meta: Set a MSRV of 1.65 and test it in CI

This is about the oldest version I was able to successfully build for
without needing changes. We could bump it easily if needed.

Since we are checking the build at this old version, there isn't really
a need to test with both stable and beta. Delete these from the matrix
so we are only testing on nightly, which will let us use unstable
features when needed.

Author: tgross35

.github/workflows/main.yaml

@@ -17,30 +17,43 @@ jobs:
     timeout-minutes: 20
     strategy:
       matrix:
-        build: [stable, beta, nightly]
         include:
-          - build: stable
-            os: ubuntu-24.04
-            rust: stable
-          - build: beta
-            os: ubuntu-24.04
-            rust: beta
           - build: nightly
             os: ubuntu-24.04
             rust: nightly
     steps:
     - uses: actions/checkout@v6
     - name: Install Rust
-      run: rustup update ${{ matrix.rust }} && rustup default ${{ matrix.rust }}
+      run: rustup update nightly --no-self-update && rustup default nightly
     - name: Build and run tests
       run: cargo test
 
+  msrv:
+    name: Check the MSRV
+    runs-on: ubuntu-24.04
+    timeout-minutes: 20
+    env:
+      RUSTFLAGS: # No need to check warnings on old MSRV, unset `-Dwarnings`
+    steps:
+    - uses: actions/checkout@v6
+    - run: |
+        msrv="$(
+          cargo metadata --no-deps --format-version 1 |
+          jq -r '.packages .[] | | select(.name == "rustc_apfloat") | .rust_version'
+        )"
+        echo "MSRV: $msrv"
+        echo "MSRV=$msrv" >> "$GITHUB_ENV"
+    - name: Install Rust
+      run: rustup update "$MSRV" --no-self-update && rustup default "$MSRV"
+    - uses: Swatinem/rust-cache@v2
+    - run: cargo build --package rustc_apfloat
+
   rustfmt:
     name: Rustfmt
     runs-on: ubuntu-24.04
     timeout-minutes: 10
     steps:
     - uses: actions/checkout@v6
     - name: Install Rust
-      run: rustup update stable && rustup default stable && rustup component add rustfmt
+      run: rustup update stable --no-self-update && rustup default stable && rustup component add rustfmt
     - run: cargo fmt --check

Cargo.toml

@@ -13,6 +13,7 @@ edition.workspace = true
 license.workspace = true
 repository = "https://github.com/rust-lang/rustc_apfloat"
 description = "Rust port of C++ llvm::APFloat library"
+rust-version = "1.65.0"
 exclude = [".github/"]
 
 [dependencies]