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Implement unstable trait impl This PR allows marking impls of stable trait with stable type as unstable. ## Approach In std/core, an impl can be marked as unstable by annotating it with ``#[unstable_feature_bound(feat_name)]``. This will add a ``ClauseKind::Unstable_Feature(feat_name)`` to the list of predicates in ``predicates_of`` . When an unstable impl's function is called, we will first iterate through all the goals in ``param_env`` to check if there is any ``ClauseKind::UnstableFeature(feat_name)`` in ``param_env``. The existence of ``ClauseKind::Unstable_Feature(feat_name)`` in ``param_env`` means an``#[unstable_feature_bound(feat_name)]`` is present at the call site of the function, so we allow the check to succeed in this case. If ``ClauseKind::UnstableFeature(feat_name)`` does not exist in ``param_env``, we will still allow the check to succeed for either of the cases below: 1. The feature is enabled through ``#[feature(feat_name)]`` outside of std / core. 2. We are in codegen because we may be monomorphizing a body from an upstream crate which had an unstable feature enabled that the downstream crate do not. For the rest of the case, it will fail with ambiguity. ## Limitation In this PR, we do not support: 1. using items that need ``#[unstable_feature_bound]`` within stable APIs 2. annotate main function with ``#[unstable_feature_bound]`` 3. annotate ``#[unstable_feature_bound]`` on items other than free function and impl ## Acknowledgement The design and mentoring are done by `@BoxyUwU`