Bump MSRV to Rust 1.38

[taiki-e]

It was released nearly 3 years ago and meets our MSRV policy.

This allows using const_in_array_repeat_expressions (rust-lang/rust#49147) and <pointer>.cast().

[taiki-e]

FYI @cuviper (because this PR affects rayon's MSRV)

Also, aside from this PR, I would like to update our MSRV policy to support 1 year instead of 6 months. Otherwise, it is impossible for rayon to actually maintain their MSRV policy (because our current MSRV policy allows us to increase the MSRV as a minor change).

[taiki-e]

bors r+

[bors]

Build succeeded:

• ci

[cuviper]

Thanks for the heads-up. I have no problem with this tiny bump. 🙂

FWIW, I'm looking at 1.56 in rayon-rs/rayon#964, but that will wait a few months yet to meet our 1-year MSRV policy.

[dhardy]

FYI this has broken rand's CI (using Rust 1.36). It's not critical since it's only through rand using rayon as a dev-dependency, but still a little annoying seeing a patch release bump the MSRV.

[taiki-e]

@dhardy

still a little annoying seeing a patch release bump the MSRV.

We had treated MSRV bump as a major change in the past, but maintaining that policy was a pain for both maintainers and users, so we changed our policy. See #444 (comment) and #503 (comment) for more.

(FYI, rust-lang/libc#2845 and rust-lang/libs-team#72 are the latest discussions on MSRV for crate widely used in the ecosystem. )

It's not critical since it's only through rand using rayon as a dev-dependency

FWIW, AFAIK, dev-deps often have much more lenient MSRV policy than the crate under test, so there are many projects that do not run cargo test on MSRV, but only run cargo build. (e.g., serde, syn, regex, tokio, futures, etc.)

[dhardy]

Committing only to support rustc versions no more than 6 months or 1 year old honestly sounds like a sensible option to me. We've seen very little concern about bumping the MSRV of rand in the past.

but still a little annoying seeing a patch release bump the MSRV

Sorry, I shouldn't complain about something already using the best apparent solution!

[tatsuya6502]

@dhardy

Just FYI. I am not sure if this is the best way, but instead of bumping the MSRV of my library, I have been working around it by pinning dependencies to some older versions in Cargo.lock when testing the MSRV.

moka-rs/moka/blob/78050701a/.github/workflows/CI.yml#L55-L67

      - name: Pin some dependencies to specific versions (MSRV only)
        if: ${{ matrix.rust == '1.51.0' }}
        # hashbrown >= v0.12 requires Rust 2021 edition.
        # native-tls >= v0.2.9 requires more recent Rust version.
        # async-global-executor >= 2.1 requires Rust 2021 edition.
        # pull-down-cmark >= 0.9.2 requires Rust 2021 edition.
        run: |
          cargo update -p dashmap --precise 5.2.0
          cargo update -p indexmap --precise 1.8.2
          cargo update -p hashbrown --precise 0.11.2
          cargo update -p native-tls --precise 0.2.8
          cargo update -p async-global-executor --precise 2.0.4
          cargo update -p pulldown-cmark --precise 0.9.1