v: fix mutable option

[felipensp]

Fix #18818

struct Abc {
        a int 
}

fn foo(mut baz ?Abc) {
        baz = Abc{a:3}
        println(baz)
        dump(baz)
}

fn main() {
	mut a := ?Abc{
                a:2
        }
	dump(a)
        foo(mut a)
        println('--')
        dump(a)
}

🤖 Generated by Copilot at 7d84ddf

This pull request fixes the C code generation for option types that are passed as mutable parameters to functions. It adds a new type flag option_mut_param_t to mark these types and adjusts the C type names, assignments, and string representations accordingly. It also fixes some bugs in the code generation for option expressions and function calls.

🤖 Generated by Copilot at 7d84ddf

• Add a new type flag option_mut_param_t to mark option types that are passed as mutable parameters or return values (link)
  • Use memcpy to copy the right expression to the data field of the option struct in assignments (link, link, link)
  • Remove the asterisk from the C type name and the variable declaration, since the option struct already contains a pointer to the data type (link, link, link, link)
  • Skip the double dereference or the reference of the option value, depending on the context, since the option struct already contains a pointer to the data type (link, link, link, link, link)
  • Add a dereference to the option value in the dump_expr method, which is used for debugging and error reporting (link)
• Modify the parse_param and parse_return_type methods in vlib/v/parser/fn.v to set the option_mut_param_t flag on option types without any pointer modifiers (link, link)
• Skip the generation of option types that end with an asterisk in the gen_option_type method in vlib/v/gen/c/cgen.v, since they are already handled by the option_mut_param_t flag (link)

[spytheman]

./v -autofree -o v2 cmd/v is failing for some reason.

[spytheman]

Do you really need that? mut is usually just a modifier, not something that is part of the type.

[spytheman]

TypeFlags are limited to just 3 bits, i.e. just 8 combinations :-|

[spytheman]

@joe-conigliaro can you please also review this?

[felipensp]

Do you really need that? mut is usually just a modifier, not something that is part of the type.

Yes. I was thinking a way to determine when an option ptr must be used as struct _option_*_ptr or a real pointer.

[spytheman]

The CI is passing, can it be reviewed/merged now?

[felipensp]

The CI is passing, can it be reviewed/merged now?

It was missing a parameter validation, checking if everything is ok yet.