fix(plugins): enforce rustc compatibility

BREAKING CHANGE: API v4+ plugin packages must declare the exact rustc version used to build the binary.

Refs #901

crates/ocs_plugin_api/ARCHITECTURE.md

@@ -54,7 +54,7 @@ The runtime enforces three gates:
 
 1. `ocs_plugin_api_version()` exported by the cdylib must be within `[API_VERSION_MIN_SUPPORTED, effective_max_api_version()]`.
 2. For v4+, `acadrust_sources_compatible(host_acadrust_source(), plugin_acadrust_source)` must be true.
-3. For v4+, when the plugin declares `rustc_version`, `rustc_versions_compatible(host_rustc_version(), plugin_rustc_version)` must be true.
+3. For v4+, the plugin must declare `rustc_version`, and `rustc_versions_compatible(host_rustc_version(), plugin_rustc_version)` must be true.
 
 ---
 

crates/ocs_plugin_api/build.rs

@@ -384,7 +384,8 @@ fn generate_version_info(out_dir: &Path) {
         .find(|p| p.name.as_str() == "acadrust")
         .expect("acadrust package in Cargo.lock");
 
-    let rustc_version = Command::new("rustc")
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| "rustc".into());
+    let rustc_version = Command::new(rustc)
         .arg("--version")
         .output()
         .ok()