Auto merge of #140399 - tiif:unstable_impl, r=lcnr,BoxyUwU

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`

compiler/rustc_attr_parsing/src/context.rs

@@ -13,7 +13,9 @@ use rustc_hir::{AttrArgs, AttrItem, AttrPath, Attribute, HashIgnoredAttrId, HirI
 use rustc_session::Session;
 use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span, Symbol, sym};
 
-use crate::attributes::allow_unstable::{AllowConstFnUnstableParser, AllowInternalUnstableParser};
+use crate::attributes::allow_unstable::{
+    AllowConstFnUnstableParser, AllowInternalUnstableParser, UnstableFeatureBoundParser,
+};
 use crate::attributes::codegen_attrs::{
     ColdParser, ExportNameParser, NakedParser, NoMangleParser, OmitGdbPrettyPrinterSectionParser,
     OptimizeParser, TargetFeatureParser, TrackCallerParser, UsedParser,
@@ -133,6 +135,7 @@ attribute_parsers!(
         Combine<AllowInternalUnstableParser>,
         Combine<ReprParser>,
         Combine<TargetFeatureParser>,
+        Combine<UnstableFeatureBoundParser>,
         // tidy-alphabetical-end
 
         // tidy-alphabetical-start