These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.69 s (backtrack: 0/20).

[ebuild  NS    ] dev-build/autoconf-2.13-r9:2.1::gentoo [2.72-r6:2.72::gentoo] USE="-test% (-verify-sig%)" 434 KiB
[ebuild  N     ] dev-libs/nspr-4.40::gentoo  USE="-debug" 986 KiB
[ebuild  N     ] dev-util/cbindgen-0.29.4::gentoo  USE="-debug -test" 6453 KiB
[ebuild  N     ] dev-lang/spidermonkey-128.14.0:128::gentoo  USE="jit -clang -debug -test" LLVM_SLOT="21 -17 -18 -19 -20" 549548 KiB

Total: 4 packages (3 new, 1 in new slot), Size of downloads: 557419 KiB