cmd/compile: print columns (not just lines) in error messages

Compiler errors now show the exact line and line byte offset (sometimes
called "column") of where an error occured. For `go tool compile x.go`:

	package p
	const c int = false
	//line foo.go:123
	type t intg

reports

	x.go:2:7: cannot convert false to type int
	foo.go:123[x.go:4:8]: undefined: intg

(Some errors use the "wrong" position for the error message; arguably
the byte offset for the first error should be 15, the position of 'false',
rathen than 7, the position of 'c'. But that is an indepedent issue.)

The byte offset (column) values are measured in bytes; they start at 1,
matching the convention used by editors and IDEs.

Positions modified by //line directives show the line offset only for the
actual source location (in square brackets), not for the "virtual" file and
line number because that code is likely generated and the //line directive
only provides line information.

Because the new format might break existing tools or scripts, printing
of line offsets can be disabled with the new compiler flag -C. We plan
to remove this flag eventually.

Fixes #10324.

Change-Id: I493f5ee6e78457cf7b00025aba6b6e28e50bb740
Reviewed-on: https://go-review.googlesource.com/37970Reviewed-by: Matthew Dempsky <mdempsky@google.com>

misc/cgo/errors/test.bash

@@ -17,7 +17,7 @@ check() {
 expect() {
 	file=$1
 	shift
-	if go build $file >errs 2>&1; then
+	if go build -gcflags=-C $file >errs 2>&1; then
 		echo 1>&2 misc/cgo/errors/test.bash: BUG: expected cgo to fail on $file but it succeeded
 		exit 1
 	fi

src/cmd/compile/internal/gc/main.go

@@ -166,6 +166,7 @@ func Main() {
 	flag.BoolVar(&compiling_runtime, "+", false, "compiling runtime")
 	obj.Flagcount("%", "debug non-static initializers", &Debug['%'])
 	obj.Flagcount("B", "disable bounds checking", &Debug['B'])
+	obj.Flagcount("C", "disable printing of columns in error messages", &Debug['C']) // TODO(gri) remove eventually
 	flag.StringVar(&localimport, "D", "", "set relative `path` for local imports")
 	obj.Flagcount("E", "debug symbol export", &Debug['E'])
 	obj.Flagfn1("I", "add `directory` to import search path", addidir)

src/cmd/compile/internal/gc/subr.go

@@ -87,7 +87,7 @@ func hcrash() {
 }
 
 func linestr(pos src.XPos) string {
-	return Ctxt.OutermostPos(pos).String()
+	return Ctxt.OutermostPos(pos).Format(Debug['C'] == 0)
 }
 
 // lasterror keeps track of the most recently issued error.

src/cmd/internal/obj/util.go

@@ -59,7 +59,7 @@ func Getgoextlinkenabled() string {
 }
 
 func (p *Prog) Line() string {
-	return p.Ctxt.OutermostPos(p.Pos).String()
+	return p.Ctxt.OutermostPos(p.Pos).Format(false)
 }
 
 var armCondCode = []string{

src/cmd/internal/src/pos.go

@@ -13,8 +13,7 @@ import "strconv"
 // position base and zero line number).
 //
 // The (line, column) values refer to a position in a file independent of any
-// position base ("absolute" position). Line numbers start at 1, column values
-// start at 0 and are byte offsets from the beginning of the line.
+// position base ("absolute" file position).
 //
 // The position base is used to determine the "relative" position, that is the
 // filename and line number relative to the position base. If the base refers
@@ -80,30 +79,42 @@ func (p Pos) AbsFilename() string { return p.base.AbsFilename() }
 func (p Pos) SymFilename() string { return p.base.SymFilename() }
 
 func (p Pos) String() string {
+	return p.Format(true)
+}
+
+// Format formats a position as "filename:line" or "filename:line:column",
+// controlled by the showCol flag.
+// If the position is relative to a line directive, the original position
+// is appended in square brackets without column (since the column doesn't
+// change).
+func (p Pos) Format(showCol bool) string {
 	if !p.IsKnown() {
 		return "<unknown line number>"
 	}
 
-	s := posString(p.Filename(), p.Line(), p.Col())
 	if b := p.base; b == b.Pos().base {
 		// base is file base (incl. nil)
-		return s
+		return format(p.Filename(), p.Line(), p.Col(), showCol)
 	}
 
 	// base is relative
-	return posString(p.RelFilename(), p.RelLine(), p.Col()) + "[" + s + "]"
+	// Print the column only for the original position since the
+	// relative position's column information may be bogus (it's
+	// typically generated code and we can't say much about the
+	// original source at that point but for the file:line info
+	// that's provided via a line directive).
+	// TODO(gri) This may not be true if we have an inlining base.
+	// We may want to differentiate at some point.
+	return format(p.RelFilename(), p.RelLine(), 0, false) +
+		"[" + format(p.Filename(), p.Line(), p.Col(), showCol) + "]"
 }
 
