Auto merge of #45879 - nikomatsakis:nll-kill-cyclic-closures, r=<try>

[WIP] move closure kind, signature into `ClosureSubsts`

Instead of using side-tables, store the closure-kind and signature in the substitutions themselves. This has two key effects:

- It means that the closure's type changes as inference finds out more things, which is very nice.
    - As a result, it avoids the need for the `freshen_closure_like` code (though we still use it for generators).
- It avoids cyclic closures calls.
    - These were never meant to be supported, precisely because they make a lot of the fancy inference that we do much more complicated. However, due to an oversight, it was previously possible -- if challenging -- to create a setup where a closure *directly* called itself (see e.g. #21410).

We have to see what the effect of this change is, though. Needs a crater run. Marking as [WIP] until that has been assessed.

r? @arielb1

Author: bors

src/librustc/dep_graph/dep_node.rs

@@ -498,9 +498,7 @@ define_dep_nodes!( <'tcx>
     [] IsAutoImpl(DefId),
     [] ImplTraitRef(DefId),
     [] ImplPolarity(DefId),
-    [] ClosureKind(DefId),
     [] FnSignature(DefId),
-    [] GenSignature(DefId),
     [] CoerceUnsizedInfo(DefId),
 
     [] ItemVarianceConstraints(DefId),

src/librustc/ich/impls_ty.rs

@@ -236,8 +236,9 @@ impl<'gcx> HashStable<StableHashingContext<'gcx>> for ty::Predicate<'gcx> {
             ty::Predicate::ObjectSafe(def_id) => {
                 def_id.hash_stable(hcx, hasher);
             }
-            ty::Predicate::ClosureKind(def_id, closure_kind) => {
+            ty::Predicate::ClosureKind(def_id, closure_substs, closure_kind) => {
                 def_id.hash_stable(hcx, hasher);
+                closure_substs.hash_stable(hcx, hasher);
                 closure_kind.hash_stable(hcx, hasher);
             }
             ty::Predicate::ConstEvaluatable(def_id, substs) => {