Added help message for missing IndexMut impl.

src/librustc_mir/borrow_check/mod.rs

@@ -1601,8 +1601,9 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         if let Some(&init_index) = first_init_index {
             // And, if so, report an error.
             let init = &self.move_data.inits[init_index];
+            let span = init.span(&self.mir);
             self.report_illegal_reassignment(
-                context, place_span, init.span(&self.mir), place_span.0
+                context, place_span, span, place_span.0
             );
         }
     }

src/librustc_mir/borrow_check/mutability_errors.rs

@@ -10,9 +10,10 @@
 
 use rustc::hir;
 use rustc::hir::Node;
-use rustc::mir::{self, BindingForm, ClearCrossCrate, Local, Location, Mir};
-use rustc::mir::{Mutability, Place, Projection, ProjectionElem, Static};
-use rustc::ty::{self, TyCtxt};
+use rustc::mir::{self, BindingForm, Constant, ClearCrossCrate, Local, Location, Mir};
+use rustc::mir::{Mutability, Operand, Place, Projection, ProjectionElem, Static, Terminator};
+use rustc::mir::TerminatorKind;
+use rustc::ty::{self, Const, DefIdTree, TyS, TyKind, TyCtxt};
 use rustc_data_structures::indexed_vec::Idx;
 use syntax_pos::Span;
 
@@ -22,7 +23,7 @@ use util::borrowck_errors::{BorrowckErrors, Origin};
 use util::collect_writes::FindAssignments;
 use util::suggest_ref_mut;
 
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub(super) enum AccessKind {
     MutableBorrow,
     Mutate,
@@ -394,6 +395,47 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
                 );
             }
 
+            Place::Projection(box Projection {
+                base: Place::Local(local),
+                elem: ProjectionElem::Deref,
+            })  if error_access == AccessKind::MutableBorrow => {
+                err.span_label(span, format!("cannot {ACT}", ACT = act));
+
+                let mpi = self.move_data.rev_lookup.find_local(*local);
+                for i in self.move_data.init_path_map[mpi].iter() {
+                    if let InitLocation::Statement(location) = self.move_data.inits[*i].location {
+                        if let Some(
+                            Terminator {
+                                kind: TerminatorKind::Call {
+                                    func: Operand::Constant(box Constant {
+                                        literal: Const {
+                                            ty: &TyS {
+                                                sty: TyKind::FnDef(id, substs),
+                                                ..
+                                            },
+                                            ..
+                                        },
+                                        ..
+                                    }),
+                                    ..
+                                },
+                                ..
+                            }
+                        ) = &self.mir.basic_blocks()[location.block].terminator {
+                            if self.tcx.parent(id) == self.tcx.lang_items().index_trait() {
+                                err.help(
+                                    &format!(
+                                        "trait `IndexMut` is required to modify indexed content, \
+                                         but it is not implemented for `{}`",
+                                         substs.type_at(0),
+                                    ),
+                                );
+                            }
+                        }
+                    }
+                }
+            }
+
             _ => {
                 err.span_label(span, format!("cannot {ACT}", ACT = act));
             }

src/test/ui/borrowck/index-mut-help.nll.stderr

@@ -3,6 +3,8 @@ error[E0596]: cannot borrow data in a `&` reference as mutable
    |
 LL |     map["peter"].clear();           //~ ERROR
    |     ^^^^^^^^^^^^ cannot borrow as mutable
+   |
+   = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `std::collections::HashMap<&str, std::string::String>`
 
 error[E0594]: cannot assign to data in a `&` reference
   --> $DIR/index-mut-help.rs:22:5
@@ -15,6 +17,8 @@ error[E0596]: cannot borrow data in a `&` reference as mutable
    |
 LL |     let _ = &mut map["peter"];      //~ ERROR
    |             ^^^^^^^^^^^^^^^^^ cannot borrow as mutable
+   |
+   = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `std::collections::HashMap<&str, std::string::String>`
 
 error: aborting due to 3 previous errors
 

src/test/ui/issues/issue-41726.nll.stderr

@@ -3,6 +3,8 @@ error[E0596]: cannot borrow data in a `&` reference as mutable
    |
 LL |         things[src.as_str()].sort(); //~ ERROR cannot borrow immutable
    |         ^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable
+   |
+   = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `std::collections::HashMap<std::string::String, std::vec::Vec<std::string::String>>`
 
 error: aborting due to previous error