cmd/vet: diagnose using Printf on a function value

Printing a function value is nearly useless outside of debugging, but
can occur by mistake when one forgets to call it. Diagnose this.

I did this myself just the other day and it arose in cl/14031.
Easy to fix and seems worthwhile.

Fixes #12295.

Change-Id: Ice125a84559f0394f7fa7272b5d31ae602b07f83
Reviewed-on: https://go-review.googlesource.com/14122Reviewed-by: Andrew Gerrand <adg@golang.org>

src/cmd/vet/print.go

@@ -447,6 +447,10 @@ func (f *File) okPrintfArg(call *ast.CallExpr, state *formatState) (ok bool) {
 	arg := call.Args[argNum]
 	if !f.matchArgType(v.typ, nil, arg) {
 		typeString := ""
+		if f.isFunctionValue(arg) {
+			f.Badf(call.Pos(), "arg %s in printf call is a function value, not a function call", f.gofmt(arg))
+			return false
+		}
 		if typ := f.pkg.types[arg].Type; typ != nil {
 			typeString = typ.String()
 		}
@@ -490,6 +494,16 @@ func (f *File) recursiveStringer(e ast.Expr) bool {
 	return f.stringers[obj]
 }
 
+// isFunctionValue reports whether the expression is a function as opposed to a function call.
+// It is almost always a mistake to print a function value.
+func (f *File) isFunctionValue(e ast.Expr) bool {
+	if typ := f.pkg.types[e].Type; typ != nil {
+		_, ok := typ.(*types.Signature)
+		return ok
+	}
+	return false
+}
+
 // argCanBeChecked reports whether the specified argument is statically present;
 // it may be beyond the list of arguments or in a terminal slice... argument, which
 // means we can't see it.
@@ -579,8 +593,11 @@ func (f *File) checkPrint(call *ast.CallExpr, name string, firstArg int) {
 		}
 	}
 	for _, arg := range args {
+		if f.isFunctionValue(arg) {
+			f.Badf(call.Pos(), "arg %s in %s call is a function value, not a function call", f.gofmt(arg), name)
+		}
 		if f.recursiveStringer(arg) {
-			f.Badf(call.Pos(), "arg %s for print causes recursive call to String method", f.gofmt(arg))
+			f.Badf(call.Pos(), "arg %s in %s call causes recursive call to String method", f.gofmt(arg), name)
 		}
 	}
 }

src/cmd/vet/testdata/print.go

@@ -195,6 +195,9 @@ func PrintfTests() {
 	et4.Error() // ok, not an error method.
 	var et5 errorTest5
 	et5.error() // ok, not an error method.
+	// Can't print a function.
+	Printf("%d", someFunction) // ERROR "arg someFunction in printf call is a function value, not a function call"
+	Println(someFunction)      // ERROR "arg someFunction in Println call is a function value, not a function call"
 	// Bug: used to recur forever.
 	Printf("%p %x", recursiveStructV, recursiveStructV.next)
 	Printf("%p %x", recursiveStruct1V, recursiveStruct1V.next)
@@ -209,6 +212,10 @@ func PrintfTests() {
 
 }
 
+// A function we use as a function value; it has no other purpose.
+func someFunction() {
+}
+
 // Printf is used by the test so we must declare it.
 func Printf(format string, args ...interface{}) {
 	panic("don't call - testing only")
@@ -297,14 +304,14 @@ func (s recursiveStringer) String() string {
 	_ = fmt.Sprintf("%v", s)  // ERROR "arg s for printf causes recursive call to String method"
 	_ = fmt.Sprintf("%v", &s) // ERROR "arg &s for printf causes recursive call to String method"
 	_ = fmt.Sprintf("%T", s)  // ok; does not recursively call String
-	return fmt.Sprintln(s)    // ERROR "arg s for print causes recursive call to String method"
+	return fmt.Sprintln(s)    // ERROR "arg s in Sprintln call causes recursive call to String method"
 }
 
 type recursivePtrStringer int
 
 func (p *recursivePtrStringer) String() string {
 	_ = fmt.Sprintf("%v", *p)
-	return fmt.Sprintln(p) // ERROR "arg p for print causes recursive call to String method"
+	return fmt.Sprintln(p) // ERROR "arg p in Sprintln call causes recursive call to String method"
 }
 
 type Formatter bool