Proposal for a MSRV policy. (#330)

This PR proposes a revised policy for dealing with minimum supported Rust
versions.

It suggests to keep the rust-version field in Cargo.toml at the oldest Rust
version that is able to build with minimal dependency versions and all
features enabled. In addition, the Rust version necessary to build with
latest dependencies is provided in the README and as a comment in
Cargo.toml. Both these versions are checked in the CI workflow.

The reasoning behind the proposal is that rust-version sets a hard limit on
the version, so it should be chosen as low as possible. While technically it
could be lower with a subset of the features selected, this feels like a
slippery slope and setting the version with all features selected feels like
an acceptable compromise.

Only documenting the version that is necessary to build with latest
dependencies isn’t ideal but at least provides some guidance as to what to
choose for people who go look for some.

.github/workflows/ci.yml

@@ -7,14 +7,14 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
-        rust: [1.78.0, stable, beta, nightly]
+        rust: [1.76.0, stable, beta, nightly]
     env:
         RUSTFLAGS: "-D warnings"
     steps:
     - name: Checkout repository
       uses: actions/checkout@v1
     - name: Install Rust
-      uses: hecrj/setup-rust-action@v1
+      uses: hecrj/setup-rust-action@v2
       with:
         rust-version: ${{ matrix.rust }}
     - if: matrix.rust == 'stable'
@@ -25,8 +25,22 @@ jobs:
       run: cargo fmt --all -- --check
     - run: cargo check --no-default-features --all-targets
     - run: cargo test --all-features
-    - if: matrix.rust == 'nightly'
+  minimal-versions:
+    name: Check minimal versions
+    runs-on: ubuntu-latest
+    env:
+        RUSTFLAGS: "-D warnings"
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v1
+    - name: Install Rust
+      uses: hecrj/setup-rust-action@v2
+      with:
+        rust-version: "1.68.2"
+    - name: Install nightly Rust
+      run: rustup install nightly
+    - name: Check with minimal-versions
       run: |
         cargo +nightly update -Z minimal-versions
-        cargo check --all-features --all-targets
-      name: Check with minimal-versions
+        cargo check --all-features --all-targets --locked
+      

.gitignore

@@ -10,8 +10,5 @@
 # Generated by Cargo
 /target/
 
-# Library, so no Cargo.lock
-Cargo.lock
-
 # Editor cruft
 *.*.swp