Use critical-section 1.0 for locking.

[Dirbaio]

This allows having no platform-specific code in rtt-target at all,
as long as the user imports the right critical-section implementation.

This is a breaking change.

TODO before merging:

• Wait for critical-section 1.0 release v1.0.0 release rust-embedded/critical-section#19
• Wait for cortex-m v0.7.6 release Add implementation for critical-section 1.0, for cortex-m v0.7.x rust-embedded/cortex-m#448

[Dirbaio]

cortex-m 0.7.6 is released with critical-section support, i've removed the [patch.crates.io]. I think this should be ready to go :)

[jannic]

Note that this is a semver breaking change, as it removes cargo features.
EDIT: Oops you already noted it in the description - I somehow missed it and only noticed it when some build failed.

[mvirkkunen]

Neat, another annoying cortex-m feature bites the dust!

Breaking semver is fine because I've wanted to clean things up for a long time anyways, for example steal defmt's idea of initializing everything before main. With critical-section I can make that happen more easily. I'll see if I can make that happen within a weekend or two, and if I can't figure out a nice way, I'll release just this change as-is as a new version because it's a good addition in and of itself.

[mvirkkunen]

Oh, the "gotta use the crate or it gets pruned" problem is back again. The example binaries fail to link without a use cortex_m;. This has happened with other crates too, in that even if your own code doesn't actually use the crate, it still needs to be use'd. The examples could be a bit artificial in that they don't actually need anything from cortex-m though. I'll fix them before merging.

Just out of interest, do you know if there is a RISC-V implementation of critical-section? I think some people were using RTT on that too.

[jannic]

Just out of interest, do you know if there is a RISC-V implementation of critical-section? I think some people were using RTT on that too.

In versions <1.0.0, some implementations were part of the critical-section crate, and one of them was for riscv.
With 1.0.0, the implementations were removed and should now be provided by platform-support crates. However, it seems like this was not done yet. Should not be difficult to add, at least for single-core, where disabling interrupts should be sufficient.

[Dirbaio]

PR for riscv critical-section at rust-embedded/riscv#110