Introduce dedicated minimum supported Rust version job

Introduce a dedicated CI job for checking that the program builds with
the advertised minimum supported Rust version. One problem that we have
with the existing check is that it builds the entire crate, including
all tests and other development artifacts. But we don't really care
whether anything development related adheres to our MSRV -- only the
main program and core extensions.

.github/workflows/test.yml

@@ -29,7 +29,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [1.56.1, stable, beta, nightly]
+        rust: [stable, beta, nightly]
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master
@@ -40,6 +40,23 @@ jobs:
       - run: cargo build --workspace --bins --tests --verbose --release
       - run: cargo test --workspace --verbose
 
+  build-minimum:
+    name: Build with minimum supported Rust version
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: sudo apt-get install libhidapi-dev
+      - name: Install Nightly Rust
+        uses: dtolnay/rust-toolchain@nightly
+      - run: cargo +nightly -Z minimal-versions update
+      - name: Install minimum Rust
+        uses: dtolnay/rust-toolchain@master
+        with:
+          # Please adjust README file when bumping version.
+          toolchain: 1.56.1
+      - name: Build
+        run: cargo build --bins --locked
+
   clippy:
     name: Lint with clippy
     runs-on: ubuntu-latest

Cargo.toml

@@ -7,6 +7,7 @@
 name = "nitrocli"
 version = "0.4.1"
 edition = "2021"
+rust-version = "1.56"
 authors = ["Daniel Mueller <[email protected]>"]
 license = "GPL-3.0-or-later"
 homepage = "https://github.com/d-e-s-o/nitrocli"
@@ -63,7 +64,7 @@ version = "0.9.0"
 version = "3.0.2"
 
 [dependencies.serde]
-version = "1.0"
+version = "1.0.103"
 features = ["derive"]
 
 [dependencies.structopt]
@@ -87,3 +88,12 @@ version = "1"
 
 [dev-dependencies.tempfile]
 version = "3.1"
+
+[dev-dependencies]
+# A set of unused dependencies that we require to force correct minimum versions
+# of transitive dependencies, for cases where our dependencies have incorrect
+# dependency specifications themselves.
+# error: cannot find macro `log` in this scope
+_log_unused = { package = "log", version = "0.4.4" }
+# error[E0635]: unknown feature `proc_macro_span_shrink`
+_proc_macro2_unused = { package = "proc-macro2", version = "1.0.60" }