Set and test a MSRV of 1.32

This is the version needed for the 2018 edition (stable in 1.31) and the
`?` operator in macros (stable in 1.32).

Author: tgross35

.github/workflows/main.yaml

@@ -14,7 +14,11 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: [stable, beta, nightly]
+        rust:
+          - "1.32" # msrv
+          - stable
+          - beta
+          - nightly
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
       - name: Install Rust ${{ matrix.rust }}
@@ -24,7 +28,11 @@ jobs:
           rustup default ${{ matrix.rust }}
           rustc -vV
       - name: Run tests
-        run: cargo test
+        run: |
+          set -eux
+          # Remove `-Dwarnings` at the MSRV since lints may be different
+          [ "${{ matrix.rust }}" = "1.32" ] && export RUSTFLAGS=""
+          cargo test
 
   rustfmt:
     name: Rustfmt

Cargo.toml

@@ -11,6 +11,7 @@ parameters. Structured like an if-else chain, the first matching branch is the
 item that gets emitted.
 """
 edition = "2018"
+rust-version = "1.32"
 
 [dependencies]
 core = { version = "1.0.0", optional = true, package = "rustc-std-workspace-core" }