Ptr align test has different behavior with build mode ReleaseSafe

[alanvoss]

Zig Version

0.14.0-dev.1660+444228865

Steps to Reproduce and Observed Behavior

I'm running this on MacOS 15.1.1.

make a file called safetest.zig with the following contents:

const std = @import("std");

pub const S = extern struct {
    a: [11]u8,
    b: u8,
    _: [4]u8 = undefined,
    c: u64,
    d: f64,

    const SIZE: usize = @sizeOf(S);

    pub fn marshal(self: *const S) *const [SIZE]u8 {
        return @alignCast(@ptrCast(self));
    }

    pub fn unmarshal(s: *const [SIZE]u8) *const S {
        return @alignCast(@ptrCast(s));
    }
};

test "test marshal/unmarshal with ReleaseSafe" {
    var s = S{
        .a = [_]u8{ 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K' },
        .b = 'X',
        .c = 12345678,
        .d = 12.345,
    };
    const marshaled = s.marshal();
    var copy: [S.SIZE]u8 align(8) = [1]u8{0} ** S.SIZE;
    // std.debug.print("anything\n", .{}); // somehow just printing eliminates the error?  huh?
    @memcpy(&copy, marshaled);
    const got = S.unmarshal(&copy);
    try std.testing.expectEqual(s, got.*);
}

run as-is with 3 different test build modes:

 > zig test safetest.zig
All 1 tests passed.
> zig test safetest.zig -O ReleaseFast
All 1 tests passed.
> zig test safetest.zig -O ReleaseSafe
slices differ. first difference occurs at index 0 (0x0)

============ expected this output: =============  len: 4 (0x4)

41 42 43 44                                       ABCD

============= instead found this: ==============  len: 4 (0x4)

00 00 00 00                                       ....

================================================

1/1 safetest.test.test marshal/unmarshal with ReleaseSafe...FAIL (TestExpectedEqual)
0 passed; 0 skipped; 1 failed.
error: the following test command failed with exit code 1:
/Users/avoss/.cache/zig/o/5355486d8b426f44920699a773957766/test --seed=0xae458817

The bug is really above, but if you care to have a second bug that is potentially related to this, simply uncommenting this line in the above:

 std.debug.print("anything\n", .{}); // somehow just printing eliminates the error?  huh?

and then running the same tests, and what I believe would be the expected behavior happens again.

> zig test safetest.zig
anything
All 1 tests passed.

> zig test safetest.zig -O ReleaseFast
anything
All 1 tests passed.

> zig test safetest.zig -O ReleaseSafe
anything
All 1 tests passed.

Expected Behavior

I would expect both versions of the test to pass in all 3 build modes each.

[rohlem]

Comparisons with undefined are illegal behavior (#63).
got.* was copied from s, so got.*._ is undefined.
Branching on an @as(bool, undefined) (from comparing the fields _ which are undefined) in/calling std.testing.expectEqual is illegal behavior.
Besides missing safety in safe build modes, I don't think there is really a Zig bug here.