Making and dropping an empty `BTreeMap` is not free.

[ruriww]

I tried this code:
https://godbolt.org/z/v594PEsr7

I expected to see this happen:

ret

Instead, this happened:

example[7a9efb3d937417e2]::drop_btree:
        push    r14
        push    rbx
        sub     rsp, 104
        mov     qword ptr [rsp + 32], 0
        mov     qword ptr [rsp + 64], 0
        mov     qword ptr [rsp + 96], 0
        lea     rdi, [rsp + 8]
        lea     rsi, [rsp + 32]
        call    <alloc[c00562a4ff4db933]::collections::btree::map::IntoIter<(), ()>>::dying_next
        cmp     qword ptr [rsp + 8], 0
        je      .LBB0_3
        lea     rbx, [rsp + 8]
        lea     r14, [rsp + 32]
.LBB0_2:
        mov     rdi, rbx
        mov     rsi, r14
        call    <alloc[c00562a4ff4db933]::collections::btree::map::IntoIter<(), ()>>::dying_next
        cmp     qword ptr [rsp + 8], 0
        jne     .LBB0_2
.LBB0_3:
        add     rsp, 104
        pop     rbx
        pop     r14
        ret

[jakedrew]

@rustbot claim

[jakedrew]

Regression introduced in 7b28036 in 1.56.0.
Testing same code in 1.55.0 produces the expected result: https://godbolt.org/z/Y5bnKe7jW

[bardiharborow]

@rustbot label +S-has-bisection