Rollup merge of #145973 - vexide:vex-std, r=tgross35

Add `std` support for `armv7a-vex-v5`

This PR adds standard library support for the VEX V5 Brain (`armv7a-vex-v5` target). It is more-or-less an updated version of the library-side work done in rust-lang/rust#131530.

This was a joint effort between me, `@lewisfm,` `@max-niederman,` `@Gavin-Niederman` and several other members of the [`vexide` project](https://github.com/vexide/).

## Background

VEXos is a fairly unconventional operating system, with user code running in a restricted enviornment with regards to I/O capabilities and whatnot. As such, several OS-dependent APIs are unsupported or have partial support (such as `std::net`, `std::process`, and most of `std::thread`). A more comprehensive list of what does or doesn't work is outlined in the [updated target documentation](https://github.com/vexide/rust/blob/vex-std/src/doc/rustc/src/platform-support/armv7a-vex-v5.md). Despite these limitations, we believe that `libstd` support on this target still has value to users, especially given the popular use of this hardware for educational purposes. For some previous discussion on this matter, see [this comment](https://github.com/rust-lang/rust/pull/131530#issuecomment-2432856841).

## SDK Linkage

VEXos doesn't really ship with an official `libc` or POSIX-style platform API (and though it does port newlib, these are stubbed on top of the underlying SDK). Instead, VEX provides their own SDK for calling platform APIs. Their official SDK is kept proprietary (with public headers), though open-source implementations exist. Following the precedent of the `armv6k-nintendo-3ds` team's work in rust-lang/rust#95897, we've opted not to directly link `libstd` to any SDK with the expectation that users will provide their own with one of the following options:
-  [`vex-sdk-download`](https://github.com/vexide/vex-sdk/tree/main/packages/vex-sdk-download), which downloads an official proprietary SDK from VEX using a build script.
- [`vex-sdk-jumptable`](https://crates.io/crates/vex-sdk-jumptable), which is a compatible, open-source reimplementation of the SDK using firmware jumps.
- [`vex-sdk-pros`](https://github.com/vexide/vex-sdk/tree/main/packages/vex-sdk-pros), which uses the [PROS kernel](https://github.com/purduesigbots/pros) as a provider for SDK functions.
- Linking their own implementation or stubbing the functions required by libstd.

 The `vex-sdk` crate used in the VEXos PAL provides `libc`-style FFI bindings for any compatible system library, so any of these options *should* work fine. A functional demo project using `vex-sdk-download` can be found [here](https://github.com/vexide/armv7a-vex-v5-demo/tree/main).

## Future Work

This PR implements virtually everything we are currently able to implement given the current capabilities of the platform. The exception to this is file directory enumeration, though the implementation of that is sufficiently [gross enough](https://github.com/vexide/vexide/blob/c6c5bad11e035cf4e51d429dca7e427210185ed4/packages/vexide-core/src/fs/mod.rs#L987) to drive us away from supporting this officially.

Additionally, I have a working branch implementing the `panic_unwind` runtime for this target, which is something that would be nice to see in the future, though given the volume of compiler changes i've deemed it out-of-scope for this PR.

2 files changed, 19 insertions, 8 deletions

diff --git a/src/doc/rustc/src/platform-support/armv7a-vex-v5.md b/src/doc/rustc/src/platform-support/armv7a-vex-v5.md
index a7da1b16f7e..3677f8931dd 100644
--- a/src/doc/rustc/src/platform-support/armv7a-vex-v5.md
+++ b/src/doc/rustc/src/platform-support/armv7a-vex-v5.md
@@ -4,7 +4,7 @@
 
 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.
+Rust support for this target is not affiliated with VEX Robotics or IFI, and does not link to any official VEX SDK.
 
 ## Target maintainers
 
@@ -17,11 +17,24 @@ This target is maintained by members of the [vexide](https://github.com/vexide)
 
 ## Requirements
 
-This target is cross-compiled and currently requires `#![no_std]`. Dynamic linking is unsupported.
+This target is cross-compiled. Dynamic linking is unsupported.
 
-When compiling for this target, the "C" calling convention maps to AAPCS with VFP registers (hard float ABI) and the "system" calling convention maps to AAPCS without VFP registers (soft float ABI).
+`#![no_std]` crates can be built using `build-std` to build `core` and `panic_abort` and optionally `alloc`. Unwinding panics are not yet supported on this target.
 
-This target generates binaries in the ELF format that may uploaded to the brain with external tools.
+`std` has only partial support due platform limitations. Notably:
+- `std::process` and `std::net` are unimplemented. `std::thread` only supports sleeping and yielding, as this is a single-threaded environment.
+- `std::time` has full support for `Instant`, but no support for `SystemTime`.
+- `std::io` has full support for `stdin`/`stdout`/`stderr`. `stdout` and `stderr` both write to to USB channel 1 on this platform and are not differentiated.
+- `std::fs` has limited support for reading or writing to files. Directory operations, file deletion, and some file opening features are unsupported and will return errors.
+- A global allocator implemented on top of `dlmalloc` is provided.
+- Modules that do not need to interact with the OS beyond allocation such as `std::collections`, `std::hash`, `std::future`, `std::sync`, etc are fully supported.
+- Random number generation and hashing is insecure, as there is no reliable source of entropy on this platform.
+
+In order to support some APIs, users are expected to provide a supporting runtime SDK for `libstd` to link against. This library may be provided either by [`vex-sdk-build`](https://github.com/vexide/vex-sdk/tree/main/packages/vex-sdk-build) (which will download an official SDK from VEX) or through an open-source implementation such as [`vex-sdk-jumptable`](https://crates.io/crates/vex-sdk-jumptable).
+
+When compiling for this target, the "C" calling convention maps to AAPCS with VFP registers (hard float ABI) and the "system" calling convention maps to AAPCS without VFP registers (softfp ABI).
+
+This target generates binaries in the ELF format that may be uploaded to the brain with external tools.
 
 ## Building the target
 
@@ -29,10 +42,7 @@ You can build Rust with support for this target by adding it to the `target` lis
 
 ## Building Rust programs
 
-Rust does not yet ship pre-compiled artifacts for this target. To compile for
-this target, you will either need to build Rust with the target enabled (see
-"Building the target" above), or build your own copy of `core` by using
-`build-std` or similar.
+Rust does not yet ship pre-compiled artifacts for this target. To compile for this target, you will either need to build Rust with the target enabled (see "Building the target" above), or build your own copy of `core` by using `build-std` or similar.
 
 When the compiler builds a binary, an ELF build artifact will be produced. Additional tools are required for this artifact to be recognizable to VEXos as a user program.
 
diff --git a/src/tools/tidy/src/deps.rs b/src/tools/tidy/src/deps.rs
index 247080102fb..c76b46ec2bf 100644
--- a/src/tools/tidy/src/deps.rs
+++ b/src/tools/tidy/src/deps.rs
@@ -578,6 +578,7 @@ const PERMITTED_STDLIB_DEPENDENCIES: &[&str] = &[
     "rustc-literal-escaper",
     "shlex",
     "unwinding",
+    "vex-sdk",
     "wasi",
     "windows-sys",
     "windows-targets",