Bug 1549792: Display flags for baldrapi.h binding generation on failu…

…re; r=froydnj

This change makes cranelift's use of bindgen more consistent with stylo's.

Differential Revision: https://phabricator.services.mozilla.com/D30238

js/src/wasm/cranelift/build.rs

@@ -30,7 +30,7 @@ fn main() {
     // Tell Cargo to regenerate the bindings if the header file changes.
     println!("cargo:rerun-if-changed=baldrapi.h");
 
-    let mut bindings = bindgen::builder()
+    let mut generator = bindgen::builder()
         .disable_name_namespacing()
         // We whitelist the Baldr C functions and get the associated types for free.
         .whitelist_function("env_.*")
@@ -70,17 +70,25 @@ fn main() {
                 .map(|s| s.to_owned())
                 .collect();
             for flag in extra_flags {
-                bindings = bindings.clang_arg(flag);
+                generator = generator.clang_arg(flag);
             }
         }
         None => {
             println!("cargo:warning=MOZ_TOPOBJDIR should be set by default, otherwise the build is not guaranted to finish.");
         }
     }
 
-    let bindings = bindings
-        .generate()
-        .expect("Unable to generate baldrapi.h bindings");
+    let command_line_opts = generator.command_line_flags();
+
+    // In case of error, bindgen prints to stderr, and the yielded error is the empty type ().
+    let bindings = generator.generate().unwrap_or_else(|_err| {
+        panic!(
+            r#"Unable to generate baldrapi.h bindings:
+- flags: {}
+"#,
+            command_line_opts.join(" "),
+        );
+    });
 
     // Write the bindings to the $OUT_DIR/bindings.rs file.
     let out_path = PathBuf::from(env::var("OUT_DIR").unwrap());