[swift-evolution] SR-104: Improve Crash-Safety when Importing Objective-C Code Without Nullability Attributes

[Fabian Ehrentraud]

Swift code accessing Objective-C methods can easily crash - if the Objective-C code does not include nullability attributes, the code is brought as implicitly unwrapped into Swift, where unsafe accesses do not produce compiler warnings.

I created issue SR-104 for this, but as suggested by Jordan Rose it should be discussed on this mailing list first.

Short example:

// Objective-C  
- (NSString *)giveMeAString { return nil; }  

// Swift  
func thisWillCrash() {  
    let string = someObjectiveCObject.giveMeAString()  
    let length = string.length // crash  
}

The ClangImporter could handle this issue in several safer ways:

1. Only import declarations that are nullability annotated (as proposed by Greg Parker)
Positive side effect would be that it would motivate to write nullability annotations.

2. Import un-annotated code as Optional
Values would need to be unwrapped every time, which does not hurt too much due to the easy use of the `?` syntactic sugar.

What do you think? Would this make Swift more safe when used together with Objective-C?