Fail DRT build quickly if lean build dir isn't present (#477)

Signed-off-by: John Kastner <[email protected]>

cedar-drt/build.rs

@@ -15,14 +15,21 @@
  */
 
 use std::env;
+use std::path::Path;
+const LEAN_BUILD_DIR: &'static str = "../cedar-lean/.lake/build/lib";
 fn main() {
     let lean_dir = env::var("LEAN_LIB_DIR").expect(
         "`LEAN_LIB_DIR` environment variable is not set! Try running `source set_env_vars.sh`",
     );
-    println!("cargo:rustc-link-search=native=../cedar-lean/.lake/build/lib");
+    // We'll need to link against some files found here later, and it's nicer to
+    // fail quickly with a helpful error message.
+    if !Path::new(LEAN_BUILD_DIR).exists() {
+        panic!("Lean build directory does not exist! Try running `( cd ../cedar-lean && ../cedar-drt/build_lean_lib.sh )`")
+    }
+    println!("cargo:rustc-link-search=native={LEAN_BUILD_DIR}");
     println!("cargo:rustc-link-search=native={lean_dir}");
     println!(
         "cargo:rustc-link-search=native=../cedar-lean/.lake/packages/batteries/.lake/build/lib"
     );
-    println!("cargo:rerun-if-changed=../cedar-lean/.lake/build/lib");
+    println!("cargo:rerun-if-changed={LEAN_BUILD_DIR}");
 }