add "the path towards stable" announcement

newsletters/2018-05-13-the-path-towards-stable.md

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+# Cortex-M library development now possible on beta and the path towards stable embedded Rust
+
+> 2018-05-13
+
+> **TL;DR**
+>
+> * 1.27 = embedded (Cortex-M) *library* development on stable
+> * 1.28 or 1.29 = embedded (Cortex-M) *application* development on stable
+> * We are making breaking changes in the ecosystem in order to move towards development on the
+>   stable channel
+> * If you are a crate maintainer / author, make sure you test your crates with the new versions of
+>   `cortex-m` (and similar) on the current `beta` release of Rust
+> * If the latest version of a dependency doesn't compile on `beta`, file an issue and/or ping us on
+>   the #rust-embedded IRC channel
+> * Thanks for your patience!
+
+We are happy to announce that *library* development for the Cortex-M targets is now possible on the
+beta channel! :tada:
+
+For example, to cross compile a library crate for the Cortex-M3 you would run these commands:
+
+``` console
+$ # switch to the beta channel
+$ rustup default beta
+
+$ # install the rust-std component (pre-compiled core) for the target
+$ rustup target add thumbv7m-none-eabi
+
+$ # get some source to build
+$ cargo clone cortex-m --vers 0.5.0 && cd cortex-m
+
+$ # then It Just Works
+$ cargo build --target thumbv7m-none-eabi
+```
+
+What about embedded *application* development? There is just a single unstable feature that prevents
+us from building a `no_std` binary on stable: the `panic_fmt` lang item. This unstable feature will
+soon be removed in favor of the `#[panic_implementation]` feature ([implementation PR]), which is
+slated for stabilization in time for the edition release (we are hoping to stabilize it as early as
+1.28 but we'll see).
+
+[implementation PR]: https://github.com/rust-lang/rust/pull/50338
+
+## Making the Cortex-M ecosystem work on stable
+
+"`no_std` binaries are *possible* on stable" is very different from "*my* embedded application
+builds on stable". The later requires all the dependencies to also compile on stable. So we are
+starting to migrate the Cortex-M ecosystem to work on stable. All the crates listed below are now
+compiling on the beta channel.
+
+Legend: `crate-name` - link to CHANGELOG - link to stabilization PR - ~removed unstable features~
+
+- [`cortex-m-quickstart`][c0] - [CHANGELOG][ch0] - [PR][p0]
+
+[c0]: https://docs.rs/cortex-m-quickstart/~0.3
+[ch0]: https://github.com/japaric/cortex-m-quickstart/blob/master/CHANGELOG.md#v030---2018-05-12
+[p0]: https://github.com/japaric/cortex-m-quickstart/pull/29
+
+- [`cortex-m-rt`][c1] - [CHANGELOG][ch1] - [PR][p1] - ~asm!~ ~core_intrinsics~ ~global_asm!~
+  ~lang_items~ ~linkage~ ~naked_functions~ ~used~
+
+[c1]: https://crates.io/crates/cortex-m-rt/0.5.0
+[ch1]: https://github.com/japaric/cortex-m-rt/blob/master/CHANGELOG.md#v050---2018-05-12
+[p1]: https://github.com/japaric/cortex-m-rt/pull/69
+
+- [`cortex-m-semihosting`][c2] - [CHANGELOG][ch2] - [PR][p2]
+
+[c2]: https://crates.io/crates/cortex-m-semihosting/0.3.0
+[ch2]: https://github.com/japaric/cortex-m-semihosting/blob/master/CHANGELOG.md#v030---2018-05-10
+[p2]: https://github.com/japaric/cortex-m-semihosting/pull/16
+
+- [`cortex-m`][c3] - [CHANGELOG][ch3] - [PR][p3] - ~asm!~ ~const_fn~
+
+[c3]: https://crates.io/crates/cortex-m/0.5.0
+[ch3]: https://github.com/japaric/cortex-m/blob/master/CHANGELOG.md#v050---2018-05-11
