internal/astutil: missing test for CL 748480

Updates golang/go#77675

Change-Id: Ie7d3d49f0a7ccc8fb4876592c95ef7d592ffb544
Reviewed-on: https://go-review.googlesource.com/c/tools/+/748481
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Hongxiang Jiang <hxjiang@golang.org>
Reviewed-by: Hongxiang Jiang <hxjiang@golang.org>

Author: adonovan

internal/astutil/stringlit_test.go

@@ -0,0 +1,100 @@
+// Copyright 2026 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.
+
+package astutil_test
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"strconv"
+	"strings"
+	"testing"
+
+	"golang.org/x/tools/internal/astutil"
+	"golang.org/x/tools/internal/testenv"
+)
+
+func TestPosInStringLiteral(t *testing.T) {
+	// Each string is Go source for a string literal with ^ marker at expected Pos.
+	tests := []string{
+		`"^abc"`,
+		`"a^bc"`,
+		`"ab^c"`,
+		`"abc^"`,
+		`"a\n^b"`,
+		`"\n^"`,
+		`"a\000^"`,
+		`"\x61^"`,
+		`"\u0061^"`,
+		`"\U00000061^"`,
+		`"€^"`,
+		`"a€^b"`,
+		"`abc^`",
+		"`a\n^b`",
+		// normalization of \r carriage returns:
+		"`a\r\n^b`",
+		"`a\r\nb\r\nc\r\n^d`",
+	}
+	for _, test := range tests {
+		// The workaround for \r requires the go1.26 fix for https://go.dev/issue/76031.
+		if strings.Contains(test, "\r") && testenv.Go1Point() < 26 {
+			continue
+		}
+
+		t.Logf("input: %#q", test)
+
+		// Parse.
+		const prefix = "package p; const _ = "
+		src := prefix + test
+		fset := token.NewFileSet()
+		f, err := parser.ParseFile(fset, "p.go", src, 0)
+		if err != nil {
+			t.Errorf("Parse: %v", err)
+			continue
+		}
+
+		// Find literal.
+		var lit *ast.BasicLit
+		ast.Inspect(f, func(n ast.Node) bool {
+			if b, ok := n.(*ast.BasicLit); ok {
+				lit = b
+				return false
+			}
+			return true
+		})
+		if lit == nil {
+			t.Errorf("No literal")
+			continue
+		}
+
+		// Find index of marker within logical value.
+		value, err := strconv.Unquote(lit.Value)
+		if err != nil {
+			t.Errorf("Unquote: %v", err)
+			continue
+		}
+		index := strings.Index(value, "^")
+		if index < 0 {
+			t.Errorf("Value %q contains no marker", value)
+			continue
+		}
+
+		// Convert logical index to file position.
+		pos, err := astutil.PosInStringLiteral(lit, index)
+		if err != nil {
+			t.Errorf("PosInStringLiteral(%d): %v", index, err)
+			continue
+		}
+
+		// Check that cut offset in original src file is before marker.
+		offset := fset.Position(pos).Offset
+		before, after := src[:offset], src[offset:]
+		t.Logf("\t%q :: %q", before, after)
+		if !strings.HasPrefix(after, "^") {
+			t.Errorf("no marker at cut point")
+			continue
+		}
+	}
+}