go/printer: form feed is wrongly treated as a newline

This is an unrealistic edge-case, found by a fuzzer.

Both of the go sources in the reproducer below, are parsed successfully, but the printed source (with go/printer) contains a syntax error:

func TestReproducer(t *testing.T) { cases := []string{ "package A\nimport(\"a\"/*\f*/\n\"bb\")", "package A\nfunc test() {\"a\"/*\f*/\n\"bb\"}",	} for _, src := range cases { fset := token.NewFileSet() f, err := parser.ParseFile(fset, "test.go", src, parser.ParseComments|parser.SkipObjectResolution) if err != nil { t.Fatal(err)	} ast.Print(fset, f) var out strings.Builder if err := Fprint(&out, fset, f); err != nil { t.Fatal(err)	} _, err = parser.ParseFile(fset, "test.go", out.String(), parser.ParseComments|parser.SkipObjectResolution) if err != nil { t.Logf("source:\n%s\nformatted as:\n%s", src, out.String()) t.Error(err) // test.go:4:11: expected ';', found "bb" (and 1 more errors)	}	} }

[mateusz@arch go (master)]$ go test -run NotNewl -v go/printer -v === RUN TestFormFeedNotNewline printer_test.go:887: source: package A import("a"/* */ "bb") formatted as: package A import ( "a" /* */"bb" ) printer_test.go:888: test.go:4:11: expected ';', found "bb" (and 1 more errors) printer_test.go:887: source: package A func test() {"a"/* */ "bb"} formatted as: package A func test() { "a" /* */"bb" } printer_test.go:888: test.go:4:11: expected ';', found "bb" (and 1 more errors)

This issue happens in the (*printer).print method.

go/src/go/printer/printer.go

Lines 1006 to 1020 in b521ebb

     if !p.impliedSemi {  
   n := nlimit(next.Line - p.pos.Line)  
   // don't exceed maxNewlines if we already wrote one  
   if wroteNewline && n == maxNewlines {  
   n = maxNewlines - 1  
  	}  
   if n > 0 {  
   ch := byte('\n')  
   if droppedFF {  
   ch = '\f' // use formfeed since we dropped one before  
  	}  
   p.writeByte(ch, n)  
   impliedSemi = false  
  	}  
   }  
 

specifically here:

go/src/go/printer/printer.go

Line 1007 in b521ebb

n := nlimit(next.Line - p.pos.Line)

next.Line is equal to 3 and p.pos.Line is also 3, which causes zero newlines to be inserted, this happens because the (*printer).writeString method treats \f as a newline and incorrectly increases the p.pos.Line field.

go/src/go/printer/printer.go

Lines 312 to 334 in b521ebb

     for i := 0; i < len(s); i++ {  
   // Raw string literals may contain any character except back quote (`).  
   if ch := s[i]; ch == '\n' || ch == '\f' {  
   // account for line break  
   nlines++  
   li = i  
   // A line break inside a literal will break whatever column  
   // formatting is in place; ignore any further alignment through  
   // the end of the line.  
   p.endAlignment = true  
  	}  
   }  
   p.pos.Offset += len(s)  
   if nlines > 0 {  
   p.pos.Line += nlines  
   p.out.Line += nlines  
   c := len(s) - li  
   p.pos.Column = c  
   p.out.Column = c  
   } else {  
   p.pos.Column += len(s)  
   p.out.Column += len(s)  
   }  
 

This was a nightmare to debug, but it seems like a fix is a one-liner, will send a CL.

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go/printer: form feed is wrongly treated as a newline #69858

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	if !p.impliedSemi {
	n := nlimit(next.Line - p.pos.Line)
	// don't exceed maxNewlines if we already wrote one
	if wroteNewline && n == maxNewlines {
	n = maxNewlines - 1
	}
	if n > 0 {
	ch := byte('\n')
	if droppedFF {
	ch = '\f' // use formfeed since we dropped one before
	}
	p.writeByte(ch, n)
	impliedSemi = false
	}
	}

	for i := 0; i < len(s); i++ {
	// Raw string literals may contain any character except back quote (`).
	if ch := s[i]; ch == '\n' \|\| ch == '\f' {
	// account for line break
	nlines++
	li = i
	// A line break inside a literal will break whatever column
	// formatting is in place; ignore any further alignment through
	// the end of the line.
	p.endAlignment = true
	}
	}
	p.pos.Offset += len(s)
	if nlines > 0 {
	p.pos.Line += nlines
	p.out.Line += nlines
	c := len(s) - li
	p.pos.Column = c
	p.out.Column = c
	} else {
	p.pos.Column += len(s)
	p.out.Column += len(s)
	}

go/printer: form feed is wrongly treated as a newline #69858

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