> Rust code that type-puns an unaligned pointer into an integer would crash on MIPS or SPARC just like the C code would.

Actually you can enforce that on Intel too. I do that for some hash functions in debugging mode, to avoid valgrind slowdown.

e.g. https://github.com/perl11/cperl/blob/master/cpan/Digest-MD5/...

    #if defined(U32_ALIGNMENT_REQUIRED) && defined(__GNUC__) && (defined(__x86_64__) || defined(__i386))
        /* Generate SIGBUS on unaligned access even on x86:
           Set AC in EFLAGS. See http://orchistro.tistory.com/206
           Also see https://sourceforge.net/p/predef/wiki/Architectures/
           for possible other compilers. Here only GNU C: gcc, clang, icc.
           MSVC would be nice also. */
    #ifdef __x86_64__
        __asm__("pushf\n"
                "orl $0x40000, (%rsp)\n"
                "popf");
    #else
        __asm__("pushf\n"
                "orl $0x40000, (%esp)\n"
                "popf");
    #endif
    #endif

This way you won't get the SPARC/MIPS surprises debian maintainers are struggling with. If it doesn't align, copy it temp. It's still faster.