go b/src/context/context.go
index 36f83c7b5b..77298f6531 100644
--- a/src/context/context.go
+++ b/src/context/context.go
@@ -49,7 +49,6 @@ package context
 
 import (
 	"errors"
-	"fmt"
 	"internal/reflectlite"
 	"sync"
 	"time"
@@ -338,8 +337,19 @@ func (c *cancelCtx) Err() error {
 	return err
 }
 
+type stringer interface {
+	String() string
+}
+
+func contextName(c Context) string {
+	if s, ok := c.(stringer); ok {
+		return s.String()
+	}
+	return reflectlite.TypeOf(c).String()
+}
+
 func (c *cancelCtx) String() string {
-	return fmt.Sprintf("%v.WithCancel", c.Context)
+	return contextName(c.Context) + ".WithCancel"
 }
 
 // cancel closes c.done, cancels each of c's children, and, if
@@ -420,7 +430,9 @@ func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
 }
 
 func (c *timerCtx) String() string {
-	return fmt.Sprintf("%v.WithDeadline(%s [%s])", c.cancelCtx.Context, c.deadline, time.Until(c.deadline))
+	return contextName(c.cancelCtx.Context) + ".WithDeadline(" +
+		c.deadline.String() + " [" +
+		time.Until(c.deadline).String() + "])"
 }
 
 func (c *timerCtx) cancel(removeFromParent bool, err error) {
@@ -481,8 +493,23 @@ type valueCtx struct {
 	key, val interface{}
 }
 
+// stringify tries a bit to stringify v, without using fmt, since we don't
+// want context depending on the unicode tables. This is only used by
+// *valueCtx.String().
+func stringify(v interface{}) string {
+	if s, ok := v.(stringer); ok {
+		return s.String()
+	}
+	if s, ok := v.(string); ok {
+		return s
+	}
+	return "<not Stringer>"
+}
+
 func (c *valueCtx) String() string {
-	return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)
+	return contextName(c.Context) + ".WithValue(type " +
+		reflectlite.TypeOf(c.key).String() +
+		", val " + stringify(c.val) + ")"
 }
 
 func (c *valueCtx) Value(key interface{}) interface{} {
diff --git a/src/context/context_test.go b/src/context/context_test.go
index f73f2837b8..0cec169915 100644
--- a/src/context/context_test.go
+++ b/src/context/context_test.go
@@ -343,7 +343,7 @@ func XTestValues(t testingT) {
 	c1 := WithValue(Background(), k1, "c1k1")
 	check(c1, "c1", "c1k1", "", "")
 
-	if got, want := fmt.Sprint(c1), `context.Background.WithValue(1, "c1k1")`; got != want {
+	if got, want := fmt.Sprint(c1), `context.Background.WithValue(type context.key1, val c1k1)`; got != want {
 		t.Errorf("c.String() = %q want %q", got, want)
 	}