Rollup merge of #154481 - ferrocene:hoverbear/flag-off-unused-code, r=wesleywiser

Mark a function only used in nightly as nightly only

If you run `./x.py test rustc_next_trait_solver` you'll currently see a failure:

```
warning: method `merge` is never used
  --> compiler/rustc_abi/src/callconv.rs:38:8
   |
25 | impl HomogeneousAggregate {
   | ------------------------- method in this implementation
...
38 |     fn merge(self, other: HomogeneousAggregate) -> Result<HomogeneousAggregate, Heterogeneous> {
   |        ^^^^^
   |
   = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: `rustc_abi` (lib) generated 1 warning
```

This is because the usages are behind a nightly feature flag:

https://github.com/rust-lang/rust/blob/fda6d37bb88ee12fd50fa54d15859f1f91b74f55/compiler/rustc_abi/src/callconv.rs#L52

https://github.com/rust-lang/rust/blob/fda6d37bb88ee12fd50fa54d15859f1f91b74f55/compiler/rustc_abi/src/callconv.rs#L131

https://github.com/rust-lang/rust/blob/fda6d37bb88ee12fd50fa54d15859f1f91b74f55/compiler/rustc_abi/src/callconv.rs#L167

This does the flag off.

Test on `main` and this branch:

```
./x.py test rustc_next_trait_solver
```

compiler/rustc_abi/src/callconv.rs

@@ -35,6 +35,7 @@ pub fn unit(self) -> Option<Reg> {
     /// Try to combine two `HomogeneousAggregate`s, e.g. from two fields in
     /// the same `struct`. Only succeeds if only one of them has any data,
     /// or both units are identical.
+    #[cfg(feature = "nightly")]
     fn merge(self, other: HomogeneousAggregate) -> Result<HomogeneousAggregate, Heterogeneous> {
         match (self, other) {
             (x, HomogeneousAggregate::NoData) | (HomogeneousAggregate::NoData, x) => Ok(x),

compiler/rustc_index/src/bit_set.rs

@@ -1,6 +1,4 @@
 use std::marker::PhantomData;
-#[cfg(not(feature = "nightly"))]
-use std::mem;
 use std::ops::{Bound, Range, RangeBounds};
 use std::rc::Rc;
 use std::{fmt, iter, slice};