Remove unsafety from sys/unsupported and add deny(unsafe_op_in_unsafe_fn).

[m-ou-se]

Replacing UnsafeCells by a Cells simplifies things and makes the mutex and rwlock implementations safe. Other than that, only unsafety in strlen() contained unsafe code.

@rustbot modify labels: +F-unsafe-block-in-unsafe-fn +C-cleanup

[rust-highfive]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

Could you add a comment on both introduced Cells that they're for platforms without threads so there's no need for synchronization? The UnsafeCell would've wanted that comment too, I feel.

It seems a bit unfortunate that we implement Sync for the Mutex/RwLock in sys/unsupported -- do we really need to do that? It's obviously untrue... it seems like maybe ideally Rust would provide a cfg(no_threads) or whatever for us.

[m-ou-se]

Sure. Will add a comment.

It's obviously untrue...

Well they're safe to share between all the threads your program can have on those platforms: just the one main thread. ^^'

Without Sync, you couldn't use one in a static like on other platforms.

[m-ou-se]

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

[Mark-Simulacrum]

Yes, I mean that it would be better if (for example) the compiler removed Sync bounds on statics, for example, on single-threaded platforms. But that would require lang approval and design :)

@bors r+

[bors]

📌 Commit b26aa5d has been approved by Mark-Simulacrum

[JohnTitor]

Failed in https://github.com/rust-lang-ci/rust/runs/1248784774:

warning: dropping unsupported crate type `dylib` for target `wasm32-unknown-unknown`

error: unnecessary `unsafe` block
  --> library/std/src/sys/wasm/../unsupported/common.rs:43:5
   |
43 |     unsafe {
   |     ^^^^^^ unnecessary `unsafe` block
   |
   = note: `-D unused-unsafe` implied by `-D warnings`

error: aborting due to previous error; 1 warning emitted

[RUSTC-TIMING] std test:false 2.295
error: could not compile `std`

@bors r-

[m-ou-se]

Ah, apparently sys/wasm takes some stuff directly from sys/unsupported. Then sys/unsupported/mod.rs gets bypassed and the deny(unsafe_op_in_unsafe_fn) attribute does not apply.

[m-ou-se]

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

Checked locally that std now compiles for wasm32-unknown-unknown. I only applied the attribute to the one mondule where it was necessary. Looks like sys/wasm will be applying the attribute to everything soon as well: #74477

[jyn514]

@bors r=Mark-Simulacrum rollup=iffy

[bors]

📌 Commit af414dc has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit af414dc with merge 772eaeb38d5203c0efb95201b52b2bbaa780c734...

[bors]

💔 Test failed - checks-actions

[JohnTitor]

@bors retry
Seems the failure is due to GitHub outage (it's resolved now): https://www.githubstatus.com/incidents/nq2s4gt7xs1w

[m-ou-se]

Ah, thanks! Was just about to post a comment because I didn't understand what failed.

By the way, looks like all the relevant platforms for this PR passed their tests, so rollup should be fine now. ^^