ci: downgrade hashbrown rather than limiting indexmap (petgraph#714)

Pinning `indexmap = "~2.5.0"` for MSRV prevents users with more current
compilers from using the latest petgraph and indexmap together, even
though they would otherwise work fine. Instead, dependencies can just be
downgraded as needed for CI testing, and users who need the older
compiler support can also manage this in their own lock files.

Author: cuviper

.github/workflows/ci.yml

@@ -37,6 +37,11 @@ jobs:
           profile: minimal
           toolchain: ${{ matrix.rust }}
           override: true
+      - name: MSRV downgrade
+        if: matrix.rust == '1.64.0'
+        run: |
+          cargo generate-lockfile
+          cargo update -p hashbrown --precise 0.15.0
       - name: Build
         run: |
           cargo build --verbose --no-default-features

Cargo.toml

@@ -35,7 +35,7 @@ debug = true
 
 [dependencies]
 fixedbitset = { version = "0.5.7", default-features = false }
-indexmap = "~2.5.0"
+indexmap = "2.5.0"
 quickcheck = { optional = true, version = "0.8", default-features = false }
 serde = { version = "1.0", optional = true }
 serde_derive = { version = "1.0", optional = true }