[BUG] Mojo build crashes when using Optional and struct

[ericqu]

Bug description

The following code crashes the 'mojo build' on Apple M1.

from collections import Optional

@value
struct Tree:
    var left :Optional[Tree]
    var right :Optional[Tree]

fn main() -> None :
    var t = Tree(Optional[Tree](None), Optional[Tree](None))

I was experimenting with Optional, so I likely did something wrong, although I hoped for a compiler error, not a crash.
I wonder if it is related to the recursive nature of the struct, although it seems a natural way to implement something like a tree.

Steps to reproduce

the source code is provided in the description above. Then I execute mojo build src/test_bin.mojo and it gives:

[47079:11093979:20240515,231828.233868:WARNING crash_report_exception_handler.cc:257] UniversalExceptionRaise: (os/kern) failure (5)
zsh: illegal hardware instruction  mojo build src/test_bin.mojo

or sometimes:

[47132:11096302:20240515,232253.957242:WARNING crash_report_exception_handler.cc:257] UniversalExceptionRaise: (os/kern) failure (5)
zsh: bus error  mojo build src/test_bin.mojo

I did also try on the playground and got the following crash details
mojo-stack.txt

It looks like there are other issues reported with Optional, although they don't crash the compiler.
Let me know if any other information could help.

System information

- What OS did you do install Mojo on ? MacOs with Apple M1
- Provide version information for Mojo by pasting the output of `mojo -v` : mojo 24.3.0 (9882e19d)
- Provide Modular CLI version by pasting the output of `modular -v` : modular 0.8.0 (39a426b5)

[Ryul0rd]

This is because your struct is recursive. A struct has all its fields stored contiguously in memory and the total size is the sum of the size of those fields. This means you can't have a struct with a field that has the same type as the struct itself because you get infinite recursion trying to determine the size. You get around this by using pointers since a pointer has fixed size regardless of what's being pointed to.

The fact that you get a crash instead of a useful error is still an issue though. The mojo vscode plugin also crashes if you attempt this.

[Mogball]

This should be fixed in the next release

[Mogball]

The crash -- it should emit an error

[ericqu]

Thanks for looking into this, and my apologies for the belated answer.

I would point out that the following, suggested, code gives a similar output in the same version of Mojo and the Playground:

from collections import Optional
from memory.unsafe_pointer import UnsafePointer

@value
struct Tree:
    var item: Optional[UnsafePointer[Tree]]

fn main() -> None:
    var t = Tree(None)

I am unsure what the next release means (a bit off-topic: I tried to install nightly/mojo. However, it failed, so I couldn't try the code with nightly if it meant the next version on nightly). It would be nice to have a nightly Playground so that a regular user could test if a minimal reproducible example is fixed.