Set rust MSRV to 1.66

Also update CI to test it and ensure future compatibility

.github/workflows/rust.yml

@@ -27,7 +27,8 @@ jobs:
         # informationally check the latest as well.  We should
         # regularly update the known good once we know that tests
         # pass on it
-        rust-toolchain: [ 1.68, stable, nightly ]
+        # 1.66 is our MSRV, so we should keep testing on it
+        rust-toolchain: [ 1.68, stable, nightly, 1.66 ]
     steps:
     - uses: actions/checkout@v3
     - name: Setup

Cargo.toml

@@ -8,6 +8,9 @@ license = "MIT"
 repository = "https://github.com/dburgener/cascade"
 readme = "README.md"
 keywords = [ "selinux" ]
+# Rust 1.66 is required by BTreeSet::pop_first()
+# https://doc.rust-lang.org/stable/std/collections/struct.BTreeSet.html#method.pop_first
+rust-version = "1.66"
 
 [build-dependencies]
 lalrpop = "0.19"