Improve makefile, document utils MSRV policy (#6390)

Fixes #3707

I didn't end up trying to generalize the CI. We can figure that out
separately if we want.


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Author: Manishearth

.github/workflows/build-test.yml

@@ -76,12 +76,8 @@ jobs:
     - name: Check
       run: cargo make ci-job-msrv-${{ matrix.behavior }}
 
-  msrv-zerofrom:
+  msrv-utils:
     runs-on: ubuntu-latest
-    # Defined as a matrix so that features can start immediately, but
-    # be killed if check fails.
-    strategy:
-      fail-fast: true
     # Set CARGO_HTTP_MULTIPLEXING=false to work around crates.io curl bug:
     # <https://rust-lang.zulipchat.com/#narrow/stream/246057-t-cargo/topic/timeout.20investigation>
     env:
@@ -99,7 +95,7 @@ jobs:
 
     # Actual job
     - name: Check
-      run: cargo make ci-job-msrv-zerofrom
+      run: cargo make ci-job-msrv-utils
 
   # ci-job-test
   test:

Makefile.toml

@@ -65,7 +65,15 @@ exec --fail-on-error rustup install 1.81 --profile minimal
 exec --fail-on-error cargo +1.81 check --all-targets --all-features
 '''
 
-[tasks.ci-job-msrv-zerofrom]
+[tasks.ci-job-msrv-utils]
+description = "Check utils that have different MSRVs"
+category = "ICU4X Development"
+dependencies = [
+    # This list should be kept in sync with the list in rust_versions.md
+    "msrv-zerofrom",
+]
+
+[tasks.msrv-zerofrom]
 description = "Run MSRV tests for the zerofrom MSRV job"
 category = "ICU4X Development"
 env = { ICU4X_DATA_DIR = "../stubdata" }

documents/process/rust_versions.md

@@ -13,6 +13,16 @@ While we hope that old ICU4X versions will always compile, this is not always go
 
 The main ICU4X library does not use nightly; however nightly is required for building our FreeRTOS port since it involves building a no_std binary. We shall attempt to keep such ports (and any future nightly-only ICU4X features) building on nightly compilers in a range of 3-6 months in the past and the future, however we cannot make concrete guarantees here.
 
+
+### Utils crates with different MSRVs
+
+Our "utils" crates (yoke, zerovec, zerofrom, etc) are not versioned alongside ICU4X: they may have major version updates between minor ICU4X releases, and they may choose to have minor version updates between major ICU4X releases.
+
+By default, these crates have the same MSRV as their corresponding ICU4X release, however at times we may give them a lower MSRV for a wider range of compatability.
+
+This is done on an as-requested basis and is best-effort.
+
+Currently, `zerofrom` is the only crate with a lower MSRV.
 
 ## For ICU4X developers