bridge: Mark error-throwing paths as #[cold]

Tiny code size improvement, possibly also a tiny happy-path
performance improvement. (Branch predictors these days are very good,
but it still makes a difference how the code is laid out in the
binary.)

Author: jrose-signal

rust/bridge/shared/types/src/ffi/error.rs

@@ -186,6 +186,11 @@ impl SignalFfiError {
     pub fn downcast_ref<T: FfiError>(&self) -> Option<&T> {
         (*self.0).upcast_as_any().downcast_ref()
     }
+
+    #[cold]
+    pub fn into_raw_box_for_ffi(self) -> *mut Self {
+        Box::into_raw(Box::new(self))
+    }
 }
 
 /// SignalFfiError is a typed wrapper around a Box, and as such it's reasonable for it to have the

rust/bridge/shared/types/src/ffi/futures.rs

@@ -95,7 +95,7 @@ impl<T: ResultTypeInfo + std::panic::UnwindSafe> ResultReporter for FutureResult
                 std::mem::forget(value);
             }
             Err(err) => (promise.complete)(
-                Box::into_raw(Box::new(err)),
+                err.into_raw_box_for_ffi(),
                 std::ptr::null(),
                 promise.context,
             ),

rust/bridge/shared/types/src/ffi/mod.rs

@@ -308,11 +308,9 @@ pub fn run_ffi_safe<F: FnOnce() -> Result<(), SignalFfiError> + std::panic::Unwi
         Err(r) => Err(UnexpectedPanic(r).into()),
     };
 
-    // When ThinBox is stabilized, we can return that instead of double-boxing.
-    // (Unfortunately, Box<dyn MyTrait> is two pointers wide and not FFI-safe.)
     match result {
         Ok(()) => std::ptr::null_mut(),
-        Err(e) => Box::into_raw(Box::new(e)),
+        Err(e) => e.into_raw_box_for_ffi(),
     }
 }
 

rust/bridge/shared/types/src/jni/mod.rs

@@ -102,6 +102,7 @@ impl<'a, T> From<JavaCompletableFuture<'a, T>> for JObject<'a> {
     }
 }
 
+#[cold]
 fn convert_to_exception<'a, 'env, F>(env: &'a mut JNIEnv<'env>, error: SignalJniError, consume: F)
 where
     F: 'a + FnOnce(&'a mut JNIEnv<'env>, Result<JThrowable<'a>, BridgeLayerError>, SignalJniError),
@@ -896,6 +897,7 @@ impl JniError for RateLimitChallenge {
 ///
 /// Exceptions thrown in callbacks will be rethrown; all other errors will be mapped to an
 /// appropriate Java exception class and thrown.
+#[cold]
 fn throw_error(env: &mut JNIEnv, error: SignalJniError) {
     convert_to_exception(env, error, |env, throwable, error| match throwable {
         Err(failure) => log::error!("failed to create exception for {error}: {failure}"),