Add project proposal for documenting and testing MSRV in all WG crates

projects/in-progress/0443-test-msrv.md

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+# Metadata
+
+* Task Shepherd: @adamgreig
+* Contributors: Newcomers welcome!
+* Relevant repository: All WG repositories
+
+# Background
+
+All of the Embedded WG's projects should have a defined "Minimum Support Rust
+Version" (MSRV). This is the oldest version of the Rust compiler we guarantee
+will work with the project. Our MSRV policy is defined
+[in ops/msrv.md](https://github.com/rust-embedded/wg/blob/master/ops/msrv.md),
+along with how we should document and test it.
+
+However, many of our projects do not document or test an MSRV, which means we
+effectively do not know what the MSRV is and might accidentally raise it at
+any time. This has been flagged recently in a few projects where the MSRV was
+surprisingly high and untested.
+
+# Suggested Task
+
+This project would ensure all WG repositories which:
+
+* Contain a Rust project
+* Test that Rust project using a CI system
+
+define a specific MSRV, test it automatically, and document it. The MSRV should
+be defined in line with the WG policy of being at least one less than the
+current stable Rust version.
+
+# Detailed Work
+
+Our specific goal is to ensure that each project:
+
+* Documents the MSRV in the crate-level documentation, for example in `lib.rs`:
+
+```markdown
+# Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.31 and up. It *might*
+compile with older versions but that may change in any new patch release.
+```
+
+* Tests the MSRV in CI, for example:
+
+```yaml
+matrix:
+  include:
+    # MSRV
+    - env: TARGET=thumbv7-none-eabi
+      rust: 1.31.0
+    # ...
+```
+
+These repositories have been identified as meeting the project criteria:
+
+* [ ] [alloc-cortex-m](https://github.com/rust-embedded/alloc-cortex-m)
+* [ ] [arm-dcc](https://github.com/rust-embedded/arm-dcc)
+* [ ] [bare-metal](https://github.com/rust-embedded/bare-metal)
+* [ ] [cargo-binutils](https://github.com/rust-embedded/cargo-binutils)
+* [ ] [cortex-a](https://github.com/rust-embedded/cortex-a)
+* [ ] [cortex-m-rt](https://github.com/rust-embedded/cortex-m-rt)
+* [ ] [cortex-m-semihosting](https://github.com/rust-embedded/cortex-m-semihosting)
+* [ ] [cortex-m](https://github.com/rust-embedded/cortex-m)
+* [ ] [cortex-r](https://github.com/rust-embedded/patterns)
+* [ ] [cross](https://github.com/rust-embedded/cross)
+* [ ] [embedded-hal](https://github.com/rust-embedded/embedded-hal)
+* [ ] [fixedvec-rs](https://github.com/rust-embedded/fixedvec-rs)
+* [ ] [gpio-cdev](https://github.com/rust-embedded/gpio-cdev)
+* [ ] [gpio-utils](https://github.com/rust-embedded/gpio-utils)
+* [ ] [itm](https://github.com/rust-embedded/itm)
+* [ ] [linux-embedded-hal](https://github.com/rust-embedded/linux-embedded-hal)
+* [ ] [msp430-rt](https://github.com/rust-embedded/msp430-rt)
+* [ ] [msp430](https://github.com/rust-embedded/msp430)
+* [ ] [mutex-trait](https://github.com/rust-embedded/mutex-trait)
+* [ ] [panic-itm](https://github.com/rust-embedded/panic-itm)
+* [ ] [panic-semihosting](https://github.com/rust-embedded/panic-semihosting)
+* [ ] [r0](https://github.com/rust-embedded/r0)
+* [ ] [register-rs](https://github.com/rust-embedded/register-rs)
+* [ ] [riscv-rt](https://github.com/rust-embedded/riscv-rt)
+* [ ] [riscv](https://github.com/rust-embedded/riscv)
+* [ ] [rust-i2cdev](https://github.com/rust-embedded/rust-i2cdev)
+* [ ] [rust-spidev](https://github.com/rust-embedded/rust-spidev)
+* [ ] [rust-sysfs-gpio](https://github.com/rust-embedded/rust-sysfs-gpio)
+* [ ] [rust-sysfs-pwm](https://github.com/rust-embedded/rust-sysfs-pwm)
+* [ ] [svd2rust](https://github.com/rust-embedded/svd2rust)
+* [ ] [svd](https://github.com/rust-embedded/svd)
+
+# Discussion / Unresolved Questions
+
+* Do we want to also include a MSRV badge in repository READMEs? See
+  [rand](https://github.com/rust-random/rand) for an example.
+* Do we want to update the WG policy to stable-2, i.e.
+  [#427](https://github.com/rust-embedded/wg/issues/427)?
+
+# References
+
+* WG MSRV policy: [msrv.md](https://github.com/rust-embedded/wg/blob/master/ops/msrv.md)
+* Original discussion about MSRV policy: [#285](https://github.com/rust-embedded/wg/issues/285)
+* PR adding WG MSRV policy: [#304](https://github.com/rust-embedded/wg/pull/304)
+* Issue around guidance for MSRV: [#382](https://github.com/rust-embedded/wg/issues/382)
+* Recent issue around rethinking the WG MSRV: [#427](https://github.com/rust-embedded/wg/issues/427)
+* PR adding MSRV to cortex-m-rt: [#262](https://github.com/rust-embedded/cortex-m-rt/pull/262)
+* PR adding MSRV to cortex-m: [#159](https://github.com/rust-embedded/cortex-m/pull/159)