From afd38e95208725f6d96730d64a311f6deda07d4e Mon Sep 17 00:00:00 2001
From: LeSeulArtichaut <leseulartichaut@gmail.com>
Date: Sat, 1 Feb 2020 02:05:38 +0100
Subject: [PATCH] Suggest removing `&mut` when encountering mutability error

---
 .../diagnostics/mutability_errors.rs          | 22 ++++++++++++++-----
 .../ui/borrowck/issue-68697-remove-mut-ref.rs | 15 +++++++++++++
 .../issue-68697-remove-mut-ref.stderr         | 12 ++++++++++
 3 files changed, 43 insertions(+), 6 deletions(-)
 create mode 100644 src/test/ui/borrowck/issue-68697-remove-mut-ref.rs
 create mode 100644 src/test/ui/borrowck/issue-68697-remove-mut-ref.stderr

diff --git a/src/librustc_mir/borrow_check/diagnostics/mutability_errors.rs b/src/librustc_mir/borrow_check/diagnostics/mutability_errors.rs
index 563ff1112c3a6..bba3bdb3e927a 100644
--- a/src/librustc_mir/borrow_check/diagnostics/mutability_errors.rs
+++ b/src/librustc_mir/borrow_check/diagnostics/mutability_errors.rs
@@ -257,12 +257,22 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 assert_eq!(local_decl.mutability, Mutability::Not);
 
                 err.span_label(span, format!("cannot {ACT}", ACT = act));
-                err.span_suggestion(
-                    local_decl.source_info.span,
-                    "consider changing this to be mutable",
-                    format!("mut {}", self.local_names[local].unwrap()),
-                    Applicability::MachineApplicable,
-                );
+
+                if let ty::Ref(_, _, Mutability::Mut) = local_decl.ty.kind {
+                    err.span_suggestion(
+                        span,
+                        "remove the unnecessary `&mut` here",
+                        self.local_names[local].unwrap().to_string(),
+                        Applicability::MaybeIncorrect,
+                    );
+                } else {
+                    err.span_suggestion(
+                        local_decl.source_info.span,
+                        "consider changing this to be mutable",
+                        format!("mut {}", self.local_names[local].unwrap()),
+                        Applicability::MachineApplicable,
+                    );
+                }
             }
 
             // Also suggest adding mut for upvars
diff --git a/src/test/ui/borrowck/issue-68697-remove-mut-ref.rs b/src/test/ui/borrowck/issue-68697-remove-mut-ref.rs
new file mode 100644
index 0000000000000..1cfb5cd2843af
--- /dev/null
+++ b/src/test/ui/borrowck/issue-68697-remove-mut-ref.rs
@@ -0,0 +1,15 @@
+// Regression test for #68697: Suggest removing `&mut x`
+// when `x: &mut T` and `&mut T` is expected type
+
+struct A;
+
+fn bar(x: &mut A) {}
+
+fn foo(x: &mut A) {
+    bar(&mut x);
+    //~^ ERROR: cannot borrow `x` as mutable
+    //~| HELP: remove the unnecessary `&mut` here
+    //~| SUGGESTION: x
+}
+
+fn main() {}
diff --git a/src/test/ui/borrowck/issue-68697-remove-mut-ref.stderr b/src/test/ui/borrowck/issue-68697-remove-mut-ref.stderr
new file mode 100644
index 0000000000000..fc57a94fdd4cd
--- /dev/null
+++ b/src/test/ui/borrowck/issue-68697-remove-mut-ref.stderr
@@ -0,0 +1,12 @@
+error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
+  --> $DIR/issue-68697-remove-mut-ref.rs:6:9
+   |
+LL |     bar(&mut x);
+   |         ^^^^^^
+   |         |
+   |         cannot borrow as mutable
+   |         help: remove the unnecessary `&mut` here: `x`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.