Add CI job checking library builds with oldest allowed dependencies (#854)

* Add CI job checking library builds with oldest allowed dependencies

Prevents under-specifying our dependencies. Ensure the semver specs we
write in our Cargo.toml are not allowing older versions than we actually
build with

* Run minimal-versions CI against MSRV and move to variable

Author: faern

.github/workflows/ci.yaml

@@ -10,6 +10,7 @@ on:
 name: tests
 env:
   CARGO_TERM_COLOR: always
+  MSRV: 1.80.0
 
 jobs:
   ci:
@@ -19,7 +20,7 @@ jobs:
       matrix:
         rust:
           - 1.87.0 # Stable release as of 2025-05-17
-          - 1.80.0 # MSRV
+          - $MSRV # MSRV
 
     steps:
       - uses: actions/checkout@v4
@@ -98,3 +99,23 @@ jobs:
 
       - name: check typos
         uses: crate-ci/[email protected]
+
+
+  # Make sure the library builds with all dependencies downgraded to their
+  # oldest versions allowed by the semver spec. This ensures we have not
+  # under-specified any dependency
+  minimal-versions:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install needed Rust toolchain versions
+        run: |
+          rustup install $MSRV
+          rustup install nightly
+
+      - name: Downgrade dependencies to minimal versions
+        run: cargo +nightly update -Z minimal-versions
+
+      - name: Compile with minimal versions
+        run: cargo +$MSRV build --all-targets --locked

Cargo.toml

@@ -9,6 +9,7 @@ name = "criterion"
 # * Update version numbers in the book;
 version = "0.6.0"
 edition = "2021"
+# Update the MSRV variable in .github/workflows/ci.yml when changing this.
 rust-version = "1.80"
 
 description = "Statistics-driven micro-benchmarking library"