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

Calculating dependencies ... done!
Dependency resolution took 1.75 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  NS    ] llvm-core/llvm-21.1.8:21/21.1::gentoo [22.1.1:22/22.1::gentoo] USE="binutils-plugin libffi zstd -debug (-debuginfod) -doc -exegesis -libedit -test -verify-sig -xml -z3" LLVM_TARGETS="(AArch64) (AMDGPU) (ARM) (AVR) (BPF) (Hexagon) (Lanai) (LoongArch) (MSP430) (Mips) (NVPTX) (PowerPC) (RISCV) (SPIRV) (Sparc) (SystemZ) (VE) (WebAssembly) (X86) (XCore) -ARC -CSKY -DirectX -M68k -Xtensa" 155530 KiB
[ebuild  NS    ] llvm-core/llvm-toolchain-symlinks-21:21::gentoo [22:22::gentoo] USE="-multilib-symlinks -native-symlinks" 0 KiB
[ebuild  N     ] dev-lang/spidermonkey-140.17.0:140/140.11.0::gentoo  USE="jit -clang -debug -test" LLVM_SLOT="21 -20" 622715 KiB

Total: 5 packages (3 new, 2 in new slots), Size of downloads: 785682 KiB