exp/template: plain actions with declarations should produce no output.

This is already the behavior for pipelines producing values for if, with, and range.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/4808050

src/pkg/exp/template/doc.go

@@ -158,7 +158,8 @@ The initialization has syntax
 
 	$variable := pipeline
 
-where $variable is the name of the variable.
+where $variable is the name of the variable. An action that declares a
+variable produces no output.
 
 If a "range" action initializes a variable, the variable is set to the
 successive elements of the iteration.  Also, a "range" may declare two

src/pkg/exp/template/exec.go

@@ -99,7 +99,11 @@ func (s *state) walk(dot reflect.Value, n node) {
 	case *actionNode:
 		s.line = n.line
 		// Do not pop variables so they persist until next end.
-		s.printValue(n, s.evalPipeline(dot, n.pipe))
+		// Also, if the action declares variables, don't print the result.
+		val := s.evalPipeline(dot, n.pipe)
+		if len(n.pipe.decl) == 0 {
+			s.printValue(n, val)
+		}
 	case *ifNode:
 		s.line = n.line
 		s.walkIfOrWith(nodeIf, dot, n.pipe, n.list, n.elseList)

src/pkg/exp/template/exec_test.go

@@ -197,7 +197,7 @@ var execTests = []execTest{
 	{"$ int", "{{$}}", "123", 123, true},
 	{"$.I", "{{$.I}}", "17", tVal, true},
 	{"$.U.V", "{{$.U.V}}", "v", tVal, true},
-	{"declare in action", "{{$x := $.U.V}},{{$x}}", "v,v", tVal, true},
+	{"declare in action", "{{$x := $.U.V}}{{$x}}", "v", tVal, true},
 
 	// Pointers.
 	{"*int", "{{.PI}}", "23", tVal, true},
@@ -312,7 +312,7 @@ var execTests = []execTest{
 	{"with empty interface, struct field", "{{with .Empty4}}{{.V}}{{end}}", "UinEmpty", tVal, true},
 	{"with $x int", "{{with $x := .I}}{{$x}}{{end}}", "17", tVal, true},
 	{"with $x struct.U.V", "{{with $x := $}}{{$x.U.V}}{{end}}", "v", tVal, true},
-	{"with variable and action", "{{with $x := $}}{{$y := $.U.V}},{{$y}}{{end}}", "v,v", tVal, true},
+	{"with variable and action", "{{with $x := $}}{{$y := $.U.V}}{{$y}}{{end}}", "v", tVal, true},
 
 	// Range.
 	{"range []int", "{{range .SI}}-{{.}}-{{end}}", "-3--4--5-", tVal, true},
@@ -331,7 +331,7 @@ var execTests = []execTest{
 	{"range $x MSIone", "{{range $x := .MSIone}}<{{$x}}>{{end}}", "<1>", tVal, true},
 	{"range $x $y MSIone", "{{range $x, $y := .MSIone}}<{{$x}}={{$y}}>{{end}}", "<one=1>", tVal, true},
 	{"range $x PSI", "{{range $x := .PSI}}<{{$x}}>{{end}}", "<21><22><23>", tVal, true},
-	{"declare in range", "{{range $x := .PSI}}<{{$foo:=$x}}>{{end}}", "<21><22><23>", tVal, true},
+	{"declare in range", "{{range $x := .PSI}}<{{$foo:=$x}}{{$x}}>{{end}}", "<21><22><23>", tVal, true},
 
 	// Cute examples.
 	{"or as if true", `{{or .SI "slice is empty"}}`, "[3 4 5]", tVal, true},