Add build warning only if CRANELIFT_VERBOSE is set to anything;

lib/codegen/build.rs

@@ -23,8 +23,11 @@ extern crate cranelift_codegen_meta as meta;
 use meta::isa::Isa;
 use std::env;
 use std::process;
+use std::time::Instant;
 
 fn main() {
+    let start_time = Instant::now();
+
     let out_dir = env::var("OUT_DIR").expect("The OUT_DIR environment variable must be set");
     let target_triple = env::var("TARGET").expect("The TARGET environment variable must be set");
     let cranelift_targets = env::var("CRANELIFT_TARGETS").ok();
@@ -92,6 +95,14 @@ fn main() {
             process::exit(1);
         }
     }
+
+    if let Ok(_) = env::var("CRANELIFT_VERBOSE") {
+        println!(
+            "cargo:warning=Build step took {:?}.",
+            Instant::now() - start_time
+        );
+        println!("cargo:warning=Generated files are in {}", out_dir);
+    }
 }
 
 fn identify_python() -> &'static str {