introduce new fallback algorithm

We now fallback type variables using the following rules:

* Construct a coercion graph `A -> B` where `A` and `B` are unresolved
  type variables or the `!` type.
* Let D be those variables that are reachable from `!`.
* Let N be those variables that are reachable from a variable not in
D.
* All variables in (D \ N) fallback to `!`.
* All variables in (D & N) fallback to `()`.

compiler/rustc_middle/src/ty/sty.rs

@@ -1672,6 +1672,14 @@ impl<'tcx> TyS<'tcx> {
         matches!(self.kind(), Infer(TyVar(_)))
     }
 
+    #[inline]
+    pub fn ty_vid(&self) -> Option<ty::TyVid> {
+        match self.kind() {
+            &Infer(TyVar(vid)) => Some(vid),
+            _ => None,
+        }
+    }
+
     #[inline]
     pub fn is_ty_infer(&self) -> bool {
         matches!(self.kind(), Infer(_))