Do not suggest to make mut binding external to Fn closure

src/librustc_borrowck/borrowck/mod.rs

@@ -842,10 +842,32 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                         if let mc::NoteClosureEnv(upvar_id) = err.cmt.note {
                             let node_id = self.tcx.hir.hir_to_node_id(upvar_id.var_id);
                             let sp = self.tcx.hir.span(node_id);
-                            match self.tcx.sess.codemap().span_to_snippet(sp) {
-                                Ok(snippet) => {
+                            let fn_closure_msg = "`Fn` closures cannot capture their enclosing \
+                                                  environment for modifications";
+                            match (self.tcx.sess.codemap().span_to_snippet(sp), &err.cmt.cat) {
+                                (_, &Categorization::Upvar(mc::Upvar {
+                                    kind: ty::ClosureKind::Fn, ..
+                                })) => {
+                                    db.note(fn_closure_msg);
+                                    // we should point at the cause for this closure being
+                                    // identified as `Fn` (like in signature of method this
+                                    // closure was passed into)
+                                }
+                                (Ok(ref snippet), ref cat) => {
                                     let msg = &format!("consider making `{}` mutable", snippet);
-                                    db.span_suggestion(sp, msg, format!("mut {}", snippet));
+                                    let suggestion = format!("mut {}", snippet);
+
+                                    if let &Categorization::Deref(ref cmt, _) = cat {
+                                        if let Categorization::Upvar(mc::Upvar {
+                                            kind: ty::ClosureKind::Fn, ..
+                                        }) = cmt.cat {
+                                            db.note(fn_closure_msg);
+                                        } else {
+                                            db.span_suggestion(sp, msg, suggestion);
+                                        }
+                                    } else {
+                                        db.span_suggestion(sp, msg, suggestion);
+                                    }
                                 }
                                 _ => {
                                     db.span_help(sp, "consider making this binding mutable");

src/test/ui/suggestions/fn-closure-mutable-capture.rs

@@ -0,0 +1,20 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub fn bar<F: Fn()>(_f: F) {}
+
+pub fn foo() {
+    let mut x = 0;
+    bar(move || x = 1);
+    //~^ ERROR cannot assign to captured outer variable in an `Fn` closure
+    //~| NOTE `Fn` closures cannot capture their enclosing environment for modifications
+}
+
+fn main() {}

src/test/ui/suggestions/fn-closure-mutable-capture.stderr

@@ -0,0 +1,15 @@
+error[E0594]: cannot assign to captured outer variable in an `Fn` closure
+  --> $DIR/fn-closure-mutable-capture.rs:15:17
+   |
+15 |     bar(move || x = 1);
+   |                 ^^^^^
+   |
+   = note: `Fn` closures cannot capture their enclosing environment for modifications
+help: consider changing this closure to take self by mutable reference
+  --> $DIR/fn-closure-mutable-capture.rs:15:9
+   |
+15 |     bar(move || x = 1);
+   |         ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+