Remove #![feature()] for if_let, while_let, and tuple_indexing

[nrc]

Needs a snapshot after #19472 lands

[abonander]

Looks like most of the work has been done in #19517

[csouth3]

Ah, yep. Didn't even see this issue! After #19472 makes it into a snapshot, my PR should close this issue. I'll make double sure before then that I'm getting all of them.

[mdinger]

You'll want to remove them from the list of compiler features as well

[csouth3]

Pull request has been updated, thanks!

[lgrz]

These feature gates (if_let, tuple_indexing) look liked they were left in, they're supposed to be removed right?

https://github.com/rust-lang/rust/blob/master/src/libcollections/lib.rs#L27
https://github.com/rust-lang/rust/blob/master/src/librustc_borrowck/lib.rs#L19-L20

[csouth3]

Oops, looks like the two in borrowck were added after my PR was r+'ed and I didn't see them. I'd be happy to go take them out, but it looks like #19741 actually takes care of these last three as part of the second commit, so it may potentially be better to just wait for that to land so I don't get in the way of it.

[lgrz]

Ah thanks, I wasn't aware they were already taken care of.