Point to definition when modifying field of immutable variable

Given a file

```rust
struct S {
    x: i32,
}
fn foo() {
    let s = S { x: 42 };
    s.x += 1;
}
fn bar(s: S) {
    s.x += 1;
}
```

Provide the following output:

```rust
error: cannot assign to immutable field `s.x`
 --> $DIR/issue-35937.rs:16:5
  |
5 |     let s = S { x: 42 };
  |         - consider changing this to `mut s`
6 |     s.x += 1;
  |     ^^^^^^^^ cannot mutably borrow immutable field

error: cannot assign to immutable field `s.x`
 --> $DIR/issue-35937.rs:20:5
  |
8 | fn bar(s: S) {
  |        - consider changing this to `mut s`
9 |     s.x += 1;
  |     ^^^^^^^^ cannot mutably borrow immutable field
```

Follow up to rust-lang#40445. Fix rust-lang#35937.

Author: estebank

src/librustc_borrowck/borrowck/mod.rs

@@ -688,6 +688,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
 
                 match err.cause {
                     MutabilityViolation => {
+                        local_def = err.cmt.get_def().map(|nid| self.tcx.hir.span(nid));
                         format!("cannot assign to {}", descr)
                     }
                     BorrowViolation(euv::ClosureCapture(_)) => {
@@ -1132,6 +1133,7 @@ before rustc 1.16, this temporary lived longer - see issue #39283 \
             }
         }
     }
+
     pub fn append_loan_path_to_string(&self,
                                       loan_path: &LoanPath<'tcx>,
                                       out: &mut String) {

src/test/ui/did_you_mean/issue-35937.rs

@@ -0,0 +1,31 @@
+// Copyright 2017 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.
+
+struct Foo {
+    pub v: Vec<String>
+}
+
+fn main() {
+    let f = Foo { v: Vec::new() };
+    f.v.push("cat".to_string());
+}
+
+
+struct S {
+    x: i32,
+}
+fn foo() {
+    let s = S { x: 42 };
+    s.x += 1;
+}
+
+fn bar(s: S) {
+    s.x += 1;
+}

src/test/ui/did_you_mean/issue-35937.stderr

@@ -0,0 +1,26 @@
+error: cannot borrow immutable field `f.v` as mutable
+  --> $DIR/issue-35937.rs:17:5
+   |
+16 |     let f = Foo { v: Vec::new() };
+   |         - consider changing this to `mut f`
+17 |     f.v.push("cat".to_string());
+   |     ^^^ cannot mutably borrow immutable field
+
+error: cannot assign to immutable field `s.x`
+  --> $DIR/issue-35937.rs:26:5
+   |
+25 |     let s = S { x: 42 };
+   |         - consider changing this to `mut s`
+26 |     s.x += 1;
+   |     ^^^^^^^^ cannot mutably borrow immutable field
+
+error: cannot assign to immutable field `s.x`
+  --> $DIR/issue-35937.rs:30:5
+   |
+29 | fn bar(s: S) {
+   |        - consider changing this to `mut s`
+30 |     s.x += 1;
+   |     ^^^^^^^^ cannot mutably borrow immutable field
+
+error: aborting due to 3 previous errors
+