fmt/print: give %p priority, analogous to %T

Fixes #1024.

R=rsc
CC=golang-dev
https://golang.org/cl/1961042

src/pkg/fmt/fmt_test.go

@@ -95,6 +95,15 @@ type S struct {
 	g G // a struct field that GoStrings
 }
 
+// A type with a String method with pointer receiver for testing %p
+type P int
+
+var pValue P
+
+func (p *P) String() string {
+	return "String(p)"
+}
+
 var b byte
 
 var fmttests = []fmtTest{
@@ -294,11 +303,6 @@ var fmttests = []fmtTest{
 	fmtTest{"%x", I(23), `3c32333e`},
 	fmtTest{"%d", I(23), `%d(string=<23>)`},
 
-	// %p on non-pointers
-	fmtTest{"%p", make(chan int), "PTR"},
-	fmtTest{"%p", make(map[int]int), "PTR"},
-	fmtTest{"%p", make([]int, 1), "PTR"},
-
 	// go syntax
 	fmtTest{"%#v", A{1, 2, "a", []int{1, 2}}, `fmt_test.A{i:1, j:0x2, s:"a", x:[]int{1, 2}}`},
 	fmtTest{"%#v", &b, "(*uint8)(PTR)"},
@@ -354,6 +358,17 @@ var fmttests = []fmtTest{
 	fmtTest{"%T", intVal, "int"},
 	fmtTest{"%6T", &intVal, "  *int"},
 
+	// %p
+	fmtTest{"p0=%p", new(int), "p0=PTR"},
+	fmtTest{"p1=%s", &pValue, "p1=String(p)"}, // String method...
+	fmtTest{"p2=%p", &pValue, "p2=PTR"},       // ... not called with %p
+
+	// %p on non-pointers
+	fmtTest{"%p", make(chan int), "PTR"},
+	fmtTest{"%p", make(map[int]int), "PTR"},
+	fmtTest{"%p", make([]int, 1), "PTR"},
+	fmtTest{"%p", 27, "%p(int=27)"}, // not a pointer at all
+
 	// erroneous things
 	fmtTest{"%d", "hello", "%d(string=hello)"},
 	fmtTest{"no args", "hello", "no args?(extra string=hello)"},

src/pkg/fmt/print.go

@@ -462,13 +462,14 @@ func (p *pp) fmtBytes(v []byte, verb int, goSyntax bool, depth int, value interf
 	}
 }
 
-func (p *pp) fmtUintptrGetter(field interface{}, value reflect.Value, verb int, sharp bool) bool {
+func (p *pp) fmtPointer(field interface{}, value reflect.Value, verb int, goSyntax bool) {
 	v, ok := value.(uintptrGetter)
-	if !ok {
-		return false
+	if !ok { // reflect.PtrValue is a uintptrGetter, so failure means it's not a pointer at all.
+		p.badVerb(verb, field)
+		return
 	}
 	u := v.Get()
-	if sharp {
+	if goSyntax {
 		p.add('(')
 		p.buf.WriteString(reflect.Typeof(field).String())
 		p.add(')')
@@ -482,7 +483,6 @@ func (p *pp) fmtUintptrGetter(field interface{}, value reflect.Value, verb int,
 	} else {
 		p.fmt0x64(uint64(u))
 	}
-	return true
 }
 
 var (
@@ -503,10 +503,14 @@ func (p *pp) printField(field interface{}, verb int, plus, goSyntax bool, depth
 	}
 
 	// Special processing considerations.
-	// %T (the value's type) is special; we always do it first.
-	if verb == 'T' {
+	// %T (the value's type) and %p (its address) are special; we always do them first.
+	switch verb {
+	case 'T':
 		p.printField(reflect.Typeof(field).String(), 's', false, false, 0)
 		return false
+	case 'p':
+		p.fmtPointer(field, reflect.NewValue(field), verb, goSyntax)
+		return false
 	}
 	// Is it a Formatter?
 	if formatter, ok := field.(Formatter); ok {
@@ -606,12 +610,8 @@ func (p *pp) printField(field interface{}, verb int, plus, goSyntax bool, depth
 		return verb == 's'
 	}
 
-	value := reflect.NewValue(field)
 	// Need to use reflection
-	// Special case for reflection values that know how to print with %p.
-	if verb == 'p' && p.fmtUintptrGetter(field, value, verb, goSyntax) { // TODO: is this goSyntax right?
-		return false
-	}
+	value := reflect.NewValue(field)
 
 BigSwitch:
 	switch f := value.(type) {
@@ -753,10 +753,7 @@ BigSwitch:
 		}
 		p.fmt0x64(uint64(v))
 	case uintptrGetter:
-		if p.fmtUintptrGetter(field, value, verb, goSyntax) {
-			break
-		}
-		p.unknownType(f)
+		p.fmtPointer(field, value, verb, goSyntax)
 	default:
 		p.unknownType(f)
 	}