Add RedundantTLet cop

[dejmedus]

• closes Suggested cop: remove T.let when its signature matches the param signature in initialize #346

Sorbet can infer the type of initialize method instance variables when they are assigned to parameters already typed by signatures. In cases where a T.let assertion is:

• top level inside an initialize method
• assigned to an instance variable
• typed the same as in sig params

we can DRY up code by removing it

sig { params(foo: String) }
def initialize(foo:)
- @foo = T.let(foo, String)
+ @foo = foo
end

The cop will display Sorbet/RedundantTLet: Unnecessary T.let. The instance variable type is inferred from the signature and optionally autocorrect

Demo

tlet-demo.mov

[Morriar]

This is looking good, let's add a few tests to encode current behaviour with more complex type expressions then we can merge it 👍

[Morriar]

Let's add a few example with more complicated type syntax:

sig { params(a: Foo[Bar], b: T.any(A, B), c: T.proc.void).void }
def initialize(proc)
  @a = T.let(a, Foo[Foo]) # error
  @b = T.let(b, T.any(B, A)) # it's equivalent, should it be ok or error?
  @c = T.let(c, T.proc.returns(Integer)) # ok
end

[dejmedus]

Added a few more examples:)

@ b = T.let(b, T.any(B, A)) # it's equivalent, should it be ok or error?

Right now the cop is checking for syntactical equivalence, not semantical, so T.any(A, B) and T.any(B, A) (or T.proc.params(x: A, y: B) and T.proc.params(y: B, x: A)) are treated as different and left alone

I think it would be possible to manually check for parity in these cases but the solutions I looked at felt a bit fragile. I would lean towards not warning/autocorrecting. What do you think?

[KaanOzkan]

LGTM