Auto merge of #54942 - matthewjasper:wf-type-annotations, r=nikomatsakis

[NLL] Check user types are well-formed

Also contains a change of span for AscribeUserType.
I'm not quite sure if this was what @nikomatsakis was thinking.

Closes #54620

r? @nikomatsakis

Author: bors

src/librustc/mir/mod.rs

@@ -710,7 +710,7 @@ pub struct LocalDecl<'tcx> {
     /// e.g. via `let x: T`, then we carry that type here. The MIR
     /// borrow checker needs this information since it can affect
     /// region inference.
-    pub user_ty: Option<CanonicalTy<'tcx>>,
+    pub user_ty: Option<(CanonicalTy<'tcx>, Span)>,
 
     /// Name of the local, used in debuginfo and pretty-printing.
     ///

src/librustc/mir/visit.rs

@@ -735,7 +735,7 @@ macro_rules! make_mir_visitor {
                     local,
                     source_info: *source_info,
                 });
-                if let Some(user_ty) = user_ty {
+                if let Some((user_ty, _)) = user_ty {
                     self.visit_user_ty(user_ty);
                 }
                 self.visit_source_info(source_info);