Auto merge of #55069 - matthewjasper:explain-free-region-liveness, r=nikomatsakis

[NLL] Use new region infer errors when explaining borrows

Use the new free region infer errors for explaining borrows

This gives at least some explanation for why a borrow is expected to
last for a certain free region. Also:

* Reports E0373: "closure may outlive the current function" with NLL.
* Special cases the case of returning a reference to (or value referencing) a local variable or temporary (E0515).
* Special case assigning a reference to a local variable in a closure to a captured variable. (E0521)

Closes #51026 - `regions-nested-fns-2.rs` isn't changed to that diagnostic, since that would not be the correct error here.
Closes #51169
cc #53882 - The error is (IMO) better now, but it could be better when we trace lifetimes in these error messages.

r? @nikomatsakis cc @pnkfelix

Author: bors

src/librustc/ich/impls_mir.rs

@@ -557,6 +557,8 @@ impl_stable_hash_for!(struct mir::ClosureOutlivesRequirement<'tcx> {
 
 impl_stable_hash_for!(enum mir::ConstraintCategory {
     Return,
+    UseAsConst,
+    UseAsStatic,
     TypeAnnotation,
     Cast,
     ClosureBounds,

src/librustc/mir/mod.rs

@@ -2725,6 +2725,8 @@ pub struct ClosureOutlivesRequirement<'tcx> {
 #[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash, RustcEncodable, RustcDecodable)]
 pub enum ConstraintCategory {
     Return,
+    UseAsConst,
+    UseAsStatic,
     TypeAnnotation,
     Cast,