Fix std compilation error for wasi+atomics

[tomaka]

Fix #109727

It seems that the unsupported/once.rs module isn't meant to exist at the same time as the futex module, as they have conflicting definitions.

I've solved this by defining the once module only if not(target_feature = "atomics").
The wasm32-unknown-unknown target similarly only defines the once module if not(target_feature = "atomics").

As show in this block of code, the sys::once module doesn't need to exist if all(target_arch = "wasm32", target_feature = "atomics").

[rustbot]

r? @joshtriplett

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[tomaka]

As a heads up, I'll probably open another PR afterwards to make wasm32-wasi work properly with atomics on. I'm still trying to figure out how to try out my locally-built std.

[jyn514]

@tomaka right now you need to also build the compiler to use a local std in a dev toolchain, see #101691. you can run x build std --stage 1 and that should work (follow the instructions in https://rustc-dev-guide.rust-lang.org/building/suggested.html#configuring-rustup-to-use-nightly). you can also set download-rustc = true to avoid having to build the whole compiler and that should work if you pass --stage 2 instead of 1.

[tomaka]

I'll probably open another PR afterwards to make wasm32-wasi work properly with atomics on

Or maybe not.
Thread-safe wasi seems to be a ton of history, and it's a bit too overwhelming for me to figure out what the situation is.
Since there seems to be some movement, I'll just wait.

Nonetheless, this small PR is still worth merging, since it moves the situation from "doesn't compile" to "compiles".

[jyn514]

How can I test this change? RUSTFLAGS_BOOTSTRAP="-C target-features=+atomics,+bulk-memory" x build std --target=wasm32-wasi tells me I'm missing a wasi-root key:

thread 'main' panicked at 'Target "wasm32-wasi" does not have a "wasi-root" key', compile.rs:276:17

[jyn514]

ah, using check instead worked:

; RUSTFLAGS_BOOTSTRAP="-C target-feature=+atomics,+bulk-memory" x check std --target=wasm32-wasi

@bors r+ rollup

[bors]

📌 Commit 01c4f31 has been approved by jyn514

It is now in the queue for this repository.