From 37e046337216f326d5cd551ea0d12463919c13d0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Mart=C3=AD?= <mvdan@mvdan.cc>
Date: Sun, 28 Jan 2024 16:15:32 +0000
Subject: [PATCH] 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                     |  26 ++--
 testdata/script/linedirectives.txtar | 173 +++++++++++++++++++++++++++
 2 files changed, 192 insertions(+), 7 deletions(-)
 create mode 100644 testdata/script/linedirectives.txtar

diff --git a/format/format.go b/format/format.go
index aaae194..7316eb7 100644
--- a/format/format.go
+++ b/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:
diff --git a/testdata/script/linedirectives.txtar b/testdata/script/linedirectives.txtar
new file mode 100644
index 0000000..91f4291
--- /dev/null
+++ b/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