> toolchains

Are what cargo, rustc, etc. are expected to run on. You probably meant target.

> i686-unknown-none

Is admittedly a missing target. `x86_64-unknown-none` specifies stuff like `extern "C"`'s ABI (per https://doc.rust-lang.org/rustc/platform-support/x86_64-unkn... ) which is a lot less universal/appropriate for i686, where AFAIK everyone chooses their own different incompatible ABIs - which might be the reason it's not provided? Usually you want to pick an i686-unknown-* target that aligns more closely with your own needs (e.g. your desired object/library/binary file format, abi, bootloader, ...?)

    C:\local>rustup target list | findstr i686
    i686-linux-android
    i686-pc-windows-gnu
    i686-pc-windows-gnullvm
    i686-pc-windows-msvc (installed)
    i686-unknown-freebsd
    i686-unknown-linux-gnu
    i686-unknown-linux-musl
    i686-unknown-uefi

If, truly, none of them are appropriate for your needs, that's when it's time to use a custom target (per https://doc.rust-lang.org/rustc/targets/custom.html ) and `build-std` (per https://doc.rust-lang.org/cargo/reference/unstable.html#buil... .) Using a toolchain file to pin your nightly rustc version might be appropriate (per https://rust-lang.github.io/rustup/overrides.html#the-toolch... .)

The last time I played with custom targets was on https://github.com/MaulingMonkey/rust-opendingux-test/tree/m... , using the old `xargo` instead of `build-std`. Notes.md details modifications made to make things work.