checker: fix nested struct reference type field initialized check. (fix: #15741)

[shove70]

1. Fix compiler accepts uninitialized reference in nested structs #15741
2. Add test.

struct ContainsRef {
	a &int = unsafe { 0 }
	b &int
}

struct Outer {
	c ContainsRef
}

fn main() {
	o := Outer{}
	dump(o)
}

output:

vlib/v/checker/tests/struct_ref_fields_uninitialized_err.vv:11:7: error: reference field `Outer.c.b` must be initialized
    9 | 
   10 | fn main() {
   11 |     o := Outer{}
      |          ~~~~~~~
   12 |     dump(o)
   13 | }

[yuyi98]

It may be necessary that references in the structure are not initialized by default and must be initialized when used.

[shove70]

It may be necessary that references in the structure are not initialized by default and must be initialized when used.

Oh, may be. sdl/examples, ui module, and many other references have not been initialized.
The work may be in the opposite direction.
:)

[yuyi98]

@shove70 You did a good work! There are some rules to be clarified here.

[shove70]

@shove70 You did a good work! There are some rules to be clarified here.

Thank you for your encouragement! :)

[ghost]

Wouldn't it be better to make unsafe{ nil } the default initialization value for reference field as well as the default initialization for structure fields?

[shove70]

@spytheman

Please help me review these three PRs, because they will affect the CI check of this PR. Thank you very much.

vlang/sdl#406
vlang/ui#482
vlang/ved#149

[spytheman]

It looks good to me, but it effectively requires, that all struct fields of a reference type, require that = unsafe { nil }, which is more of a language change imho, not just a fix.
@medvednikov what do you think?

[medvednikov]

I'm personally fine with it. I agree it's a big change though.

[medvednikov]

@shove70 can you make it a warning for now, for a transitionary period?

3 months later we'll make it an error.

[shove70]

medvednikov

OK, this is better. I'll deal with it. :)

[larpon]

@shove70 great job!

I just found this edge case - where I think it's reporting the wrong result:

module main

struct Ref {}

struct EmbedStruct {
    ref &Ref
}

struct Struct {
    EmbedStruct
}

fn main() {
    ref := &Ref{}
    _ := Struct{
        ref: ref
    }
}

Outputs:

t.v:16:7: warning: reference field `Struct.EmbedStruct.ref` must be initialized
   14 | fn main() {
   15 |     ref := &Ref{}
   16 |     _ := Struct{
      |          ~~~~~~~
   17 |         ref: ref
   18 |     }

Which is wrong IMO, since the ref field of the embedded struct is being initialized.