Provide better borrow checker error message

src/librustc_borrowck/borrowck/mod.rs

@@ -896,7 +896,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                 };
 
                 self.note_and_explain_mutbl_error(&mut db, &err, &error_span);
-                self.note_immutability_blame(&mut db, err.cmt.immutability_blame());
+                self.note_immutability_blame(&mut db, err.cmt.immutability_blame(), error_span);
                 db.emit();
             }
             err_out_of_scope(super_scope, sub_scope, cause) => {
@@ -1102,7 +1102,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                                                             Origin::Ast)
             }
         };
-        self.note_immutability_blame(&mut err, blame);
+        self.note_immutability_blame(&mut err, blame, span);
 
         if is_closure {
             err.help("closures behind references must be called via `&mut`");
@@ -1181,15 +1181,26 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
 
     fn note_immutability_blame(&self,
                                db: &mut DiagnosticBuilder,
-                               blame: Option<ImmutabilityBlame>) {
+                               blame: Option<ImmutabilityBlame>,
+                               error_span: Span) {
         match blame {
             None => {}
             Some(ImmutabilityBlame::ClosureEnv(_)) => {}
             Some(ImmutabilityBlame::ImmLocal(node_id)) => {
                 let let_span = self.tcx.hir.span(node_id);
-                if let ty::BindByValue(..) = self.local_binding_mode(node_id) {
-                    if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(let_span) {
-                        let (_, is_implicit_self) = self.local_ty(node_id);
+                if let ty::BindByValue(bind_value_mut) = self.local_binding_mode(node_id) {
+                    let (local_node_ty, is_implicit_self) = self.local_ty(node_id);
+                    if let Some(local_node_ty) = local_node_ty {
+                        match (local_node_ty.node, self.tcx.sess.codemap().span_to_snippet(let_span)) {
+                            (hir::Ty_::TyPtr(ref ty_ptr_mutability), Ok(snippet)) if ty_ptr_mutability.mutbl == bind_value_mut => {
+                                    db.span_suggestion(
+                                        error_span,
+                                        "consider removing the `&mut`, as it is an immutable binding to a mutable reference",
+                                        snippet);
+                            },
+                            _ => {}
+                        }
+                    } else if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(let_span) {
                         if is_implicit_self && snippet != "self" {
                             // avoid suggesting `mut &self`.
                             return

src/test/ui/unnecessary_mut_reference.rs

@@ -0,0 +1,18 @@
+struct User {
+    name: String,
+}
+
+fn main() {
+    let mut user1 = User{
+        name: String::from("Kurtis"),
+    };
+    mutator_part1(&mut user1);
+}
+
+fn mutator_part1(user: &mut User) {
+    mutator_part2(&mut user);
+}
+
+fn mutator_part2(user: &mut User) {
+    user.name = String::from("Steve");
+}

src/test/ui/unnecessary_mut_reference.stderr

@@ -0,0 +1,9 @@
+error[E0596]: cannot borrow immutable argument `user` as mutable
+  --> unnecessary_mut_reference.rs:13:24
+   |
+12 | fn mutator_part1(user: &mut User) {
+13 |     mutator_part2(&mut user);
+   |                        ^^^^ cannot borrow mutably
+   |                   --------- consider removing the `&mut`, as it is an immutable binding to a mutable reference
+
+error: aborting due to previous error