Add target thumbv7a-uwp-windows-msvc

[bdbai]

Add target spec for thumbv7a-uwp-windows-msvc, so that libraries written in Rust will have a chance to run on ARM-based devices with Windows 10.

So far I managed to create a proof-of-concept library for Universal Windows Platform apps to consume and it worked on a Windows Phone. However, building a standalone executable seemed troublesome due to LLVM ERROR: target does not implement codeview register mapping stuff (see also #52659 (comment) ).

Steps to test:

1. Clone and build this version

git clone https://github.com/bdbai/rust.git
cd rust
python x.py build -i --target thumbv7a-uwp-windows-msvc --stage 1 src/libstd
rustup toolchain link arm-uwp-stage1 .\build\x86_64-pc-windows-msvc\stage1\

2. Create a new library crate

cargo new --lib arm-uwp-test
cd arm-uwp-test

3. Change crate-type in Cargo.toml to staticlib

[lib]
crate-type=["staticlib"]

4. Replace the following code in src/lib.rs

#[no_mangle]
pub extern "system" fn call_rust() -> i32 {
    2333
}

5. Build the crate

cargo +arm-uwp-stage1 build -v --target thumbv7a-uwp-windows-msvc

6. arm-uwp-test.lib should appear in target\thumbv7a-uwp-windows-msvc\debug

To consume this library:

1. Make sure Visual Studio 2017 and Windows 10 SDK (10.0.17134 or above) are installed
2. Create a new Blank App (C++/WinRT) in Visual Studio 2017 (Visual Studio 2019 cannot deploy UWP apps to Windows Phone)
3. Go to Property Pages, and then Linker->Input->Additional Dependencies, add arm-uwp-test.lib produced just now
4. Manually declare function prototypes in MainPage.h

extern "C" {
    int call_rust();
}

5. Replace the ClickHandler part in MainPage.cpp

myButton().Content(box_value(std::to_wstring(call_rust())));

6. Build and deploy this app to an ARM device running Windows 10. The app should run and show 2333 when the button is clicked.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @estebank (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[crlf0710]

Doesn't really know who this should be assigned... I'll randomly reassign. Also cc. @retep998 here.

r? @hanna-kruppe

[retep998]

I don't have an ARM device with Windows so I am unable to test this change. The target spec looks fine otherwise.

[hanna-kruppe]

I can rubber-stamp this on behalf of @retep998 if they don't have r+ permissions, but I have neither the expertise nor (currently) the time to properly review this.

[Dylan-DPC-zz]

r? @Mark-Simulacrum

[Mark-Simulacrum]

I am also unfortunately not the right person for this. I'm going to say r? @joshtriplett as they've done some work in the targets area. I'm inclined to just r+ per @retep998's comment, though.

[joshtriplett]

This looks reasonable to me, and based on the review provided by others already:

@bors r+

[bors]

📌 Commit 5bfb7e7 has been approved by joshtriplett