panic diffing map with struct value type

[bmizerany]

Go Version

go version go1.18.2 darwin/arm64

Current kr.dev/diff commit 6c69dcc

I'm using diff.Test with a map[string]someStruct and seeing this panic:

diff % go test
--- FAIL: TestBug (0.00s)
panic: reflect.Value.UnsafeAddr of unaddressable value [recovered]
	panic: reflect.Value.UnsafeAddr of unaddressable value

goroutine 321 [running]:
testing.tRunner.func1.2({0x10119ed80, 0x1011ea1d0})
	/usr/local/go/src/testing/testing.go:1389 +0x1c8
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1392 +0x384
panic({0x10119ed80, 0x1011ea1d0})
	/usr/local/go/src/runtime/panic.go:838 +0x204
reflect.Value.UnsafeAddr(...)
	/usr/local/go/src/reflect/value.go:2528
kr.dev/diff.access({0x10119e380?, 0x1400029fbc0?, 0x1400029fbc0?})
	/Users/bmizerany/src/diff/diff.go:494 +0x90
kr.dev/diff.(*differ).walk(0x1400029d580, {0x1011ecd20, 0x140002e24e0}, {0x1011b7f60?, 0x1400029fbc0?, 0x1400029fbc8?}, {0x1011b7f60?, 0x1400029fbc8?, 0x14000050b68?}, 0x1, ...)
	/Users/bmizerany/src/diff/diff.go:330 +0x239c
kr.dev/diff.(*differ).walk(0x1400029d580, {0x1011ecd20, 0x140002e2340}, {0x1011adc40?, 0x14000010bd8?, 0x1011ba660?}, {0x1011adc40?, 0x14000010be0?, 0x14000043e78?}, 0x1, ...)
	/Users/bmizerany/src/diff/diff.go:360 +0x2110
kr.dev/diff.(*differ).each(0x1400029d580, {0x1011adc40, 0x140002e40c0}, {0x1011adc40?, 0x140002e40f0})
	/Users/bmizerany/src/diff/diff.go:242 +0x27c
kr.dev/diff.Test({0x1011eb408, 0x140002dc680}, 0x101131ee0?, {0x1011adc40, 0x140002e40c0}, {0x1011adc40, 0x140002e40f0}, {0x0, 0x0, 0x0})
	/Users/bmizerany/src/diff/diff.go:75 +0x138
kr.dev/diff_test.TestBug(0x140002dc680)
	/Users/bmizerany/src/diff/diff_test.go:388 +0xe4
testing.tRunner(0x140002dc680, 0x1011e94d0)
	/usr/local/go/src/testing/testing.go:1439 +0x110
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1486 +0x300
exit status 2
FAIL	kr.dev/diff	0.160s

A minimal test to reproduce:

func TestBug(t *testing.T) {
	type T struct {
		F int
	}

	got := map[string]T{
		"k": {},
	}

	want := map[string]T{
		"k": {},
	}

	diff.Test(t, t.Errorf, got, want)
}

[bmizerany]

This change to diff_test.go produces the same result:

NOTE: The ab(...) version passes, but the non-copy panics.

--- a/diff_test.go
+++ b/diff_test.go
@@ -62,6 +62,11 @@ func TestEqual(t *testing.T) {
                ab(make(chan int)),
                ab(unsafe.Pointer(new(int))),
                ab(unsafe.Pointer(nil)),
+               ab(map[int]struct{ F int }{0: {}}),
+               {
+                       map[int]struct{ F int }{0: {}},
+                       map[int]struct{ F int }{0: {}},
+               },
        }

[bmizerany]

Changing the value to a pointer causes the test to pass:

{
    map[int]*struct{ F int }{0: {}},
    map[int]*struct{ F int }{0: {}},
},