chore(ci): decouple MSRV from the development toolchain

.github/workflows/ci.yml

@@ -68,11 +68,27 @@ jobs:
         id: definitions
         run: |
           rustup show active-toolchain
-          export MSRV=$(rustup show active-toolchain | awk -F'-' '{print $1}')
+          # The MSRV is declared by the `rust-version` field of each crate. It is
+          # deliberately not tied to the toolchain anyone happens to be building
+          # with; jobs that need a specific toolchain install and override it
+          # themselves.
+          export MSRV=$(grep -m1 '^rust-version' quic/s2n-quic/Cargo.toml | sed -E 's/^rust-version *= *"(.+)"/\1/')
           if [ -z "$MSRV" ]; then
             echo "Error: MSRV did not parse correctly"
             exit 1
           fi
+          # The MSRV above is the floor s2n-quic promises to its users, so no
+          # crate may declare a lower `rust-version` than it. A crate requiring a
+          # higher one is allowed: the xdp and wireshark workspaces have needed
+          # that in the past when their dependencies outpaced the repository
+          # MSRV. Nothing pins these together anymore, so check it explicitly
+          # rather than silently testing a version some crate does not support.
+          export LOWEST=$(git ls-files '*Cargo.toml' | xargs grep -h '^rust-version' | sed -E 's/^rust-version *= *"(.+)"/\1/' | sort -V | head -1)
+          if [ "$LOWEST" != "$MSRV" ]; then
+            echo "Error: a crate declares rust-version $LOWEST, below the $MSRV MSRV of s2n-quic"
+            git ls-files '*Cargo.toml' | xargs grep -H '^rust-version' | sort -u
+            exit 1
+          fi
           echo "msrv=$MSRV"
           echo "msrv=$MSRV" >> $GITHUB_OUTPUT
           export RAW_VERSIONS="stable beta $MSRV"

.github/workflows/dependencies.yml

@@ -58,9 +58,6 @@ jobs:
         with:
           submodules: true
 
-      - name: "Remove rust-toolchain"
-        run: rm rust-toolchain
-
       - uses: EmbarkStudios/cargo-deny-action@v2
         with:
           arguments: --all-features --config .github/config/cargo-deny.toml

CONTRIBUTING.md

@@ -40,6 +40,18 @@ GitHub provides additional document on [forking a repository](https://help.githu
 [creating a pull request](https://help.github.com/articles/creating-a-pull-request/).
 
 
+## Development toolchain
+
+`s2n-quic` is developed against a recent stable Rust toolchain, along with the `clippy` and `rustfmt` components:
+
+```
+rustup toolchain install stable --component clippy,rustfmt
+```
+
+This is separate from the minimum supported Rust version. The MSRV is declared by the `rust-version` field of each crate and is verified in CI, so there is no need to develop against it directly. Clippy is configured with the MSRV through the `msrv` key in `.clippy.toml`, which reports uses of standard library APIs that are newer than the MSRV allows.
+
+Building the eBPF programs under `tools/xdp/ebpf` requires the nightly toolchain pinned by `tools/xdp/ebpf/rust-toolchain.toml`, which `cargo xtask` selects automatically.
+
 ## Finding contributions to work on
 Looking at the existing issues is a great way to find something to contribute on. As our projects, by default, use the default GitHub issue labels (enhancement/bug/duplicate/help wanted/invalid/question/wontfix), looking at any 'help wanted' issues is a great place to start.