Register extern struct/union symbols

[borup3]

Description

Please see the 2nd commit. I don't think it's a correct change, but I noticed that extern struct/unions were still emitted into the C output as a typedef which causes double-typedefs for structs defined in a C header. I don't see any unit tests using extern struct/union to verify its behavior so I'm not sure if this is intended or a bug. It makes my use-case below work.

This registers extern struct and extern union as known extern symbols to quench the warning "assuming external C struct". I didn't change the branch for extern fn because they don't seem to have this issue.

#include "c_header_that_defines_dmon_structs.h"

extern struct dmon_watch_id;
extern struct dmon_action;

alias FsCallback = fn(dmon_watch_id, dmon_action, const char*, const char*, const char*, void*);

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[Burnett01]

If I remember correctly there was a reason behind emitting those warnings when the typechecker got implemented.

If we want to keep the current typechecker behaivour, we should lax the warning with a flag instead.

If we don't want to keep the current typechecker behaivour, your PR is a sufficient fix I think.

This is related to #116

[borup3]

I think there's 2 concerns there.

1 is a warning/error/diagnostic if a symbol is unknown. This PR is not about this.

2 is that an extern symbol shouldn't have to be unknown. It should be possible to define it in Zen C as "This type will exist." and that's what this PR is about.