Auto merge of #120019 - lcnr:fn-wf, r=BoxyUwU

fix fn/const items implied bounds and wf check (rebase)

A rebase of #104098, see that PR for discussion. This is pretty much entirely the work of `@aliemjay.` I received his permission for this rebase.

---

These are two distinct changes (edit: actually three, see below):
1. Wf-check all fn item args. This is a soundness fix.
Fixes #104005

2. Use implied bounds from impl header in borrowck of associated functions/consts. This strictly accepts more code and helps to mitigate the impact of other breaking changes.
Fixes #98852
Fixes #102611

The first is a breaking change and will likely have a big impact without the the second one. See the first commit for how it breaks libstd.

Landing the second one without the first will allow more incorrect code to pass. For example an exploit of #104005 would be as simple as:
```rust
use core::fmt::Display;

trait ExtendLt<Witness> {
    fn extend(self) -> Box<dyn Display>;
}

impl<T: Display> ExtendLt<&'static T> for T {
    fn extend(self) -> Box<dyn Display> {
        Box::new(self)
    }
}

fn main() {
    let val = (&String::new()).extend();
    println!("{val}");
}
```

The third change is to to check WF of user type annotations before normalizing them (fixes #104764, fixes #104763). It is mutually dependent on the second change above: an attempt to land it separately in #104746 caused several crater regressions that can all be mitigated by using the implied from the impl header. It is also necessary for the soundness of associated consts that use the implied bounds of impl header. See #104763 and how the third commit fixes the soundness issue in `tests/ui/wf/wf-associated-const.rs` that was introduces by the previous commit.

r? types

compiler/rustc_trait_selection/src/traits/query/type_op/ascribe_user_type.rs

@@ -63,13 +63,16 @@ fn relate_mir_and_user_ty<'tcx>(
     user_ty: Ty<'tcx>,
 ) -> Result<(), NoSolution> {
     let cause = ObligationCause::dummy_with_span(span);
+    ocx.register_obligation(Obligation::new(
+        ocx.infcx.tcx,
+        cause.clone(),
+        param_env,
+        ty::ClauseKind::WellFormed(user_ty.into()),
+    ));
+
     let user_ty = ocx.normalize(&cause, param_env, user_ty);
     ocx.eq(&cause, param_env, mir_ty, user_ty)?;
 
-    // FIXME(#104764): We should check well-formedness before normalization.
-    let predicate =
-        ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(user_ty.into())));
-    ocx.register_obligation(Obligation::new(ocx.infcx.tcx, cause, param_env, predicate));
     Ok(())
 }
 
@@ -113,31 +116,38 @@ fn relate_mir_and_user_args<'tcx>(
         ocx.register_obligation(Obligation::new(tcx, cause, param_env, instantiated_predicate));
     }
 
+    // Now prove the well-formedness of `def_id` with `substs`.
+    // Note for some items, proving the WF of `ty` is not sufficient because the
+    // well-formedness of an item may depend on the WF of gneneric args not present in the
+    // item's type. Currently this is true for associated consts, e.g.:
+    // ```rust
+    // impl<T> MyTy<T> {
+    //     const CONST: () = { /* arbitrary code that depends on T being WF */ };
+    // }
+    // ```
+    for arg in args {
+        ocx.register_obligation(Obligation::new(
+            tcx,
+            cause.clone(),
+            param_env,
+            ty::ClauseKind::WellFormed(arg),
+        ));
+    }
+
     if let Some(UserSelfTy { impl_def_id, self_ty }) = user_self_ty {
+        ocx.register_obligation(Obligation::new(
+            tcx,
+            cause.clone(),
+            param_env,
+            ty::ClauseKind::WellFormed(self_ty.into()),
+        ));
+
         let self_ty = ocx.normalize(&cause, param_env, self_ty);
         let impl_self_ty = tcx.type_of(impl_def_id).instantiate(tcx, args);
         let impl_self_ty = ocx.normalize(&cause, param_env, impl_self_ty);
 
         ocx.eq(&cause, param_env, self_ty, impl_self_ty)?;
-        let predicate = ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(
-            impl_self_ty.into(),
-        )));
-        ocx.register_obligation(Obligation::new(tcx, cause.clone(), param_env, predicate));
     }
 
-    // In addition to proving the predicates, we have to
-    // prove that `ty` is well-formed -- this is because
-    // the WF of `ty` is predicated on the args being
-    // well-formed, and we haven't proven *that*. We don't
-    // want to prove the WF of types from  `args` directly because they
-    // haven't been normalized.
-    //
-    // FIXME(nmatsakis): Well, perhaps we should normalize
-    // them?  This would only be relevant if some input
-    // type were ill-formed but did not appear in `ty`,
-    // which...could happen with normalization...
-    let predicate =
-        ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(ty.into())));
-    ocx.register_obligation(Obligation::new(tcx, cause, param_env, predicate));
     Ok(())
 }

compiler/rustc_trait_selection/src/traits/query/type_op/prove_predicate.rs

@@ -30,6 +30,23 @@ impl<'tcx> super::QueryTypeOp<'tcx> for ProvePredicate<'tcx> {
             }
         }
 
+        if let ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(arg)) =
+            key.value.predicate.kind().skip_binder()
+        {
+            match arg.as_type()?.kind() {
+                ty::Param(_)
+                | ty::Bool
+                | ty::Char
+                | ty::Int(_)
+                | ty::Float(_)
+                | ty::Str
+                | ty::Uint(_) => {
+                    return Some(());
+                }
+                _ => {}
+            }
+        }
+
         None
     }