+[p3]: https://github.com/japaric/cortex-m/pull/88
+
+- [`embedded-hal`][c4] - [CHANGELOG][ch4] - [PR][p4] - ~never_type~
+
+[c4]: https://crates.io/crates/embedded-hal/0.2.0
+[ch4]: https://github.com/japaric/embedded-hal/blob/master/CHANGELOG.md#v020---2018-05-12
+[p4]: https://github.com/japaric/embedded-hal/pull/80
+
+- [`f3`][c5] - [CHANGELOG][ch5] - [PR][p5]
+
+[c5]: https://crates.io/crates/f3/0.6.0
+[ch5]: https://github.com/japaric/f3/blob/master/CHANGELOG.md#v060---2018-05-12
+[p5]: https://github.com/japaric/f3/pull/93
+
+- [`l3gd20`][c6] - [CHANGELOG][ch6] - [PR][p6] - ~Unsized~
+
+[c6]: https://crates.io/crates/l3gd20/0.2.0
+[ch6]: https://github.com/japaric/l3gd20/blob/master/CHANGELOG.md#v020---2018-05-12
+[p6]: https://github.com/japaric/l3gd20/pull/4
+
+- [`lsm303dlhc`][c7] - [CHANGELOG][ch7] - [PR][p7] - ~Unsized~
+
+[c7]: https://crates.io/crates/lsm303dlhc/0.2.0
+[ch7]: https://github.com/japaric/lsm303dlhc/blob/master/CHANGELOG.md#v020---2018-05-12
+[p7]: https://github.com/japaric/lsm303dlhc/pull/4
+
+- [`stm32f103xx`][c8] - [CHANGELOG][ch8] - [PR][p8] - ~const_fn~ ~global_asm!~ ~try_from~
+  ~use_extern_macros~ ~used~
+
+[c8]: https://crates.io/crates/stm32f103xx/0.10.0
+[ch8]: https://github.com/japaric/stm32f103xx/blob/master/CHANGELOG.md#v0100---2018-05-12
+[p8]: https://github.com/japaric/stm32f103xx/pull/24
+
+- [`stm32f30x-hal`][c9] - [CHANGELOG][ch9] - [PR][p9] - ~never_type~
+
+[c9]: https://crates.io/crates/stm32f30x-hal/0.2.0
+[ch9]: https://github.com/japaric/stm32f30x-hal/blob/master/CHANGELOG.md#v020---2018-05-12
+[p9]: https://github.com/japaric/stm32f30x-hal/pull/25
+
+- [`stm32f30x`][c10] - [CHANGELOG][ch10] - [PR][p10] - ~const_fn~ ~global_asm!~ ~try_from~
+  ~use_extern_macros~ ~used~
+
+[c10]: https://crates.io/crates/stm32f30x/0.7.0
+[ch10]: https://github.com/japaric/stm32f30x/blob/master/CHANGELOG.md#v070---2018-05-12
+[p10]: https://github.com/japaric/stm32f30x/pull/16
+
+- [`svd2rust`][c11] (output of) - [CHANGELOG][ch11] - [PR][p11] - ~const_fn~ ~global_asm!~
+  ~try_from~ ~use_extern_macros~ ~used~
+
+[c11]: https://crates.io/crates/svd2rust/0.13.0
+[ch11]: https://github.com/japaric/svd2rust/blob/master/CHANGELOG.md#v0130---2018-05-12
+[p11]: https://github.com/japaric/svd2rust/pull/203
+
+There's a lot of breaking changes there but the most visible one is `cortex-m-rt` because it changes
+how applications are structured. Check the [`cortex-m-quickstart`][c0] template for instructions on
+how to set up an embedded application using the latest versions of everything.
+
+There are still a lot of crates that will need to be updated to work on beta. Among the more general
+purpose ones we have [`heapless`] and [`cortex-m-rtfm`], which needs to be update to work with
+`cortex-m-rt` v0.5.0.
+
+[`heapless`]: https://crates.io/crates/heapless
+[`cortex-m-rtfm`]: https://crates.io/crates/cortex-m-rtfm
+
+### How you can help us
+
+Try to compile your Cortex-M crate using the beta channel! If your crate is an application the build
+will certainly fail, but check if all the dependencies, excluding the [panic handler crate][phc],
+compile on beta. If any dependency doesn't compile on the beta channel let the author know, but
+first check if there's a new version of the crate that already compiles on the beta channel. And if
+you feel up to the task you could even send a PR to the dependency to make it compile on beta -- the
+list of stabilization PRs in the previous section may give you a clue on how to move the crate to
+beta.
