SI-6881 Detect reference equality when comparing streams

[szeiger]

== already covers this case. We override equals in Stream to do
the same when equals is called directly. This takes care of identical
streams.

To support short-circuiting equality checks on elements prepended to
identical streams we also override sameElements in Cons to treat
the case where both sides are Cons separately.

Tests in StreamTest.test_reference_equality.

Review by @Ichoran

JIRA: https://issues.scala-lang.org/browse/SI-6881

[szeiger]

Fixed stupid mistake...

[SethTisue]

LGTM

I assume the use of a @tailrec helper is primarily about avoiding stack overflow, rather than primarily about saving a few CPU cycles here and there. you might add a comment to that effect before merging?

it took me a while to convince myself that a stack-safe sameElements actually needs to be this complicated, but I convinced myself. I think it could be improved sliiiiiightly by doing the eq check at the outer level of sameElements rather than inside the inner match. then in the case where I call sameElements directly (not from ==) and this and that are the same cons cell, you avoid an unnecessary comparison of the head values. but whatevs, it's a really tiny nitpick, feel free to change it or merge as-is, as you see fit

[szeiger]

Doing the eq check at the outer level of sameElements is not sufficient because there is no recursion on sameElements anymore. Tails are checked with consEq instead, so we need to perform the eq check there.

[SethTisue]

Doing the eq check at the outer level of sameElements is not sufficient

oops, I meant to say at the outer level of consEq. anyway, nbd

[szeiger]

AFAIR I didnt't do that to avoid unnecessary comparisons when sameElements is called from equals or ==.