it would, just without standard library support, as the default stdlib imports system APIs that older versions don't provide. Other than those and a "supported OS version" filed in the PE header there isn't really much else in your way (hence my Rust9x project)

likely no_std, and running via Win32s. Rust doesn't have support for segmented memory, so native 16-bit would be much harder

With Windows 3.1, you're probably talking about having to compile to the Win16 interface, which would be a bit of a challenge.

LLVM's x86 backend doesn't really support 16-bit compilation (the best it gets to is supporting 32-bit in a 16-bit code segment). There's also no support for the Win16-based object file formats. So you'd probably be best off creating your own entirely new backend for a Win16 environment... which probably is easier than you'd think, given that you could ignore anything more complicated than the actual Intel 386 or 486 ISA. (I've actually started sketching out what the IR for such a backend would look like, mostly in terms of LLVM address space mappings).

Win16 is also kind of hard to target for Rust because it's a segmented memory model, and Rust's pointer model doesn't really cope with that well--it only supports one pointer type, and also requires that sizeof(ptr) == sizeof(usize) which is questionable for far pointers. If you're writing your own backend, you could just map everything to huge pointers at the programmer level and have optimization passes that convert them down to far or near pointers if sufficient.

Or you could stuff all of that, and just make a 32-bit app and wrap it in a shell loader that thunks all of the OS calls to 16-bit calls and vice-versa.

Blunks.

Rust still supports Intel Macs just fine. The x86_64-apple-darwin target was recently downgraded to tier 2, which just means that some automated tests will no longer run as part of the standard dev cycle.

MSVC appears to have dropped support for Windows 3.1 in 2003 and Windows 9x in 2008. It might still be able to generate support for Windows XP and Vista if you scrounge up an appropriate SDK, but the default CRT it builds with sure doesn't support those platforms.