add platform docs

Author: Tropix126

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

@@ -159,6 +159,7 @@ target | std | notes
 `armv7-unknown-linux-musleabihf` | ✓ | Armv7-A Linux with musl 1.2.3, hardfloat
 [`armv7-unknown-linux-ohos`](platform-support/openharmony.md) | ✓ | Armv7-A OpenHarmony
 [`armv7a-none-eabi`](platform-support/arm-none-eabi.md) | * | Bare Armv7-A
+[`armv7a-vex-v5`](platform-support/armv7a-vex-v5.md) | * | Armv7-A Cortex-A9 VEX V5 Brain, vexOS
 [`armv7r-none-eabi`](platform-support/armv7r-none-eabi.md) | * | Bare Armv7-R
 [`armv7r-none-eabihf`](platform-support/armv7r-none-eabi.md) | * | Bare Armv7-R, hardfloat
 `i586-pc-windows-msvc` | * | 32-bit Windows w/o SSE [^x86_32-floats-x87]

src/doc/rustc/src/platform-support/armv7a-vex-v5.md

@@ -0,0 +1,91 @@
+# `armv7a-vex-v5`
+
+**Tier: 3**
+
+
+Allows compiling user programs for the [VEX V5 Brain](https://www.vexrobotics.com/276-4810.html), a microcontroller for educational and competitive robotics.
+
+Rust support for this target is not affiliated with VEX Robotics or IFI, and is not derived from nor used with any official VEX SDK.
+
+## Target maintainers
+
+This target is maintained by members of the [vexide](https://github.com/vexide) organization:
+
+- [@Tropix126](https://github.com/Tropix126)
+- [@Gavin-Niederman](https://github.com/Gavin-Niederman)
+- [@max-niederman](https://github.com/max-niederman)
+- [@doinkythederp](https://github.com/doinkythederp)
+
+## Requirements
+
+This target is cross-compiled. Dynamic linking is unsupported.
+
+`#![no_std]` crates can be built using `build-std` to build `core` and optionally
+`alloc`. Unwinding panics are not yet supported.
+
+`std` is partially implemented, but many modules (such as `thread`, `process`, `net`, etc...) will return errors. An allocator is provided along with partial support for the `time`, `env` and `io` modules. Filesystem operations over SDCard through `std::fs` are partially supported within the restrictions of the user enviornment (e.g. directories cannot be created, filesystem objects cannot be removed).
+
+This target generates binaries in the ELF format that may uploaded to the brain with external tools.
+
+## Building the target
+
+Rust does not ship pre-compiled artifacts for this target. You can use `build-std` feature to build ELF binaries with `std` support.
+
+`.cargo/config.toml`:
+
+```toml
+[build]
+target = "armv7a-vex-v5"
+
+[unstable]
+build-std = ["std", "panic_abort"]
+build-std-features = ["compiler-builtins-mem"]
+```
+
+## Building Rust programs
+
+The recommended way to build artifacts that can be installed and run on V5 Brain is by using the [cargo-v5](https://github.com/vexide/cargo-v5) tool. This tool wraps the `cargo build` command by supplying arguments necessary to build the target, while also providing uploading functionality.
+
+To install the tool run:
+
+```sh
+cargo install cargo-v5
+```
+
+The following fields in your project's `Cargo.toml` are read by `cargo-v5` to configure upload behavior:
+
+```toml
+[package.metadata.v5]
+# Slot number to upload the user program to. This should be from 1-8.
+slot = 1
+# Program icon/thumbnail that will be displayed on the dashboard.
+icon = "cool-x"
+# Use gzip compression when uploading binaries.
+compress = true
+```
+
+To build an uploadable BIN file using the release profile, run:
+
+```sh
+cargo v5 build --release
+```
+
+Programs can also be directly uploaded to the brain over a USB connection after building:
+
+```sh
+cargo v5 upload --release
+```
+
+## Testing
+
+Binaries built for this target can be run in an emulator (such as [vex-v5-qemu](https://github.com/vexide/vex-v5-qemu)), or uploaded to a device over a USB or bluetooth connection.
+
+The default Rust test runner is not supported.
+
+The Rust test suite for `library/std` is not yet supported.
+
+## Cross-compilation toolchains and C code
+
+This target can be cross-compiled from any host.
+
+This target does not link to C libraries. OS calls are implemented in rust through the [vex-sdk](https://github.com/vexide/vex-sdk) crate. No `libc` or crt0 implementation is present on this target.