MultiArrayList can have unexpected results when combining zero sized fields and optional pointers

[Hejsil]

Zig Version

0.10.0-dev.3885+a7661f115d

Steps to Reproduce

test.zig

const std = @import("std");

const testing = std.testing;

test {
    const S = struct { key: u32, value: void };

    const allocator = std.testing.allocator;
    var list = std.MultiArrayList(S){};
    defer list.deinit(allocator);

    try list.append(allocator, .{ .key = 0, .value = {} });
    const a: ?*const void = &list.items(.value)[0];
    try testing.expect(a != null);
}

$ zig test test.zig

Expected Behavior

All tests pass. We take a pointer to an existing value in the values slices of the list. We store then store this non null pointer in an optional pointer.

Actual Behavior

Test [1/1] test_0... FAIL (TestUnexpectedResult)
/usr/lib/zig/std/testing.zig:351:14: 0x21168f in expect (test)
    if (!ok) return error.TestUnexpectedResult;
             ^
./test.zig:14:5: 0x2117a0 in test_0 (test)
    try testing.expect(a != null);
    ^
0 passed; 0 skipped; 1 failed.
error: the following test command failed with exit code 1:
zig-cache/o/85be9248e4b75e6a5942ec409fcfeca7/test /usr/bin/zig

[Hejsil]

Basically, the problem here is that list.items(.value) returns a slice that points to address 0x0:

const std = @import("std");

const testing = std.testing;

test {
    const S = struct { key: u32, value: void };

    const allocator = std.testing.allocator;
    var list = std.MultiArrayList(S){};
    defer list.deinit(allocator);

    try list.append(allocator, .{ .key = 0, .value = {} });
    std.debug.print("{*}\n", .{list.items(.value).ptr});
}

Test [1/1] test_0... void@0
All 1 tests passed.

[Vexu]

Reduction:

test {
    std.debug.print("{*}\n", .{foo()});
}
fn foo() []void {
    const ptr: [*]void = undefined;
    var len: usize = 6;
    return ptr[0..len];
}

[Hejsil]

Also of note is this: https://github.com/ziglang/zig/issues/1831#issuecomment-722129239