Ruby

I can't imagine what it should do when foo == nil.
Should be a syntax error probably?

Hmmm, may be possible to interpret that code like as foo&.then {|recv| for recv.bar in []; end}.
Just an idea.

Given for foo.bar in []; end is valid and does for.bar = element,
I think for foo&.bar in []; end could simply do for&.bar = element which is the same as for.bar = element unless foo.nil? (NIL_P(foo) to be precise).

Changing the syntax for for variables sounds more risky and likely to unexpectedly disallow some new lhs forms when the lhs rule is updated, and the for_var rule is forgotten to be updated too.

IOW I think the segfault could be fixed here without changing what is valid syntax.
And given this segfault probably affects older versions too there it seems better to fix the segfault than to change syntax.

Agree with @Eregon (Benoit Daloze). The code should be more or less equivalent to:

for temp in [1, 2, 3]
  foo&.bar = temp
end

...which is totally valid and foo&.bar = is a no-op (semantically).

@nobu (Nobuyoshi Nakada) it's the same possibilities in for, rescue, and multi-write. As in:

for (foo, @foo, @@foo, $foo, Foo, Foo::Foo, foo.foo, foo[foo]) in bar; end

begin; rescue => foo; end
begin; rescue => @foo; end
begin; rescue => @@foo; end
begin; rescue => $foo; end
begin; rescue => Foo; end
begin; rescue => Foo::Foo; end
begin; rescue => foo.foo; end
begin; rescue => foo[foo]; end

(foo, @foo, @@foo, $foo, Foo, Foo::Foo, foo.foo, foo[foo]) = bar