unconvert: don't panic on source that uses cgo

Package loader parses the cgo-processed source,
so offsets and column numbers don't match the original
source.  This commit changes the internal code to track
the exploded token.Position values instead of raw integer
offsets, so that unconvert can at least report line numbers
of unnecessary conversions in cgo-using source files.

Updates #3.

unconvert.go

@@ -25,15 +25,14 @@ import (
 	"unicode"
 
 	"github.com/kisielk/gotool"
-	"golang.org/x/tools/container/intsets"
 	"golang.org/x/tools/go/loader"
 )
 
 // Unnecessary conversions are identified by the position
 // of their left parenthesis within a source file.
 
-func apply(file string, edits *intsets.Sparse) {
-	if edits.IsEmpty() {
+func apply(file string, edits editSet) {
+	if len(edits) == 0 {
 		return
 	}
 
@@ -46,7 +45,7 @@ func apply(file string, edits *intsets.Sparse) {
 	// Note: We modify edits during the walk.
 	v := editor{edits: edits, file: fset.File(f.Package)}
 	ast.Walk(&v, f)
-	if !edits.IsEmpty() {
+	if len(edits) != 0 {
 		log.Printf("%s: missing edits %s", file, edits)
 	}
 
@@ -64,7 +63,7 @@ func apply(file string, edits *intsets.Sparse) {
 }
 
 type editor struct {
-	edits *intsets.Sparse
+	edits editSet
 	file  *token.File
 }
 
@@ -87,20 +86,21 @@ func (e *editor) Visit(n ast.Node) ast.Visitor {
 }
 
 func (e *editor) rewrite(f *ast.Expr) {
-	n, ok := (*f).(*ast.CallExpr)
+	call, ok := (*f).(*ast.CallExpr)
 	if !ok {
 		return
 	}
-	off := e.file.Offset(n.Lparen)
-	if !e.edits.Has(off) {
+
+	pos := e.file.Position(call.Lparen)
+	if _, ok := e.edits[pos]; !ok {
 		return
 	}
-	*f = n.Args[0]
-	e.edits.Remove(off)
+	*f = call.Args[0]
+	delete(e.edits, pos)
 }
 
-func print(name string, edits *intsets.Sparse) {
-	if edits.IsEmpty() {
+func print(name string, edits editSet) {
+	if len(edits) == 0 {
 		return
 	}
 
@@ -109,15 +109,7 @@ func print(name string, edits *intsets.Sparse) {
 		log.Fatal(err)
 	}
 
-	fset := token.NewFileSet()
-	f, err := parser.ParseFile(fset, name, buf, 0)
-	if err != nil {
-		log.Fatal(err)
-	}
-
-	file := fset.File(f.Package)
-	for _, p := range edits.AppendTo(nil) {
-		pos := file.Position(file.Pos(p))
+	for pos := range edits {
 		fmt.Printf("%s:%d:%d: unnecessary conversion\n", pos.Filename, pos.Line, pos.Column)
 		if *flagV {
 			line := lineForOffset(buf, pos.Offset)
@@ -145,7 +137,7 @@ func lineForOffset(buf []byte, off int) []byte {
 	if sol < 0 {
 		sol = 0
 	} else {
-		sol += 1
+		sol++
 	}
 	eol := bytes.IndexByte(buf[off:], '\n')
 	if eol < 0 {
@@ -188,7 +180,7 @@ func main() {
 		return
 	}
 
-	var m map[string]*intsets.Sparse
+	var m map[string]editSet
 	if *flagAll {
 		m = mergeEdits(importPaths)
 	} else {
@@ -214,7 +206,7 @@ func main() {
 		sort.Strings(files)
 		found := false
 		for _, f := range files {
-			if !m[f].IsEmpty() {
+			if len(m[f]) != 0 {
 				found = true
 			}
 			print(f, m[f])
@@ -267,12 +259,16 @@ var plats = [...]struct {
 	{"windows", "amd64"},
 }
 
-func mergeEdits(importPaths []string) map[string]*intsets.Sparse {
-	m := make(map[string]*intsets.Sparse)
+func mergeEdits(importPaths []string) map[string]editSet {
+	m := make(map[string]editSet)
 	for _, plat := range plats {
 		for f, e := range computeEdits(importPaths, plat.goos, plat.goarch, false) {
 			if e0, ok := m[f]; ok {
-				e0.IntersectionWith(e)
+				for k := range e0 {
+					if _, ok := e[k]; !ok {
+						delete(e0, k)
+					}
+				}
 			} else {
 				m[f] = e
 			}
@@ -287,7 +283,7 @@ func (noImporter) Import(path string) (*types.Package, error) {
 	panic("golang.org/x/tools/go/loader said this wouldn't be called")
 }
 
-func computeEdits(importPaths []string, os, arch string, cgoEnabled bool) map[string]*intsets.Sparse {
+func computeEdits(importPaths []string, os, arch string, cgoEnabled bool) map[string]editSet {
 	ctxt := build.Default
 	ctxt.GOOS = os
 	ctxt.GOARCH = arch
@@ -306,7 +302,7 @@ func computeEdits(importPaths []string, os, arch string, cgoEnabled bool) map[st
 
 	type res struct {
 		file  string
-		edits *intsets.Sparse
+		edits editSet
 	}
 	ch := make(chan res)
 	var wg sync.WaitGroup
@@ -316,9 +312,9 @@ func computeEdits(importPaths []string, os, arch string, cgoEnabled bool) map[st
 			wg.Add(1)
 			go func() {
 				defer wg.Done()
-				v := visitor{pkg: pkg, file: conf.Fset.File(file.Package)}
+				v := visitor{pkg: pkg, file: conf.Fset.File(file.Package), edits: make(editSet)}
 				ast.Walk(&v, file)
-				ch <- res{v.file.Name(), &v.edits}
+				ch <- res{v.file.Name(), v.edits}
 			}()
 		}
 	}
@@ -327,7 +323,7 @@ func computeEdits(importPaths []string, os, arch string, cgoEnabled bool) map[st
 		close(ch)
 	}()
 
-	m := make(map[string]*intsets.Sparse)
+	m := make(map[string]editSet)
 	for r := range ch {
 		m[r.file] = r.edits
 	}
@@ -339,10 +335,12 @@ type step struct {
 	i int
 }
 
+type editSet map[token.Position]struct{}
+
 type visitor struct {
 	pkg   *loader.PackageInfo
 	file  *token.File
-	edits intsets.Sparse
+	edits editSet
 	path  []step
 }
 
@@ -398,7 +396,7 @@ func (v *visitor) unconvert(call *ast.CallExpr) {
 		return
 	}
 
-	v.edits.Insert(v.file.Offset(call.Lparen))
+	v.edits[v.file.Position(call.Lparen)] = struct{}{}
 }
 
 // isSafeContext reports whether the current context requires