Not all processors of a certain architecture will have SIMD processing units, and using a SIMD instruction which is not supported will trigger undefined behavior.
To allow building safe, portable programs, the Rust compiler will not, by default,
generate any sort of vector instructions, unless it can statically determine
they are supported. For example, on AMD64, SSE2 support is architecturally guaranteed.
x86_64-apple-darwin target enables up to SSSE3. The get a defintive list of
which features are enabled by default on various platforms, refer to the target
specifications in the compiler's source code.