Improve generated extern "C" function signatures

[madsmtm]

• Allow marking functions as safe.
• Convert bool to/from Bool.
• Convert *mut T in return types to Option<Retained<T>> (using either Retained::retain_autoreleased or Retained::from_raw depending on the memory management rules of the function).

See also #638 (comment).

[steven-joruk]

I'd like to implement this, the conversions seem simple enough.

Am I right in thinking that by "Allow marking functions as safe" you mean after a function has had a conversion, mark it as safe if all of its types are now safe?

[madsmtm]

That'd be really nice!

Am I right in thinking that by "Allow marking functions as safe" you mean after a function has had a conversion, mark it as safe if all of its types are now safe?

No, that'd still not be sound, since the function could do anything, we generally cannot automatically verify that there aren't pre-conditions that are not encoded in the types.

What I meant was to add an option to translation-config.toml, so that we can manually mark methods (like MTLCreateSystemDefaultDevice) as safe.

So in the end, I'm envisioning something like this, you can use it as a reference when doing the implementation:

# Example config
fn.NSMouseInRect.unsafe = false
fn.NSStringFromPoint.unsafe = false

Generates:

// Not changed, because none of the types need to be, and it's not marked safe.
extern "C" {
    #[cfg(feature = "NSString")]
    pub fn NSRectFromString(a_string: &NSString) -> NSRect;
}

pub extern "C" fn NSEqualPoints(a_point: NSPoint, a_rect: NSRect, flipped: bool) -> bool {
    extern "C" {
        fn NSMouseInRect(a_point: NSPoint, a_rect: NSRect, flipped: Bool) -> Bool;
    }
    unsafe { NSMouseInRect(a_point, a_rect, Bool::new(flipped)).to_bool() }
}

// Still marked `unsafe`, it has a pointer parameter
#[cfg(feature = "NSString")]
pub unsafe extern "C" fn NSDecimalString(
    dcm: NonNull<NSDecimal>,
    locale: Option<&AnyObject>,
) -> Retained<NSString> {
    extern "C" {
        fn NSDecimalString(
            dcm: NonNull<NSDecimal>,
            locale: Option<&AnyObject>,
        ) -> NonNull<NSString>;
    }
    // Retain because `NSDecimalString` is not a "copy" nor "create" function
    unsafe { Retained::retain_autoreleased(NSDecimalString(dcm, locale)) }
}