format: ignore //line directives when computing positions

Otherwise, if the line directives give us line numbers
which don't make any sense given the real file's line numbers,
our use of MergeLines can cause a panic or just bad formatting.

Fixes #288.

format/format.go

@@ -106,7 +106,7 @@ func File(fset *token.FileSet, file *ast.File, opts Options) {
 	}
 	opts.LangVersion = "v" + m[2] + "." + m[3]
 	f := &fumpter{
-		File:    fset.File(file.Pos()),
+		file:    fset.File(file.Pos()),
 		fset:    fset,
 		astFile: file,
 		Options: opts,
@@ -179,7 +179,7 @@ var rxOctalInteger = regexp.MustCompile(`\A0[0-7_]+\z`)
 type fumpter struct {
 	Options
 
-	*token.File
+	file *token.File
 	fset *token.FileSet
 
 	astFile *ast.File
@@ -227,7 +227,7 @@ func (f *fumpter) addNewline(at token.Pos) {
 	offset := f.Offset(at)
 
 	// TODO: replace with the new Lines method once we require Go 1.21 or later
-	field := reflect.ValueOf(f.File).Elem().FieldByName("lines")
+	field := reflect.ValueOf(f.file).Elem().FieldByName("lines")
 	n := field.Len()
 	lines := make([]int, 0, n+1)
 	for i := 0; i < n; i++ {
@@ -246,7 +246,7 @@ func (f *fumpter) addNewline(at token.Pos) {
 	if offset >= 0 {
 		lines = append(lines, offset)
 	}
-	if !f.SetLines(lines) {
+	if !f.file.SetLines(lines) {
 		panic(fmt.Sprintf("could not set lines to %v", lines))
 	}
 }
@@ -255,7 +255,7 @@ func (f *fumpter) addNewline(at token.Pos) {
 // up on the same line.
 func (f *fumpter) removeLines(fromLine, toLine int) {
 	for fromLine < toLine {
-		f.MergeLine(fromLine)
+		f.file.MergeLine(fromLine)
 		toLine--
 	}
 }
@@ -266,6 +266,18 @@ func (f *fumpter) removeLinesBetween(from, to token.Pos) {
 	f.removeLines(f.Line(from)+1, f.Line(to))
 }
 
+func (f *fumpter) Position(p token.Pos) token.Position {
+	return f.file.PositionFor(p, false)
+}
+
+func (f *fumpter) Line(p token.Pos) int {
+	return f.Position(p).Line
+}
+
+func (f *fumpter) Offset(p token.Pos) int {
+	return f.file.Offset(p)
+}
+
 type byteCounter int
 
 func (b *byteCounter) Write(p []byte) (n int, err error) {
@@ -295,14 +307,14 @@ func (f *fumpter) lineEnd(line int) token.Pos {
 	if line < 1 {
 		panic("illegal line number")
 	}
-	total := f.LineCount()
+	total := f.file.LineCount()
 	if line > total {
 		panic("illegal line number")
 	}
 	if line == total {
 		return f.astFile.End()
 	}
-	return f.LineStart(line+1) - 1
+	return f.file.LineStart(line+1) - 1
 }
 
 // rxCommentDirective covers all common Go comment directives:

