You can indeed use intrinsics to make a SIMD library in plain safe stable rust today to some extent; that just isn't what core::simd does; rather, on the Rust-s

You can indeed use intrinsics to make a SIMD library in plain safe stable rust today to some extent; that just isn't what core::simd does; rather, on the Rust-side it's all target-agnostic and LLVM (or whatever other backend) handles deciding how to lower any given op to the target architecture. e.g. all core::simd addition ends up invoking the single function [1] which is then directly handled by rustc. But these architecture-agnostic intrinsics are unstable[2] (as they're only there as a building block for core::simd), and you can't manually use "#[rustc_intrinsic]" & co in stable rust either. [1]: https://github.com/rust-lang/rust/blob/b01cc1cf01ed12adb2595c4696321ee722ef545e/library/core/src/intrinsics/simd.rs#L65-L67 https://github.com/rust-lang/rust/blob/b01cc1cf01ed12adb2595... [2]: https://github.com/rust-lang/rust/blob/b01cc1cf01ed12adb2595c4696321ee722ef545e/library/core/src/intrinsics/mod.rs#L48-L54 https://github.com/rust-lang/rust/blob/b01cc1cf01ed12adb2595...