Mark a function only used in nightly as nightly only

[Hoverbear]

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:

rust/compiler/rustc_abi/src/callconv.rs

Line 52 in fda6d37

#[cfg(feature = "nightly")]

rust/compiler/rustc_abi/src/callconv.rs

Line 131 in fda6d37

result = result.merge(field.homogeneous_aggregate(cx)?)?;

rust/compiler/rustc_abi/src/callconv.rs

Line 167 in fda6d37

result = result.merge(variant_result)?;

This does the flag off.

Test on main and this branch:

./x.py test rustc_next_trait_solver

[rustbot]

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