fmt/print: honor Formatter in Print, Println.

Rearrange code to clarify handling of %T, Formatter, GoStringer, and Stringer.

R=rsc
CC=golang-dev
https://golang.org/cl/1973043

src/pkg/fmt/fmt_test.go

@@ -78,6 +78,23 @@ type C struct {
 	B
 }
 
+type F int
+
+func (f F) Format(s State, c int) {
+	Fprintf(s, "<%c=F(%d)>", c, int(f))
+}
+
+type G int
+
+func (g G) GoString() string {
+	return Sprintf("GoString(%d)", int(g))
+}
+
+type S struct {
+	f F // a struct field that Formats
+	g G // a struct field that GoStrings
+}
+
 var b byte
 
 var fmttests = []fmtTest{
@@ -322,6 +339,15 @@ var fmttests = []fmtTest{
 	fmtTest{"%v", renamedComplex64(3 + 4i), "(3+4i)"},
 	fmtTest{"%v", renamedComplex128(4 - 3i), "(4-3i)"},
 
+	// Formatter
+	fmtTest{"%x", F(1), "<x=F(1)>"},
+	fmtTest{"%x", G(2), "2"},
+	fmtTest{"%+v", S{F(4), G(5)}, "{f:<v=F(4)> g:5}"},
+
+	// GoStringer
+	fmtTest{"%#v", G(6), "GoString(6)"},
+	fmtTest{"%#v", S{F(7), G(8)}, "fmt_test.S{f:<v=F(7)>, g:GoString(8)}"},
+
 	// %T
 	fmtTest{"%T", (4 - 3i), "complex"},
 	fmtTest{"%T", renamedComplex128(4 - 3i), "fmt_test.renamedComplex128"},
@@ -546,3 +572,22 @@ func TestBlankln(t *testing.T) {
 		t.Errorf("got %q expected %q", got, expect)
 	}
 }
+
+
+// Check Formatter with Sprint, Sprintln, Sprintf
+func TestFormatterPrintln(t *testing.T) {
+	f := F(1)
+	expect := "<v=F(1)>\n"
+	s := Sprint(f, "\n")
+	if s != expect {
+		t.Errorf("Sprint wrong with Formatter: expected %q got %q\n", expect, s)
+	}
+	s = Sprintln(f)
+	if s != expect {
+		t.Errorf("Sprintln wrong with Formatter: expected %q got %q\n", expect, s)
+	}
+	s = Sprintf("%v\n", f)
+	if s != expect {
+		t.Errorf("Sprintf wrong with Formatter: expected %q got %q\n", expect, s)
+	}
+}

src/pkg/fmt/print.go

@@ -493,18 +493,44 @@ var (
 )
 
 func (p *pp) printField(field interface{}, verb int, plus, goSyntax bool, depth int) (wasString bool) {
-	if field != nil {
-		switch {
-		default:
-			if stringer, ok := field.(Stringer); ok {
-				p.printField(stringer.String(), verb, plus, goSyntax, depth)
-				return false // this value is not a string
-			}
-		case goSyntax:
-			if stringer, ok := field.(GoStringer); ok {
-				p.printField(stringer.GoString(), verb, plus, goSyntax, depth)
-				return false // this value is not a string
-			}
+	if field == nil {
+		if verb == 'T' || verb == 'v' {
+			p.buf.Write(nilAngleBytes)
+		} else {
+			p.badVerb(verb, field)
+		}
+		return false
+	}
+
+	// Special processing considerations.
+	// %T (the value's type) is special; we always do it first.
+	if verb == 'T' {
+		p.printField(reflect.Typeof(field).String(), 's', false, false, 0)
+		return false
+	}
+	// Is it a Formatter?
+	if formatter, ok := field.(Formatter); ok {
+		formatter.Format(p, verb)
+		return false // this value is not a string
+
+	}
+	// Must not touch flags before Formatter looks at them.
+	if plus {
+		p.fmt.plus = false
+	}
+	// If we're doing Go syntax and the field knows how to supply it, take care of it now.
+	if goSyntax {
+		p.fmt.sharp = false
+		if stringer, ok := field.(GoStringer); ok {
+			// Print the result of GoString unadorned.
+			p.fmtString(stringer.GoString(), 's', false, field)
+			return false // this value is not a string
+		}
+	} else {
+		// Is it a Stringer?
+		if stringer, ok := field.(Stringer); ok {
+			p.printField(stringer.String(), verb, plus, false, depth)
+			return false // this value is not a string
 		}
 	}
 
@@ -580,15 +606,6 @@ func (p *pp) printField(field interface{}, verb int, plus, goSyntax bool, depth
 		return verb == 's'
 	}
 
-	if field == nil {
-		if verb == 'v' {
-			p.buf.Write(nilAngleBytes)
-		} else {
-			p.badVerb(verb, field)
-		}
-		return false
-	}
-
 	value := reflect.NewValue(field)
 	// Need to use reflection
 	// Special case for reflection values that know how to print with %p.
@@ -802,33 +819,8 @@ func (p *pp) doPrintf(format string, a []interface{}) {
 		field := a[fieldnum]
 		fieldnum++
 
-		// %T is special; we always do it here.
-		if c == 'T' {
-			// the value's type
-			if field == nil {
-				p.buf.Write(nilAngleBytes)
-				break
-			}
-			p.printField(reflect.Typeof(field).String(), 's', false, false, 0)
-			continue
-		}
-
-		// Try Formatter (except for %T).
-		if field != nil {
-			if formatter, ok := field.(Formatter); ok {
-				formatter.Format(p, c)
-				continue
-			}
-		}
-
 		goSyntax := c == 'v' && p.fmt.sharp
-		if goSyntax {
-			p.fmt.sharp = false
-		}
 		plus := c == 'v' && p.fmt.plus
-		if plus {
-			p.fmt.plus = false
-		}
 		p.printField(field, c, plus, goSyntax, 0)
 	}