Module functions can access uninitialized main.swift globals

[tristanlabelle]

Description
Global variables used in main.swift are visible in functions of the module, which allows their access while uninitialized

Steps to reproduce

class Foo { let message: String = "oh no" }
func getFoo() -> Foo { foo }
let foo = getFoo()
print(foo.message)

This happens as well if getFoo() is moved to a different file in the same module.

Expected behavior
The code should fail to compile because getFoo() accesses an uninitialized foo. More generally, variables in main.swift should probably be treated like local variables in a function rather than like global variables.

Environment

• Swift compiler version info:

compnerd.org Swift version 5.11-dev (LLVM ee13eccb18b0dc4, Swift a94d22e017bd6a4)
Target: x86_64-unknown-windows-msvc
(built from main on 2023-10-27)

• Xcode version info: N/A
• Deployment target: Windows 11

[atrick]

Any global initializer that refers to the same global attempts to reenter to the initializer. Since the initializer can only be called once, subsequent attempts crash inside libdispatch.dylib_dispatch_once_wait.

I think the compiler should at least attempt to recognize cyclic initialization and report a compile-time error.

[eeckstein]

Yeah, we could try to diagnose this error. Though it would be non-trivial because it needs interprocedural analysis.

Also note that global variables in non parse-as-library mode are broken in general because you can access them before initialization - even without such a cycle. In non parse-as-library mode this test program crashes with a segfault. In parse-as-library mode it crashes "as expected" with a trap.