Add CastNullableToNonNullableType rule

[atulgpt]

Fixes #3484

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	Messages
📖	Thanks for adding a new rule to Detekt ❤️

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[atulgpt]

Hi @BraisGabin I have created this PR for #3484 but not able to add you as reviewer

[BraisGabin]

To make ci happy you will need to run ./gradlew apiDump and commit the changes that this command will generate.

[cortinico]

Suggested change

	* problems in your code. The compliant code would be that which will correctly check
	* for two things (nullability and type) and not just one (cast).
	* problems in your code. The compliant code would either us smart-casting or would check the type before performing the cast.

[atulgpt]

Hi, @cortinico changing the message to * problems in your code. The compliant code would either us smart-casting or would check the type before performing the cast. doesn't actually reflect the intention of the rule.
Original code bar as String asserts two things(nullability as well as type assertions) at once. And suggested fix is to assert both the cases differently or handle both the cases

[cortinico]

I'm unsure I get the point of this rule at this point.

The problem of bar as String is that it will fail at runtime if the two types are incompatible. Using a if (bar is String) ... else ... would not instead.

Your rule is instead suggesting to fail for two different scenarios (null or non compatible types) with different error messages, right?.

[BraisGabin]

The point of this rule is that a cast can hide a !! behind it. That's a potential error and what this rule does is to point it out to ensure that this is not an error but a decission by the devolver. If you really want to cast directly from Any? to String you should be fine with foo!! as String. If you didn't want to do that, well, then you need to rewrite your code in any other way that doesn't produces a null pointer exception.

To me this rule should not be too opinated. Just point to the potential error and its up to the dev to decide how to fix it. If the problem here are the compilant part we can add 3 different ways to make the code compilant because it depends a lot to what the user wants on each case.

If you are casting your code is smelly. This rule at least ensure that the cast doesn't hide any extra surprise.

[cortinico]

If would suggest something like:

val x = if (bar is String) ... else ...

[atulgpt]

Hi @cortinico wouldn't val x = if (bar is String) ... else ... defeat the purpose of the rule? As if type of bar is Any? then the check bar is String can be false due to two reasons

1. bar being null
2. bar is not the type of String

hence there will be no clear and separate handling of null and wrong type. The initial idea was The idea is to write code that shows that it could fail for two things (nullability and type) and not just one (cast). at the issue description

@BraisGabin let me know your thoughts as well

[BraisGabin]

I would vote to add two alternatives:

val x = bar!! as String
val x = bar as String?

Or if !! is so bad (I really like this operator):

val x = checkNotNull(bar) as String
val x = bar as String?

The idea of this rule is to make clear that there is a cast from nullable to non-nullable. The idea is to avoid it use as String? or make it completely clear. For that second part I think that !! does its job.

[atulgpt]

Hi, @BraisGabin I personally use (bar ?: error("null assertion message")) as String as I always prefer some custom error message. But I am also okay with val x = checkNotNull(bar) as String

I don't like !! much also this also gets flagged by UnsafeCallOnNullableType

[cortinico]

hence there will be no clear and separate handling of null and wrong type

Which might very much be outside the scope of the rule.
The user receives a bar: Any? and wants to know if it can be used as a String.

Your code suggestion is too opinionated in failing on null state, which the user might not want to do.

[atulgpt]

Hi @BraisGabin / @cortinico added below compliant block

* <compliant>
 * fun foo(bar: Any?) {
 *     val x = checkNotNull(bar) as String
 * }
 *
 * // Alternative
 * fun foo(bar: Any?) {
 *     val x = (bar ?: error("null assertion message")) as String
 * }
 * </compliant>

[BraisGabin]

That works for me

[BraisGabin]

LGTM

[cortinico]

(sorry for the late review)