ci: resolve MSRV from Cargo.toml

.github/workflows/ci.yml

@@ -10,6 +10,19 @@ env:
   CARGO_TERM_COLOR: always
 
 jobs:
+  resolve:
+    runs-on: ubuntu-latest
+    outputs:
+      MSRV: ${{ steps.resolve-msrv.outputs.MSRV }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+      - name: resolve MSRV
+        id: resolve-msrv
+        run: echo MSRV=`python -c 'import tomllib; print(tomllib.load(open("Cargo.toml", "rb"))["package"]["rust-version"])'` >> $GITHUB_OUTPUT
+
   fmt:
     runs-on: ubuntu-latest
     steps:
@@ -30,7 +43,7 @@ jobs:
       - run: cargo clippy --all
 
   build:
-    needs: [fmt] # don't wait for clippy as fails rarely and takes longer
+    needs: [resolve, fmt] # don't wait for clippy as fails rarely and takes longer
     name: python${{ matrix.python-version }} ${{ matrix.os }} rust-${{ matrix.rust}}
     runs-on: ${{ matrix.os }}
     strategy:
@@ -43,7 +56,7 @@ jobs:
         include:
           - python-version: "3.13"
             os: "ubuntu-latest"
-            rust: "1.63"
+            rust: ${{ needs.resolve.outputs.MSRV }}
           - python-version: "3.13"
             python-architecture: "arm64"
             os: "macos-latest"
@@ -66,6 +79,12 @@ jobs:
       - uses: Swatinem/rust-cache@v2
         continue-on-error: true
 
+      - if: ${{ matrix.rust == needs.resolve.outputs.MSRV }}
+        name: Set dependencies on MSRV
+        run: cargo +stable update
+        env:
+          CARGO_RESOLVER_INCOMPATIBLE_RUST_VERSIONS: fallback
+
       - name: Test
         run: cargo test --verbose