go/parser: set File{Start,End} correctly in all cases

...even when the file is empty or lacks a valid package decl.

+ test

Fixes #70162

Change-Id: Idf33998911475fe8cdfaa4786ac3ba1745f54963
Reviewed-on: https://go-review.googlesource.com/c/go/+/624655
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

src/go/ast/ast.go

@@ -1065,11 +1065,15 @@ type File struct {
 }
 
 // Pos returns the position of the package declaration.
-// (Use FileStart for the start of the entire file.)
+// It may be invalid, for example in an empty file.
+//
+// (Use FileStart for the start of the entire file. It is always valid.)
 func (f *File) Pos() token.Pos { return f.Package }
 
 // End returns the end of the last declaration in the file.
-// (Use FileEnd for the end of the entire file.)
+// It may be invalid, for example in an empty file.
+//
+// (Use FileEnd for the end of the entire file. It is always valid.)
 func (f *File) End() token.Pos {
 	if n := len(f.Decls); n > 0 {
 		return f.Decls[n-1].End()

src/go/parser/interface.go

@@ -92,6 +92,8 @@ func ParseFile(fset *token.FileSet, filename string, src any, mode Mode) (f *ast
 		return nil, err
 	}
 
+	file := fset.AddFile(filename, -1, len(text))
+
 	var p parser
 	defer func() {
 		if e := recover(); e != nil {
@@ -115,12 +117,17 @@ func ParseFile(fset *token.FileSet, filename string, src any, mode Mode) (f *ast
 			}
 		}
 
+		// Ensure the start/end are consistent,
+		// whether parsing succeeded or not.
+		f.FileStart = token.Pos(file.Base())
+		f.FileEnd = token.Pos(file.Base() + file.Size())
+
 		p.errors.Sort()
 		err = p.errors.Err()
 	}()
 
 	// parse source
-	p.init(fset, filename, text, mode)
+	p.init(file, text, mode)
 	f = p.parseFile()
 
 	return
@@ -215,7 +222,8 @@ func ParseExprFrom(fset *token.FileSet, filename string, src any, mode Mode) (ex
 	}()
 
 	// parse expr
-	p.init(fset, filename, text, mode)
+	file := fset.AddFile(filename, -1, len(text))
+	p.init(file, text, mode)
 	expr = p.parseRhs()
 
 	// If a semicolon was inserted, consume it;

src/go/parser/parser.go

@@ -65,8 +65,8 @@ type parser struct {
 	nestLev int
 }
 
-func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
-	p.file = fset.AddFile(filename, -1, len(src))
+func (p *parser) init(file *token.File, src []byte, mode Mode) {
+	p.file = file
 	eh := func(pos token.Position, msg string) { p.errors.Add(pos, msg) }
 	p.scanner.Init(p.file, src, eh, scanner.ScanComments)
 
@@ -2904,8 +2904,7 @@ func (p *parser) parseFile() *ast.File {
 		Package: pos,
 		Name:    ident,
 		Decls:   decls,
-		FileStart: token.Pos(p.file.Base()),
-		FileEnd:   token.Pos(p.file.Base() + p.file.Size()),
+		// File{Start,End} are set by the defer in the caller.
 		Imports:   p.imports,
 		Comments:  p.comments,
 		GoVersion: p.goVersion,

src/go/parser/parser_test.go

@@ -838,3 +838,23 @@ func TestParseTypeParamsAsParenExpr(t *testing.T) {
 		t.Fatalf("typeParam is a %T; want: *ast.ParenExpr", typeParam)
 	}
 }
+
+// TestEmptyFileHasValidStartEnd is a regression test for #70162.
+func TestEmptyFileHasValidStartEnd(t *testing.T) {
+	for _, test := range []struct {
+		src  string
+		want string // "Pos() FileStart FileEnd"
+	}{
+		{src: "", want: "0 1 1"},
+		{src: "package ", want: "0 1 9"},
+		{src: "package p", want: "1 1 10"},
+		{src: "type T int", want: "0 1 11"},
+	} {
+		fset := token.NewFileSet()
+		f, _ := ParseFile(fset, "a.go", test.src, 0)
+		got := fmt.Sprintf("%d %d %d", f.Pos(), f.FileStart, f.FileEnd)
+		if got != test.want {
+			t.Fatalf("src = %q: got %s, want %s", test.src, got, test.want)
+		}
+	}
+}