concurrency: update shared resource examples

[geomatsi]

Hi all,

This PR suggests the following updates for the section on concurrency:

1. Use lazy_static macro to declare shared resources
   Suggested example of shared resource declaration does not compile
   neither on stable (edition 2018) nor on nightly with the following error:
   "error[E0015]: calls in statics are limited to constant functions, tuple structs and tuple variants"
2. Add example of mutable references to shared resources

Regards,
Sergey

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @jamesmunns (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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

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[adamgreig]

Thanks for the PR!

The examples of using borrow_mut() and ref_mut() are helpful, but perhaps they could be simplified to not use ITM at all. I think keeping the examples as simple as possible helps make them easier to understand.

I'm not a fan of adding the lazy_static dependency; it's not a standard part of Rust, and it adds overhead without really solving the issue, so I don't think it's a good fit for this sort of problem. The compiler error you're getting is because we need the const-fn feature of the cortex-m crate, which then exposes Mutex::new() as a const fn, allowing it to be called in this context on stable Rust. Ideally we'd make this the default on cortex-m but I guess that might require bumping the MSRV.

[geomatsi]

Hi @adamgreen
Thanks for review ! Ok, I will remove lazy_static part and simplify mutable shared resource example as suggested.

By the way, could you please clarify how to fix the build for unmodified examples ? Switching to nightly does not help. Is it necessary to explicitly enable const-fn at the moment ?

Regards,
Sergey

[geomatsi]

BTW, dropping lazy_static means dropping the first commit of this PR. Is it ok if I force-push to this PR ?

[adamgreig]

By the way, could you please clarify how to fix the build for unmodified examples ? Switching to nightly does not help. Is it necessary to explicitly enable const-fn at the moment ?

Yep, you need to add the const-fn feature to your dependency on cortex-m in Cargo.toml:

cortex-m = {version="0.6.0", features=["const-fn"]}

Ideally we'd fix this with a new cortex-m version but it might be helpful to mention it in the book for now...

BTW, dropping lazy_static means dropping the first commit of this PR. Is it ok if I force-push to this PR ?

Sure, no problem.

[andre-richter]

Agree on dropping the lazy_static part. 👍

There's too much magic happening behind closed curtains (IIRC it has a mutex dependency itself, has a once() function, and so on) that prevent user from really understanding whats going on, but this should be the goal of this book.

[geomatsi]

Hi all,
All the requested changes have been introduced:

• remove lazy_static dependency
• simplify 'mutable shared resource' example: keep only TIM2, no need for ITM
• add note regarding const-fn feature for cortex-m crate

Regards,
Sergey

[geomatsi]

Hi @andre-richter

Thanks a lot for your review proof-reading! I applied all the suggested changes. However I did it using github web UI adding 8 more commits. Let me know if that was a bad idea. Then I will collect them into a single commit and force-push PR.

Regards,
Sergey

[andre-richter]

We should be able to squash it before merge, no worries.

Clicking the right buttons would have allowed you to accept all suggestions at one, I think you need to change to review view or something. I don't remember right now.

I'll leave it to @adamgreig to make the final call.

[andre-richter]

@geomatsi Apparently, contrary to my statement, bors cannot squash yet.
So if your offer to squash and force push still holds, kindly do so :)

[geomatsi]

Hi @andre-richter

Sure, will do so later today.

Regards,
Sergey

[geomatsi]

Hi @andre-richter , @adamgreig and all,

All fixups have been merged into a single commit. Bors looks happy.

Regards,
Sergey

[andre-richter]

bors r+

Thank you for your contribution 👍

[bors]

👎 Rejected by too few approved reviews

[andre-richter]

bors r+

[bors]

Build succeeded

• continuous-integration/travis-ci/push