Build with minimum dependency versions

So far we have called whatever minimum version of Rust builds the
current version of the crate, including the current contents of
Cargo.lock, the minimum supported Rust version. However, there is an
inherent tension between keeping said Rust version minimal while also
picking up dependency updates that may fix actual issues.
This change resolves this conundrum, by adding a CI job that builds with
the minimum version of *all* dependencies instead. That is, in effect we
ignore Cargo.lock in this job.
This change indirectly enables us to run tooling such as Dependabot on
the repository, without having to worry that an update will
inadvertently break our minimum supported version contract. It will also
make it less confusing for folks to update dependencies [0] [1].

[0] #304
[1] #287

Signed-off-by: Daniel Müller <deso@posteo.net>

Author: d-e-s-o

.github/workflows/rust.yml

@@ -11,9 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # Please adjust README and rust-version field in Cargo.toml files when
-        # bumping version.
-        rust: [stable, 1.59.0, nightly]
+        rust: [stable, nightly]
     steps:
     - name: "Set environmental variables"
       shell: bash
@@ -55,6 +53,32 @@ jobs:
       run: cargo test --verbose --workspace --exclude runqslower -- --skip test_object --skip test_tc
     - name: Run BTF tests
       run: cd libbpf-rs && cargo test --verbose -- test_object test_tc
+
+  build-minimum:
+    name: Build using minimum versions of dependencies
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install deps
+        run: sudo apt-get install -y libelf-dev
+      - name: Install Nightly Rust
+        uses: actions-rs/toolchain@v1.0.6
+        with:
+          profile: minimal
+          toolchain: nightly
+      - run: cargo +nightly -Z minimal-versions update
+      - name: Install minimum Rust
+        uses: actions-rs/toolchain@v1.0.6
+        with:
+          profile: minimal
+          # Please adjust README and rust-version field in Cargo.toml files when
+          # bumping version.
+          toolchain: 1.59.0
+          components: rustfmt
+          default: true
+      - name: Build
+        run: cargo build --verbose --workspace --exclude runqslower
+
   build-capable:
     name: Build capable example with static libelf and libz
     runs-on: ubuntu-latest

libbpf-rs/Cargo.toml

@@ -37,3 +37,10 @@ plain = "0.2.3"
 probe = "0.3"
 scopeguard = "1.1"
 serial_test = "0.5"
+
+# 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.
+_cc_unused = { package = "cc", version = "1.0.3" }
+_pkg-config_unused = { package = "pkg-config", version = "0.3.3" }
+_syn_unused = { package = "syn", version = "0.15.15" }