Specify the minimum supported Rust version

cpp/Cargo.toml

@@ -6,6 +6,7 @@ version.workspace = true
 repository.workspace = true
 edition.workspace = true
 license.workspace = true
+rust-version.workspace = true
 
 [dependencies]
 cxx = { version = "1.0.192", features = ["c++17"] }

cpp/README.md

@@ -83,3 +83,7 @@ To package the build:
 ```
 cpack --config build/CPackConfig.cmake -C RelWithDebInfo
 ```
+
+## Minimum supported Rust version
+
+The minimum supported Rust version is `1.89`. This may change at any time, but there is a CI job to check that it does not change unexpectedly.