Note when a a move/borrow error is caused by a deref coercion

Fixes #73268

When a deref coercion occurs, we may end up with a move error if the
base value has been partially moved out of. However, we do not indicate
anywhere that a deref coercion is occuring, resulting in an error
message with a confusing span.

This PR adds an explicit note to move errors when a deref coercion is
involved. We mention the name of the type that the deref-coercion
resolved to, as well as the `Deref::Target` associated type being used.

compiler/rustc_middle/src/ty/adjustment.rs

@@ -4,6 +4,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
 use rustc_macros::HashStable;
+use rustc_span::Span;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
 pub enum PointerCast {
@@ -113,6 +114,9 @@ pub enum Adjust<'tcx> {
 pub struct OverloadedDeref<'tcx> {
     pub region: ty::Region<'tcx>,
     pub mutbl: hir::Mutability,
+    /// The `Span` associated with the field access or method call
+    /// that triggered this overloaded deref.
+    pub span: Span,
 }
 
 impl<'tcx> OverloadedDeref<'tcx> {

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -606,8 +606,11 @@ impl<'a, 'tcx> Lift<'tcx> for ty::adjustment::Adjust<'a> {
 impl<'a, 'tcx> Lift<'tcx> for ty::adjustment::OverloadedDeref<'a> {
     type Lifted = ty::adjustment::OverloadedDeref<'tcx>;
     fn lift_to_tcx(&self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
-        tcx.lift(&self.region)
-            .map(|region| ty::adjustment::OverloadedDeref { region, mutbl: self.mutbl })
+        tcx.lift(&self.region).map(|region| ty::adjustment::OverloadedDeref {
+            region,
+            mutbl: self.mutbl,
+            span: self.span,
+        })
     }
 }