Support nullable parameters

[skarllot]

The issue or feature being addressed

• Support nullable as optional dependency #143

Details on the issue fix or feature implementation

• Fix handling of nullable parameters
• New diagnostics JAB0013 and JAB0014 requesting to define a default value for nullable dependency

[skarllot]

Added diagnostic test

[skarllot]

Do you think that JAB0013 should be raised even when dependency is found?

[pakrym]

Do you think that JAB0013 should be raised even when dependency is found?

I think the warning should be something like "{} parameter will never be null when constructing using a service provider. Add a default value to make the service reference optional." and fire for all nullable required parameters. It feels like there is no point in defining a nullable non-optional parameter.

Feel free to pull the warning out of this PR, I will be away from the computer for more than a week, so I'd rather get this PR in and you unblocked.

[skarllot]

I've created 2 diagnostics, one for non-registered nullable dependency and other for registered nullable dependency. And changed CodeWriter to always display type as not null.

[pakrym]

Great work! Thank you!

[pakrym]

Out it 0.8.7

[Marv51]

Hi, could I ask about the reasoning behind "JAB0014: X parameter to construct Y will never be null when constructing using a service provider."

my class that raises the warning looks like this:

public Foo(Bar? service, Bar2 service2){
}

I use Jab for my application, but construct the service manually in test cases.

The warning is absolutely correct Jab will always provide a service for the nullable parameter but for convenience in test cases this parameter is optional.
I could pretty easily rearrange the parameter to Foo(Bar2 service2, Bar? service = null) to make this warning go away, but I don't quite understand how that is an improvement?