testdata/script/linedirectives.txtar

@@ -0,0 +1,173 @@
+# Line directives can throw off our use of MergeLines.
+# We should ignore them entirely when calculating line numbers.
+# The file below is borrowed from Go's test/dwarf/linedirectives.go.
+
+exec gofumpt -w foo.go
+cmp foo.go foo.go.golden
+
+-- foo.go --
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//line foo/bar.y:4
+package main
+//line foo/bar.y:60
+func main() { 
+//line foo/bar.y:297
+	f, l := 0, 0
+//line yacctab:1
+	f, l = 1, 1
+//line yaccpar:1
+	f, l = 2, 1
+//line foo/bar.y:82
+	f, l = 3, 82
+//line foo/bar.y:90
+	f, l = 3, 90
+//line foo/bar.y:92
+	f, l = 3, 92
+//line foo/bar.y:100
+	f, l = 3, 100
+//line foo/bar.y:104
+	l = 104
+//line foo/bar.y:112
+	l = 112
+//line foo/bar.y:117
+	l = 117
+//line foo/bar.y:121
+	l = 121
+//line foo/bar.y:125
+	l = 125
+//line foo/bar.y:133
+	l = 133
+//line foo/bar.y:146
+	l = 146
+//line foo/bar.y:148
+//line foo/bar.y:153
+//line foo/bar.y:155
+	l = 155
+//line foo/bar.y:160
+
+//line foo/bar.y:164
+//line foo/bar.y:173
+
+//line foo/bar.y:178
+//line foo/bar.y:180
+//line foo/bar.y:185
+//line foo/bar.y:195
+//line foo/bar.y:197
+//line foo/bar.y:202
+//line foo/bar.y:204
+//line foo/bar.y:208
+//line foo/bar.y:211
+//line foo/bar.y:213
+//line foo/bar.y:215
+//line foo/bar.y:217
+//line foo/bar.y:221
+//line foo/bar.y:229
+//line foo/bar.y:236
+//line foo/bar.y:238
+//line foo/bar.y:240
+//line foo/bar.y:244
+//line foo/bar.y:249
+//line foo/bar.y:253
+//line foo/bar.y:257
+//line foo/bar.y:262
+//line foo/bar.y:267
+//line foo/bar.y:272
+	if l == f {
+//line foo/bar.y:277
+	panic("aie!")
+//line foo/bar.y:281
+	}
+//line foo/bar.y:285
+	return
+//line foo/bar.y:288
+//line foo/bar.y:290
+}
+//line foo/bar.y:293
+//line foo/bar.y:295
+-- foo.go.golden --
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//line foo/bar.y:4
+package main
+
+//line foo/bar.y:60
+func main() {
+//line foo/bar.y:297
+	f, l := 0, 0
+//line yacctab:1
+	f, l = 1, 1
+//line yaccpar:1
+	f, l = 2, 1
+//line foo/bar.y:82
+	f, l = 3, 82
+//line foo/bar.y:90
+	f, l = 3, 90
+//line foo/bar.y:92
+	f, l = 3, 92
+//line foo/bar.y:100
+	f, l = 3, 100
+//line foo/bar.y:104
+	l = 104
+//line foo/bar.y:112
+	l = 112
+//line foo/bar.y:117
+	l = 117
+//line foo/bar.y:121
+	l = 121
+//line foo/bar.y:125
+	l = 125
+//line foo/bar.y:133
+	l = 133
+//line foo/bar.y:146
+	l = 146
+//line foo/bar.y:148
+//line foo/bar.y:153
+//line foo/bar.y:155
+	l = 155
+//line foo/bar.y:160
+
+//line foo/bar.y:164
+//line foo/bar.y:173
+
+//line foo/bar.y:178
+//line foo/bar.y:180
+//line foo/bar.y:185
+//line foo/bar.y:195
+//line foo/bar.y:197
+//line foo/bar.y:202
+//line foo/bar.y:204
+//line foo/bar.y:208
+//line foo/bar.y:211
+//line foo/bar.y:213
+//line foo/bar.y:215
+//line foo/bar.y:217
+//line foo/bar.y:221
+//line foo/bar.y:229
+//line foo/bar.y:236
+//line foo/bar.y:238
+//line foo/bar.y:240
+//line foo/bar.y:244
+//line foo/bar.y:249
+//line foo/bar.y:253
+//line foo/bar.y:257
+//line foo/bar.y:262
+//line foo/bar.y:267
+//line foo/bar.y:272
+	if l == f {
+//line foo/bar.y:277
+		panic("aie!")
+//line foo/bar.y:281
+	}
+//line foo/bar.y:285
+	return
+//line foo/bar.y:288
+//line foo/bar.y:290
+}
+
+//line foo/bar.y:293
+//line foo/bar.y:295