-// Don't print column numbers because existing tests may not work anymore.
-// It's a variable for now so that the tests can enable it.
-// TODO(gri) fix this
-var printColumn = false
-
-// posString formats a (filename, line, col) tuple as a printable position.
-func posString(filename string, line, col uint) string {
+// format formats a (filename, line, col) tuple as "filename:line" (showCol
+// is false) or "filename:line:column" (showCol is true).
+func format(filename string, line, col uint, showCol bool) string {
 	s := filename + ":" + strconv.FormatUint(uint64(line), 10)
 	// col == colMax is interpreted as unknown column value
-	if printColumn && col < colMax {
+	if showCol && col < colMax {
 		s += ":" + strconv.FormatUint(uint64(col), 10)
 	}
 	return s

src/cmd/internal/src/pos_test.go

@@ -10,8 +10,6 @@ import (
 )
 
 func TestPos(t *testing.T) {
-	printColumn = true
-
 	f0 := NewFileBase("", "")
 	f1 := NewFileBase("f1", "f1")
 	f2 := NewLinePragmaBase(Pos{}, "f2", 10)
@@ -41,15 +39,15 @@ func TestPos(t *testing.T) {
 		{MakePos(nil, 2, 3), ":2:3", "", 2, 3, "", 2},
 		{MakePos(f0, 2, 3), ":2:3", "", 2, 3, "", 2},
 		{MakePos(f1, 1, 1), "f1:1:1", "f1", 1, 1, "f1", 1},
-		{MakePos(f2, 7, 10), "f2:16:10[:7:10]", "", 7, 10, "f2", 16},
-		{MakePos(f3, 12, 7), "f3:101:7[f1:12:7]", "f1", 12, 7, "f3", 101},
-		{MakePos(f4, 25, 1), "f4:114:1[f3:25:1]", "f3", 25, 1, "f4", 114},
+		{MakePos(f2, 7, 10), "f2:16[:7:10]", "", 7, 10, "f2", 16},
+		{MakePos(f3, 12, 7), "f3:101[f1:12:7]", "f1", 12, 7, "f3", 101},
+		{MakePos(f4, 25, 1), "f4:114[f3:25:1]", "f3", 25, 1, "f4", 114},
 
 		// positions from issue #19392
-		{MakePos(fc, 4, 0), "c.go:10:0[p.go:4:0]", "p.go", 4, 0, "c.go", 10},
-		{MakePos(ft, 7, 0), "t.go:20:0[p.go:7:0]", "p.go", 7, 0, "t.go", 20},
-		{MakePos(fv, 10, 0), "v.go:30:0[p.go:10:0]", "p.go", 10, 0, "v.go", 30},
-		{MakePos(ff, 13, 0), "f.go:40:0[p.go:13:0]", "p.go", 13, 0, "f.go", 40},
+		{MakePos(fc, 4, 0), "c.go:10[p.go:4:0]", "p.go", 4, 0, "c.go", 10},
+		{MakePos(ft, 7, 0), "t.go:20[p.go:7:0]", "p.go", 7, 0, "t.go", 20},
+		{MakePos(fv, 10, 0), "v.go:30[p.go:10:0]", "p.go", 10, 0, "v.go", 30},
+		{MakePos(ff, 13, 0), "f.go:40[p.go:13:0]", "p.go", 13, 0, "f.go", 40},
 	} {
 		pos := test.pos
 		if got := pos.String(); got != test.string {
@@ -120,8 +118,6 @@ func TestPredicates(t *testing.T) {
 }
 
 func TestLico(t *testing.T) {
-	printColumn = true
-
 	for _, test := range []struct {
 		x         lico
 		string    string
@@ -140,7 +136,7 @@ func TestLico(t *testing.T) {
 		{makeLico(lineMax+1, colMax+1), fmt.Sprintf(":%d", lineMax), lineMax, 0},
 	} {
 		x := test.x
-		if got := posString("", x.Line(), x.Col()); got != test.string {
+		if got := format("", x.Line(), x.Col(), true); got != test.string {
 			t.Errorf("%s: got %q", test.string, got)
 		}
 	}

test/run.go

@@ -585,7 +585,8 @@ func (t *test) run() {
 		t.err = fmt.Errorf("unimplemented action %q", action)
 
 	case "errorcheck":
-		cmdline := []string{"go", "tool", "compile", "-e", "-o", "a.o"}
+		// TODO(gri) remove need for -C (disable printing of columns in error messages)
+		cmdline := []string{"go", "tool", "compile", "-C", "-e", "-o", "a.o"}
 		// No need to add -dynlink even if linkshared if we're just checking for errors...
 		cmdline = append(cmdline, flags...)
 		cmdline = append(cmdline, long)