use_null_aware_elements does not trigger on chain call

[tenhobi]

Currently use_null_aware_elements works on cases such as

String? getValue() => 'abc';
final value = getValue();

final x = {
  if (value != null) 'key': value,
};

which produces

String? getValue() => 'abc';
final value = getValue();

final x = {
  'key': ?value,
};

but when we use chain calls it does not work

String? getValue() => 'abc';
final value = getValue();

final x = {
  if (value != null) 'key': value.length,
};

even through it is still valid to transform it using null aware elements to

String? getValue() => 'abc';
final value = getValue();

final x = {
  'key': ?value?.length,
};

[bwilkerson]

@chloestefantsova Is there a reason why the lint isn't triggering in this case?

[lrhn]

That's not the same code, and not the same nullable value expression.

The ?value?.length has one more null check than if (value != null) ...: value.length.

(I hope it's an extra null check that an optimizing compiler can eliminate, but I would depend on it.)

Even if (v != null) e: v is not the same as e: ?v, of e is not a trivial expression (constant or local variables). The latter evaluates e and the former does l does not

I wouldn't want the lint to force me to write code that has a different behavior than what I intended, it should only trigger when the rewrite has the completely equivalent semantics. That is, when the key expression is trivial, and the value expression is one that can be promoted, so also a trivial expression.

(Although it could recognize other promotions than !=, like if (expr case var v2?) v1: v2 which can be v1: ?expr. I don't know if that pattern is recognized.)