platform-support: add riscv32imac-unknown-xous-elf

Signed-off-by: Sean Cross <sean@xobs.io>

src/doc/rustc/src/SUMMARY.md

@@ -22,6 +22,7 @@
     - [m68k-unknown-linux-gnu](platform-support/m68k-unknown-linux-gnu.md)
     - [mips64-openwrt-linux-musl](platform-support/mips64-openwrt-linux-musl.md)
     - [nvptx64-nvidia-cuda](platform-support/nvptx64-nvidia-cuda.md)
+    - [riscv32imac-unknown-xous-elf](platform-support/riscv32imac-unknown-xous-elf.md)
     - [*-pc-windows-gnullvm](platform-support/pc-windows-gnullvm.md)
     - [*-unknown-openbsd](platform-support/openbsd.md)
     - [wasm64-unknown-unknown](platform-support/wasm64-unknown-unknown.md)

src/doc/rustc/src/platform-support.md

@@ -275,6 +275,7 @@ target | std | host | notes
 `riscv32gc-unknown-linux-gnu` |   |   | RISC-V Linux (kernel 5.4, glibc 2.33)
 `riscv32gc-unknown-linux-musl` |   |   | RISC-V Linux (kernel 5.4, musl + RISCV32 support patches)
 `riscv32im-unknown-none-elf` | * |  | Bare RISC-V (RV32IM ISA)
+[`riscv32imac-unknown-xous-elf`](platform-support/riscv32imac-unknown-xous-elf.md) | ? |  | RISC-V Xous (RV32IMAC ISA)
 `riscv32imc-esp-espidf` | ✓ |  | RISC-V ESP-IDF
 `riscv64gc-unknown-freebsd` |   |   | RISC-V FreeBSD
 `riscv64gc-unknown-linux-musl` |   |   | RISC-V Linux (kernel 4.20, musl 1.2.0)

src/doc/rustc/src/platform-support/riscv32imac-unknown-xous-elf.md

@@ -0,0 +1,50 @@
+# riscv32imac-unknown-xous-elf
+
+**Tier: 3**
+
+Xous microkernel, message-based operating system that powers devices such as Precursor and Betrusted. The operating system is written entirely in Rust, so no additional software is required to compile programs for Xous.
+
+## Target maintainers
+
+- [@xobs](https://github.com/xobs)
+
+## Requirements
+
+
+Building the target itself requires a RISC-V compiler that is supported by `cc-rs`. For example, you can use the prebuilt [xPack](https://github.com/xpack-dev-tools/riscv-none-elf-gcc-xpack/releases/latest) toolchain.
+
+Cross-compiling programs does not require any additional software beyond the toolchain. Prebuilt versions of the toolchain are available [from Betrusted](https://github.com/betrusted-io/rust/releases).
+
+## Building the target
+
+The target can be built by enabling it for a `rustc` build.
+
+```toml
+[build]
+target = ["riscv32imac-unknown-xous-elf"]
+```
+
+Make sure your C compiler is included in `$PATH`, then add it to the `config.toml`:
+
+```toml
+[target.riscv32imac-unknown-xous-elf]
+cc = "riscv-none-elf-gcc"
+ar = "riscv-none-elf-ar"
+```
+
+## Building Rust programs
+
+Rust does not yet ship pre-compiled artifacts for this target. To compile for
+this target, you will need to do one of the following:
+
+* Build Rust with the target enabled (see "Building the target" above)
+* Build your own copy of `core` by using `build-std` or similar
+* Download a prebuilt toolchain [from Betrusted](https://github.com/betrusted-io/rust/releases)
+
+## Cross-compilation
+
+This target can be cross-compiled from any host.
+
+## Testing
+
+Currently there is no support to run the rustc test suite for this target.