go/ast: add Range token.Pos to RangeStmt

For #50429

Change-Id: Idb027244f901d9f482c894b5b979a054d0f07de5
Reviewed-on: https://go-review.googlesource.com/c/go/+/426091
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>

api/next/50429.txt

@@ -0,0 +1 @@
+pkg go/ast, type RangeStmt struct, Range token.Pos #50429

src/go/ast/ast.go

@@ -754,6 +754,7 @@ type (
 		Key, Value Expr        // Key, Value may be nil
 		TokPos     token.Pos   // position of Tok; invalid if Key == nil
 		Tok        token.Token // ILLEGAL if Key == nil, ASSIGN, DEFINE
+		Range      token.Pos   // position of "range" keyword
 		X          Expr        // value to range over
 		Body       *BlockStmt
 	}

src/go/parser/parser.go

@@ -2338,6 +2338,7 @@ func (p *parser) parseForStmt() ast.Stmt {
 			Value:  value,
 			TokPos: as.TokPos,
 			Tok:    as.Tok,
+			Range:  as.Rhs[0].Pos(),
 			X:      x,
 			Body:   body,
 		}

src/go/parser/parser_test.go

@@ -697,3 +697,33 @@ func TestScopeDepthLimit(t *testing.T) {
 		}
 	}
 }
+
+// proposal #50429
+func TestRangePos(t *testing.T) {
+	testcases := []string{
+		"package p; func _() { for range x {} }",
+		"package p; func _() { for i = range x {} }",
+		"package p; func _() { for i := range x {} }",
+		"package p; func _() { for k, v = range x {} }",
+		"package p; func _() { for k, v := range x {} }",
+	}
+
+	for _, src := range testcases {
+		fset := token.NewFileSet()
+		f, err := ParseFile(fset, src, src, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		ast.Inspect(f, func(x ast.Node) bool {
+			switch s := x.(type) {
+			case *ast.RangeStmt:
+				pos := fset.Position(s.Range)
+				if pos.Offset != strings.Index(src, "range") {
+					t.Errorf("%s: got offset %v, want %v", src, pos.Offset, strings.Index(src, "range"))
+				}
+			}
+			return true
+		})
+	}
+}