Tracking Issue for const_btree_len

[LG3696]

This tracks stabilization of the const_btree_len feature, which allows calling the below in a constant expression:

• BTree{Map,Set}::{len,is_empty}() (Constantify more BTreeMap and BTreeSet functions #78581)

Previously tracked here, already stable

• BTree{Map,Set}::new() (Make BTreeMap::new and BTreeSet::new const #71839)

Steps

• Implementation Make BTreeMap::new and BTreeSet::new const #71839
• Stabilization PR

[a1phyr]

Can the following functions be added to the top statement ?

• BTreeMap::len
• BTreeMap::is_empty
• BTreeSet::len
• BTreeSet::is_empty

[tvladyslav]

@jonas-schievink @JohnTitor , could you please start FCP?
I'm about to add closing PR. There were some tests added in #71839 and #78581 but now they are gone. Should I add more tests?

[Noratrieb]

What's the state of this?

[JohnTitor]

cc @rust-lang/libs-api --^

[Amanieu]

I'm 100% in favor of making BTreeMap::new const, but len/is_empty seem a bit much given that we don't even have those as const on Vec.

So I'm starting an FCP just for constifying the new function.

@rfcbot fcp merge