Test with stable and nightly, build with 1.56

The one-month old fix in #515 has already caused problems because a
test dependency has bumped its minimum supported Rust version (MSRV).

It turns out that Textwrap builds fine on old versions of Rust. It
builds all the way back to Rust 1.56 which introduced the 2021 edition
we use. To better reflect this, we now _test_ with modern versions of
Rust and we build with the old version.

.github/workflows/build.yml

@@ -16,20 +16,21 @@ env:
 
 jobs:
 
+  # Here we check that we can run `cargo test` on modern versions of
+  # Rust. Below we check that we can build the library on an old
+  # version as well.
   ubuntu:
     name: Ubuntu
     runs-on: ubuntu-latest
     strategy:
       matrix:
         rust:
-          - '1.60'  # Current MSRV
           - stable
           - nightly
         features:
           - no default features
           - all features
           - default features
-
     steps:
       - name: Checkout repository
         uses: actions/checkout@v3
@@ -61,6 +62,34 @@ jobs:
       - name: Test with default features
         run: cargo test
 
+  msrv:
+    name: Minimum supported Rust version
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        features:
+          - no default features
+          - all features
+          - default features
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      - name: Install Rust 1.56  # MSRV from Cargo.toml
+        run: rustup default 1.56
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: Build with ${{ matrix.features }}
+        run: |
+          FLAG="${{ matrix.features }}"
+          if [[ "$FLAG" = "default features" ]]; then
+            FLAG=''  # Needs no flag
+          else
+            FLAG="--${FLAG// /-}"  # Turn 'foo bar' into '--foo-bar'
+          fi
+          cargo build $FLAG
+
   # This builds benchmarks, which are not covered above.
   build-benchmarks:
     name: Build benchmarks

Cargo.toml

@@ -10,7 +10,7 @@ keywords = ["text", "formatting", "wrap", "typesetting", "hyphenation"]
 categories = ["text-processing", "command-line-interface"]
 license = "MIT"
 edition = "2021"
-rust-version = "1.60"
+rust-version = "1.56"
 exclude = [".github/", ".gitignore", "benchmarks/", "examples/", "fuzz/", "images/"]
 
 [[example]]