+
+If you are the author of a Cortex-M device crate, a crate generated using `svd2rust`, moving it to
+stable only requires regenerating it using `svd2rust` v0.13.0. Do note that the generation process
+[has changed] for the Cortex-M target; also make sure you bump the minor version when you publish a
+new version.
+
+[has changed]: https://docs.rs/svd2rust/0.13.0/svd2rust/#usage
+
+If you are the author of a driver crate or a HAL implementation, please update your crate to use
+v0.2.0 of `embedded-hal` and test that it builds on beta. Make sure you bump the minor version when
+you publish a new version of your crate -- bumping the version of the `embedded-hal` dependency is a
+breaking change (drivers that depend on `embedded-hal` v0.1.0 can't be used with HAL implementation
+crates that depend on `embedded-hal` v0.2.0).
+
+If you are porting an application to the newest `cortex-m-*` crates you may hit these errors:
+
+- "error: requires `start` lang_item" on binary crates. You need to switch from the standard `main`
+  interface to `#![no_main]` + `entry!`. Use the [examples][qse] in the quickstart template as a
+  reference.
+
+[qse]: https://docs.rs/cortex-m-quickstart/0.3.0/cortex_m_quickstart/examples/_0_minimal/index.html
+
+- "error[E0015]: calls in statics are limited to constant functions, tuple structs and tuple
+  variants". `const fn` is not a stable feature so it's not used in the API by default. The
+  `cortex-m` provides a `"const-fn"` feature to opt into this unstable feature and expose `const`
+  functions in the API; other dependencies should / probably provide a similar feature.
+
+## What does this all mean for you?
+
+To those of you who have been doing embedded development on nightly: things will get *much easier*
+from now on. We have reduced the number of unstable features in core crates of the Cortex-M
+ecosystem to **zero**. Although you will have to continue to use nightly for application development
+you can expect zero breakage from these crates when updating the nightly compiler. There is one
+expected breakage coming soon though: `panic_fmt` will be removed as announced above. This will
+break the [panic handler crates][phc]; however, the breakage will be isolated to those crates and
+the fix will consist of simply updating the dependency version.
+
+[phc]: https://crates.io/keywords/panic-impl
+
+To those of you who have been meaning to dive into embedded Rust development: we'll soon be done
+with our "embedded Rust on stable" tasks and we'll move our focus to documenting the embedded
+development process, tooling and ecosystem. The [embedded Rust book][book] will become the resource
+to get started on embedded Rust, but it doesn't quite exist right now.
+
+[book]: https://github.com/rust-lang-nursery/embedded-wg/blob/master/books/embedded-rust-book/README.md
+
+## What about architectures other than ARM Cortex-M?
+
+The path carved for ARM Cortex-M can be followed by the other architectures (ARM Cortex-R, AVR,
+MSP430, RISCV, etc.). Each architecture presents its own extra set of challenges though:
+
+- MSP430 being the second oldest embedded / `no_std` target is the closest one to make it to stable
+  (as a tier 2 target) by the edition release. The main blocker is that `extern "msp430-interrupt"`
+  is not available on stable but I think that may be possible to work around using external C /
+  assembly files + FFI.
+
+- Both AVR and RISCV are blocked from being included in the Rust compiler due to either the
+  immaturity of the LLVM backend or LLVM backend bugs. Not many (embedded) Rust developers can also
+  hack LLVM backends so we are short handed on the AVR front; OTOH, the RISCV is being actively
+  developed by third parties so it's probably just a matter of time before we add it to rustc.
+
+- ARM Cortex-R LLVM backend is probably in good shape due to the similarities between its
+  instruction set and the ARM Cortex-M instruction set, so adding the target to the compiler should
+  be straightforward. However, the crate ecosystem is nonexistent at this point so there's a lot of
+  work to be done on that front.
+
+---
+
+This work is part of the [embedded WG][ewg] effort towards [making embedded development possible on
+the stable channel][stable]. (We are almost done! :tada:)
+
+[ewg]: https://github.com/rust-lang-nursery/embedded-wg
+[stable]: https://github.com/rust-lang-nursery/embedded-wg