Factor out suggest_ref_mut; use it in rustc_borrowck

Also teach rustc_borrowck not to show useless help messages like
"use a mutable reference instead: `x`".

src/librustc_borrowck/borrowck/mod.rs

@@ -39,6 +39,7 @@
 use rustc::ty::{self, Ty, TyCtxt};
 use rustc::ty::query::Providers;
 use rustc_mir::util::borrowck_errors::{BorrowckErrors, Origin};
+use rustc_mir::util::suggest_ref_mut;
 use rustc::util::nodemap::FxHashSet;
 
 use std::cell::RefCell;
@@ -1206,21 +1207,17 @@ fn note_immutability_blame(&self,
                 self.note_immutable_local(db, error_node_id, node_id)
             }
             Some(ImmutabilityBlame::LocalDeref(node_id)) => {
-                let let_span = self.tcx.hir.span(node_id);
                 match self.local_binding_mode(node_id) {
                     ty::BindByReference(..) => {
-                        if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(let_span) {
-                            let replace_str = if snippet.starts_with("ref ") {
-                                snippet.replacen("ref ", "ref mut ", 1)
-                            } else {
-                                snippet
-                            };
+                        let let_span = self.tcx.hir.span(node_id);
+                        let suggestion = suggest_ref_mut(self.tcx, let_span);
+                        if let Some((let_span, replace_str)) = suggestion {
                             db.span_suggestion(
                                 let_span,
                                 "use a mutable reference instead",
                                 replace_str,
                             );
-                        };
+                        }
                     }
                     ty::BindByValue(..) => {
                         if let (Some(local_ty), is_implicit_self) = self.local_ty(node_id) {

src/librustc_mir/borrow_check/mod.rs

@@ -29,8 +29,6 @@
 use rustc_data_structures::indexed_vec::Idx;
 use rustc_data_structures::small_vec::SmallVec;
 
-use core::unicode::property::Pattern_White_Space;
-
 use std::rc::Rc;
 
 use syntax_pos::Span;
@@ -46,6 +44,7 @@
 use dataflow::{MaybeInitializedPlaces, MaybeUninitializedPlaces};
 use util::borrowck_errors::{BorrowckErrors, Origin};
 use util::collect_writes::FindAssignments;
+use util::suggest_ref_mut;
 
 use self::borrow_set::{BorrowData, BorrowSet};
 use self::flows::Flows;
@@ -1861,7 +1860,7 @@ enum AccessKind {
                     ClearCrossCrate::Set(mir::BindingForm::Var(mir::VarBindingForm {
                         binding_mode: ty::BindingMode::BindByReference(_),
                         ..
-                    })) => suggest_ref_mut(self.tcx, local_decl),
+                    })) => suggest_ref_mut(self.tcx, local_decl.source_info.span),
 
                     ClearCrossCrate::Clear => bug!("saw cleared local state"),
                 };
@@ -1957,22 +1956,6 @@ fn suggest_ampmut<'cx, 'gcx, 'tcx>(
             assert_eq!(ty_mut.mutbl, hir::MutImmutable);
             (highlight_span, format!("&mut {}", ty_mut.ty))
         }
-
-        fn suggest_ref_mut<'cx, 'gcx, 'tcx>(
-            tcx: TyCtxt<'cx, 'gcx, 'tcx>,
-            local_decl: &mir::LocalDecl<'tcx>,
-        ) -> Option<(Span, String)> {
-            let hi_span = local_decl.source_info.span;
-            let hi_src = tcx.sess.codemap().span_to_snippet(hi_span).unwrap();
-            if hi_src.starts_with("ref")
-                && hi_src["ref".len()..].starts_with(Pattern_White_Space)
-            {
-                let suggestion = format!("ref mut{}", &hi_src["ref".len()..]);
-                Some((hi_span, suggestion))
-            } else {
-                None
-            }
-        }
     }
 
     /// Adds the place into the used mutable variables set

src/librustc_mir/util/mod.rs

@@ -8,6 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use core::unicode::property::Pattern_White_Space;
+use rustc::ty;
+use syntax_pos::Span;
+
 pub mod borrowck_errors;
 pub mod elaborate_drops;
 pub mod def_use;
@@ -23,3 +27,19 @@
 pub use self::pretty::{dump_enabled, dump_mir, write_mir_pretty, PassWhere};
 pub use self::graphviz::{write_mir_graphviz};
 pub use self::graphviz::write_node_label as write_graphviz_node_label;
+
+/// If possible, suggest replacing `ref` with `ref mut`.
+pub fn suggest_ref_mut<'cx, 'gcx, 'tcx>(
+    tcx: ty::TyCtxt<'cx, 'gcx, 'tcx>,
+    pattern_span: Span,
+) -> Option<(Span, String)> {
+    let hi_src = tcx.sess.codemap().span_to_snippet(pattern_span).unwrap();
+    if hi_src.starts_with("ref")
+        && hi_src["ref".len()..].starts_with(Pattern_White_Space)
+    {
+        let replacement = format!("ref mut{}", &hi_src["ref".len()..]);
+        Some((pattern_span, replacement))
+    } else {
+        None
+    }
+}

src/test/ui/rfc-2005-default-binding-mode/enum.stderr

 error[E0594]: cannot assign to immutable borrowed content `*x`
   --> $DIR/enum.rs:19:5
    |
-LL |     let Wrap(x) = &Wrap(3);
-   |              - help: use a mutable reference instead: `x`
 LL |     *x += 1; //~ ERROR cannot assign to immutable
    |     ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*x`
   --> $DIR/enum.rs:23:9
    |
-LL |     if let Some(x) = &Some(3) {
-   |                 - help: use a mutable reference instead: `x`
 LL |         *x += 1; //~ ERROR cannot assign to immutable
    |         ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*x`
   --> $DIR/enum.rs:29:9
    |
-LL |     while let Some(x) = &Some(3) {
-   |                    - help: use a mutable reference instead: `x`
 LL |         *x += 1; //~ ERROR cannot assign to immutable
    |         ^^^^^^^ cannot borrow as mutable
 

src/test/ui/rfc-2005-default-binding-mode/explicit-mut.stderr

 error[E0594]: cannot assign to immutable borrowed content `*n`
   --> $DIR/explicit-mut.rs:17:13
    |
-LL |         Some(n) => {
-   |              - help: use a mutable reference instead: `n`
 LL |             *n += 1; //~ ERROR cannot assign to immutable
    |             ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*n`
   --> $DIR/explicit-mut.rs:25:13
    |
-LL |         Some(n) => {
-   |              - help: use a mutable reference instead: `n`
 LL |             *n += 1; //~ ERROR cannot assign to immutable
    |             ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*n`
   --> $DIR/explicit-mut.rs:33:13
    |
-LL |         Some(n) => {
-   |              - help: use a mutable reference instead: `n`
 LL |             *n += 1; //~ ERROR cannot assign to immutable
    |             ^^^^^^^ cannot borrow as mutable