Point to let when modifying field of immutable variable

src/librustc/middle/mem_categorization.rs

@@ -195,6 +195,21 @@ pub struct cmt_<'tcx> {
 pub type cmt<'tcx> = Rc<cmt_<'tcx>>;
 
 impl<'tcx> cmt_<'tcx> {
+    pub fn get_def(&self) -> Option<ast::NodeId> {
+        match self.cat {
+            Categorization::Deref(ref cmt, ..) |
+            Categorization::Interior(ref cmt, _) |
+            Categorization::Downcast(ref cmt, _) => {
+                if let Categorization::Local(nid) = cmt.cat {
+                    Some(nid)
+                } else {
+                    None
+                }
+            }
+            _ => None
+        }
+    }
+
     pub fn get_field(&self, name: ast::Name) -> Option<DefId> {
         match self.cat {
             Categorization::Deref(ref cmt, ..) |

src/librustc_borrowck/borrowck/mod.rs

@@ -659,6 +659,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
     pub fn bckerr_to_diag(&self, err: &BckError<'tcx>) -> DiagnosticBuilder<'a> {
         let span = err.span.clone();
         let mut immutable_field = None;
+        let mut local_def = None;
 
         let msg = &match err.code {
             err_mutbl => {
@@ -708,6 +709,14 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                                 }
                                 None
                             });
+                        local_def = err.cmt.get_def()
+                            .and_then(|nid| {
+                                if !self.tcx.hir.is_argument(nid) {
+                                    Some(self.tcx.hir.span(nid))
+                                } else {
+                                    None
+                                }
+                            });
 
                         format!("cannot borrow {} as mutable", descr)
                     }
@@ -738,6 +747,11 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
         if let Some((span, msg)) = immutable_field {
             db.span_label(span, &msg);
         }
+        if let Some(let_span) = local_def {
+            if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(let_span) {
+                db.span_label(let_span, &format!("consider changing this to `mut {}`", snippet));
+            }
+        }
         db
     }
 
@@ -1106,6 +1120,11 @@ before rustc 1.16, this temporary lived longer - see issue #39283 \
                     } else {
                         db.span_label(*error_span, &format!("cannot borrow mutably"));
                     }
+                } else if let Categorization::Interior(ref cmt, _) = err.cmt.cat {
+                    if let mc::MutabilityCategory::McImmutable = cmt.mutbl {
+                        db.span_label(*error_span,
+                                      &"cannot mutably borrow immutable field");
+                    }
                 }
             }
         }

src/test/ui/did_you_mean/issue-39544.stderr

@@ -1,8 +1,10 @@
 error: cannot borrow immutable field `z.x` as mutable
   --> $DIR/issue-39544.rs:21:18
    |
+20 |     let z = Z { x: X::Y };
+   |         - consider changing this to `mut z`
 21 |     let _ = &mut z.x;
-   |                  ^^^
+   |                  ^^^ cannot mutably borrow immutable field
 
 error: aborting due to previous error