Rollup merge of #84682 - jackh726:transitive_bounds_rebind, r=nikomatsakis

Don't rebind in `transitive_bounds_that_define_assoc_type`

Fixes #83737
Fixes #84604

Also fixes another issue that I don't have a test for, popped up in [zulip](https://rust-lang.zulipchat.com/#narrow/stream/144729-wg-traits/topic/Duplicate.20symbol.20error.20.2384604/near/236570445)

r? `````@nikomatsakis`````

compiler/rustc_infer/src/traits/util.rs

@@ -305,9 +305,7 @@ pub fn transitive_bounds_that_define_assoc_type<'tcx>(
                     Some(assoc_name),
                 ));
                 for (super_predicate, _) in super_predicates.predicates {
-                    let bound_predicate = super_predicate.kind();
+                    let subst_predicate = super_predicate.subst_supertrait(tcx, &trait_ref);
-                    let subst_predicate = super_predicate
-                        .subst_supertrait(tcx, &bound_predicate.rebind(trait_ref.skip_binder()));
                     if let Some(binder) = subst_predicate.to_opt_poly_trait_ref() {
                         stack.push(binder.value);
                     }

compiler/rustc_middle/src/ty/flags.rs

@@ -59,6 +59,10 @@ impl FlagComputation {
     {
         let mut computation = FlagComputation::new();
 
+        if !value.bound_vars().is_empty() {
+            computation.flags = computation.flags | TypeFlags::HAS_RE_LATE_BOUND;
+        }
+
         f(&mut computation, value.skip_binder());
 
         self.add_flags(computation.flags);

src/test/ui/lifetimes/issue-83737-erasing-bound-vars.rs

@@ -0,0 +1,14 @@
+// build-pass
+// compile-flags: --edition 2018
+// compile-flags: --crate-type rlib
+
+use std::future::Future;
+
+async fn handle<F>(slf: &F)
+where
+    F: Fn(&()) -> Box<dyn for<'a> Future<Output = ()> + Unpin>,
+{
+    (slf)(&()).await;
+}
+
+fn main() {}

src/test/ui/lifetimes/issue-84604.rs

@@ -0,0 +1,9 @@
+// run-pass
+// compile-flags: -Zsymbol-mangling-version=v0
+
+pub fn f<T: ?Sized>() {}
+pub trait Frob<T: ?Sized> {}
+fn main() {
+    f::<dyn Frob<str>>();
+    f::<dyn for<'a> Frob<str>>();
+}