[c-bata]

Motivation

Trial.user_attrs of a trial returned by Study.ask() does not include the user attrs given to Study.enqueue_trial(). They are stored in the storage, but the trial object does not show them. This is different from Optuna, where Trial.user_attrs returns them.

import rustuna

study = rustuna.create_study()
study.enqueue_trial({}, user_attrs={"key": "value"})
trial = study.ask()

print(trial.user_attrs)  # {} (expected: {"key": "value"})
print(study._storage.get_trial(trial._trial_id).user_attrs)  # {"key": "value"}

This happens because Trial keeps user attrs in a cache (cached_user_attrs). The cache always starts empty and is only updated by Trial.set_user_attr(s).

Description of the changes

• When Study.ask() takes a trial from the trial queue, fill the user attr cache of the trial with the user attrs in the storage. This is the same place where the fixed params are read.
• Keep the design that caches user attrs in Trial. Only the initial value of the cache changes.
• Add a Rust test (test_ask_returns_user_attrs_of_enqueued_trial), and check trial.user_attrs in the existing Python test test_enqueue_trial_with_user_attrs, which runs for every trial queue type.