[Theoretical matter] Is it possible to compile and access a non-existent field?

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It’s not compiling. The SIGTRAP is being triggered by the compiler.

To be honest, I’m not sure why, if it is obvious that having a slice of anyopaque is not allowed, Zig doesn’t throw a compilation error. Also, I’m unsure about where exactly the SIGTRAP happens.

Regarding the example. Thank you for showing how to cast pointers (didn’t reach that part yet:)). I couldn’t get it work though with t being const:

const std = @import("std");

const T = packed struct {
    f: u8,
};
const U = packed struct {
    f: u4,
    g: u4,
};

const t: T = .{ .f = 0xAB };
const u: *const U = @ptrCast(&t);

pub fn main() !void {
    std.log.debug("{any}", .{@import("builtin").target.cpu.arch.endian()});
    std.log.debug("{X}", .{t.f});
    std.log.debug("{X} {X}", .{ u.f, u.g });
}

Throws:

file.zig:17:34: error: dereference of '*align(1:0:1) const u4' exceeds bounds of containing decl of type 'u0'
    std.log.debug("{X} {X}", .{ u.f, u.g });
                                ~^~

However, when I get rid of pointer const-ness like this:

var t: T = .{ .f = 0xAB };
const u: *U = @ptrCast(&t);

I get the expected:

debug: builtin.Endian.little
debug: AB
debug: B A