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

Calculating dependencies ... done!
Dependency resolution took 1.14 s (backtrack: 0/20).

[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-140.11.0:140/140.11.0::gentoo  USE="jit -clang -debug -test" LLVM_SLOT="21 -20" 622171 KiB

Total: 3 packages (3 new), Size of downloads: 629609 KiB