build: declare Minimum Supported Rust Version (MSRV) as 1.42

* Test build with MSRV version in CI as well.

.github/workflows/tests.yml

@@ -23,7 +23,7 @@ jobs:
           docker build . --pull \
             -f varia/Dockerfile.tests \
             --target=cargo-build \
-            --build-arg=channel=nightly \
+            --build-arg=rust=nightly \
             --tag=posixacl-nightly
       - name: Test suite
         run: docker run --rm posixacl-nightly cargo test --color=always
@@ -44,7 +44,7 @@ jobs:
           docker build . --pull \
             -f varia/Dockerfile.tests \
             --target=cargo-build \
-            --build-arg=channel=stable \
+            --build-arg=rust=stable \
             --tag=posixacl-stable
       - name: Test suite
         run: docker run --rm posixacl-stable cargo test --color=always
@@ -54,3 +54,15 @@ jobs:
         run: docker run --rm posixacl-stable cargo clippy --color=always
       - name: rustfmt
         run: docker run --rm posixacl-stable cargo fmt -- --color=always --check
+
+  rust-msrv:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Build on Rust MSRV
+        run: |
+          docker build . --pull \
+            -f varia/Dockerfile.tests \
+            --target=cargo-build \
+            --build-arg=rust=msrv \
+            --tag=posixacl-msrv

Cargo.toml

@@ -2,6 +2,7 @@
 name = "posix-acl"
 version = "1.1.0"
 edition = "2018"
+rust-version = "1.42.0"
 
 # Metadata
 authors = ["Marti Raudsepp <marti@juffo.org>"]

varia/Dockerfile.tests

@@ -1,19 +1,23 @@
 # This Dockerfile is mostly for CI, see .github/workflows/tests.yml
 # Run with --build-arg=channel=stable OR --build-arg=channel=nightly (default)
-ARG channel=nightly
+ARG rust=nightly
 
 # Using Dockerfile conditionals
 #### Base image for STABLE
 FROM rust AS cargo-base-stable
-ENV components="rustfmt clippy"
+ENV components="rustfmt clippy" buildflags="--tests"
 
 #### Base image for NIGHTLY
 FROM rustlang/rust:nightly AS cargo-base-nightly
-# clippy and rustfmt are frequently broken in nightly
-ENV components=""
+ENV components="" buildflags="--tests"
+
+#### Base image for MSRV
+FROM rust:1.42.0 AS cargo-base-msrv
+# Don't build tests: dev-dependencies are incompatible with MSRV.
+ENV components="" buildflags=""
 
 #### Common logic for base image
-FROM cargo-base-$channel AS cargo-build
+FROM cargo-base-$rust AS cargo-build
 WORKDIR /root/build
 # Make warnings fatal
 ENV RUSTFLAGS="-D warnings"
@@ -26,10 +30,10 @@ RUN if test -n "$components"; then rustup component add $components; fi
 COPY Cargo.toml ./
 RUN mkdir src/ && \
 	echo "pub fn main() {println!(\"dummy function\")}" > src/lib.rs && \
-	cargo build --lib --tests --color=always && \
+	cargo build --lib $buildflags --color=always && \
 	rm -rdv target/*/deps/posix_acl-* \
 	        target/*/.fingerprint/posix-acl-*
 
 # Do the actual build
 COPY . .
-RUN cargo build --lib --tests --color=always
+RUN cargo build --lib $buildflags --color=always