fix: Add attribute nonnull_error if returning pointer and error from Objective-C

[jefft0]

Consider the following Go code with a method that returns a byte array and an error:

package example

type TestClass struct {
}
func (req *TestClass) GetData() ([]byte, error) {
        return nil, nil
}

gobind creates an Objective-C method which returns NSData* and an error:

- (NSData* _Nullable)getData:(NSError* _Nullable* _Nullable)error {
    ....
}

The method signature seen by Swift is func getData() throws -> Data . The problem is that Data is not optional, and when the Objective-C method returns NULL the program crashes. This is the default signature as explained in the Swift documentation: https://clang.llvm.org/docs/AttributeReference.html#swift-error

But this is not appropriate for gobind since it is valid for Go to return a nil value. We don't want the crash. This pull request updates asSignature so that if the method returns an error and the method returns a nullable pointer to NSData or NSString, then add the nonnull_error attribute as explained in the Swift documentation.

With this attribute, the method signature seen by Swift is func getData() throws -> Data? . When the Go function returns nil, it is returned as a nil optional value without crashing.

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[gopherbot]

This PR (HEAD: b50c057) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/mobile/+/426595 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off)
See the Wiki page for more info