A little-known C feature: Static array indices in parameter declarations (2013)
Seems cool, but how is useful in practice? In 15 years I've never written a C function taking a fixed array size at all. I always write my functions with a dynamic size argument like    void foo(char *data, int len)

Perhaps cryptography functions with 256-bit outputs might be able to use this, but even when the array size is fixed, most crypto libraries (like OpenSSL) tend to have a size argument anyway, to encourage the caller to think about what they're doing, and for the library to check that 32 bytes are actually available, for example.