feat(ci): add OpenHarmony CI and add it to tier2

[richerfu]

What does this PR do

Add OpenHarmony target into CI and Update documents.

The official support: arm64,arm and x86_64, so i add three targets:

• aarch64-unknown-linux-ohos
• armv7-unknown-linux-ohos
• x86_64-unknown-linux-ohos

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Any idea what is the minimal Rust version required for these 3 targets?

[richerfu]

Any idea what is the minimal Rust version required for these 3 targets?

Maybe 1.70.0 and 1.73.0
for arm64 and arm: rust-lang/rust#108792
for x86_64: rust-lang/rust#113061

I want to split it a unique task and use rust with 1.79.0, how about it?

[SteveLauC]

I tend to use the workaround that Loongarch uses:

1. Specify a separate MSRV for it

   nix/.github/workflows/ci.yml

   Lines 17 to 18 in 3d4f311

   	# Rust's Loongarch support merged in 1.71.0
   	MSRV_LOONGARCH: 1.71.0

   # Minimal Rust version to support all 3 official OpenHarmony targets
   MSRV_OHOS: 1.79.0 

2. In the step that sets up Rust toolchain, use that separate MSRV if needed

   nix/.github/workflows/ci.yml

   Lines 109 to 114 in 3d4f311

   	- name: setup Rust
   	uses: dtolnay/rust-toolchain@master
   	with:
   	# Use a newer version rustc if the target is Loongarch, remove this workaround after MSRV is newer than 1.71.0
   	toolchain: "${{ matrix.target == 'loongarch64-unknown-linux-gnu' && env.MSRV_LOONGARCH || env.MSRV }}"
   	components: clippy

    # Use a newer version rustc if it is OpenHarmony, remove this workaround after MSRV is newer than 1.79.0 
    toolchain: "${{ contains(matrix.target, 'ohos')  && env.MSRV_OHOS || env.MSRV }}" 

[SteveLauC]

Maybe 1.70.0 and 1.73.0

Sorry for missing this, if 1.73 will work, we should use it (the minimal one)

[richerfu]

Maybe 1.70.0 and 1.73.0

Sorry for missing this, if 1.73 will work, we should use it (the minimal one)

1.73 is tier3, if use 1.73, we need to add them into tier3 task. I think using 1.79 be better?

[SteveLauC]

1.73 is tier3, if use 1.73, we need to add them into tier3 task

Thanks for letting me know. Right, we want the Rust tier 2 support.

According to this comment, we should use 1.78 as rust-lang/compiler-team#719 was included in 1.78

[richerfu]

1.73 is tier3, if use 1.73, we need to add them into tier3 task

Thanks for letting me know. Right, we want the Rust tier 2 support.

According to this comment, we should use 1.78 as rust-lang/compiler-team#719 was included in 1.78

So sorry that i made a mistake in remembering :( and already fixed it.

[SteveLauC]

So sorry that i made a mistake in remembering :( and already fixed it.

No need to be sorry:)

[SteveLauC]

Thanks!