fix warnings

.github/workflows/rust.yml

@@ -6,9 +6,6 @@ on:
   pull_request:
   merge_group:
 
-env:
-  RUSTFLAGS: -D warnings
-
 defaults:
   run:
     shell: bash
@@ -92,6 +89,11 @@ jobs:
         msrv="$(cargo metadata --format-version 1 --no-deps | jq -r '.packages | map(.rust_version) | map(values) | min')"
         rustup override set $msrv
         rustup component add clippy --toolchain $msrv
+    - name: Error on warnings and clippy checks
+      # Only error on warnings and checks for the msrv, because new versions of
+      # Rust will frequently add new warnings and checks.
+      if: matrix.rust == 'msrv'
+      run: echo RUSTFLAGS='-Dwarnings' >> $GITHUB_ENV
     - run: rustup update
     - run: cargo version
     - run: rustup target add ${{ matrix.sys.target }}