Reimplement some "add mut" suggestions under NLL

Specifically, `&self` -> `&mut self` and explicit `ref` -> `ref mut`.
Implicit `ref` isn't handled yet and causes an ICE.

Author: ashtneoi

src/librustc_mir/borrow_check/mod.rs

@@ -29,6 +29,8 @@ use rustc_data_structures::indexed_set::IdxSetBuf;
 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;
@@ -1837,17 +1839,45 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
             Place::Projection(box Projection {
                 base: Place::Local(local),
                 elem: ProjectionElem::Deref,
-            }) if self.mir.local_decls[*local].is_nonref_binding() =>
-            {
-                let (err_help_span, suggested_code) =
-                    find_place_to_suggest_ampmut(self.tcx, self.mir, *local);
+            }) if self.mir.local_decls[*local].is_user_variable.is_some() => {
+                let local_decl = &self.mir.local_decls[*local];
+                let (err_help_span, suggested_code) = match local_decl.is_user_variable {
+                    Some(ClearCrossCrate::Set(mir::BindingForm::ImplicitSelf)) => {
+                        suggest_ampmut_self(local_decl)
+                    },
+
+                    Some(ClearCrossCrate::Set(mir::BindingForm::Var(mir::VarBindingForm {
+                        binding_mode: ty::BindingMode::BindByValue(_),
+                        opt_ty_info,
+                        ..
+                    }))) => {
+                        if let Some(x) = try_suggest_ampmut_rhs(
+                            self.tcx, self.mir, *local,
+                        ) {
+                            x
+                        } else {
+                            suggest_ampmut_type(local_decl, opt_ty_info)
+                        }
+                    },
+
+                    Some(ClearCrossCrate::Set(mir::BindingForm::Var(mir::VarBindingForm {
+                        binding_mode: ty::BindingMode::BindByReference(_),
+                        ..
+                    }))) => {
+                        suggest_ref_mut(self.tcx, local_decl)
+                    },
+
+                    Some(ClearCrossCrate::Clear) => bug!("saw cleared local state"),
+
+                    None => bug!(),
+                };
+
                 err.span_suggestion(
                     err_help_span,
                     "consider changing this to be a mutable reference",
                     suggested_code,
                 );
 
-                let local_decl = &self.mir.local_decls[*local];
                 if let Some(name) = local_decl.name {
                     err.span_label(
                         span,
@@ -1874,13 +1904,16 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         err.emit();
         return true;
 
-        // Returns the span to highlight and the associated text to
-        // present when suggesting that the user use an `&mut`.
-        //
+        fn suggest_ampmut_self<'cx, 'gcx, 'tcx>(
+            local_decl: &mir::LocalDecl<'tcx>,
+        ) -> (Span, String) {
+            (local_decl.source_info.span, "&mut self".to_string())
+        }
+
         // When we want to suggest a user change a local variable to be a `&mut`, there
         // are three potential "obvious" things to highlight:
         //
-        // let ident [: Type] [= RightHandSideExresssion];
+        // let ident [: Type] [= RightHandSideExpression];
         //     ^^^^^    ^^^^     ^^^^^^^^^^^^^^^^^^^^^^^
         //     (1.)     (2.)              (3.)
         //
@@ -1889,48 +1922,58 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         // for example, if the RHS is present and the Type is not, then the type is going to
         // be inferred *from* the RHS, which means we should highlight that (and suggest
         // that they borrow the RHS mutably).
-        fn find_place_to_suggest_ampmut<'cx, 'gcx, 'tcx>(
+        //
+        // This implementation attempts to emulate AST-borrowck prioritization
+        // by trying (3.), then (2.) and finally falling back on (1.).
+
+        fn try_suggest_ampmut_rhs<'cx, 'gcx, 'tcx>(
             tcx: TyCtxt<'cx, 'gcx, 'tcx>,
             mir: &Mir<'tcx>,
             local: Local,
-        ) -> (Span, String) {
-            // This implementation attempts to emulate AST-borrowck prioritization
-            // by trying (3.), then (2.) and finally falling back on (1.).
+        ) -> Option<(Span, String)> {
             let locations = mir.find_assignments(local);
             if locations.len() > 0 {
                 let assignment_rhs_span = mir.source_info(locations[0]).span;
                 let snippet = tcx.sess.codemap().span_to_snippet(assignment_rhs_span);
                 if let Ok(src) = snippet {
-                    // pnkfelix inherited code; believes intention is
-                    // highlighted text will always be `&<expr>` and
-                    // thus can transform to `&mut` by slicing off
-                    // first ASCII character and prepending "&mut ".
                     if src.starts_with('&') {
                         let borrowed_expr = src[1..].to_string();
-                        return (assignment_rhs_span, format!("&mut {}", borrowed_expr));
+                        return Some((assignment_rhs_span, format!("&mut {}", borrowed_expr)));
                     }
                 }
             }
+            None
+        }
 
-            let local_decl = &mir.local_decls[local];
-            let highlight_span = match local_decl.is_user_variable {
+        fn suggest_ampmut_type<'tcx>(
+            local_decl: &mir::LocalDecl<'tcx>,
+            opt_ty_info: Option<Span>,
+        ) -> (Span, String) {
+            let highlight_span = match opt_ty_info {
                 // if this is a variable binding with an explicit type,
                 // try to highlight that for the suggestion.
-                Some(ClearCrossCrate::Set(mir::BindingForm::Var(mir::VarBindingForm {
-                    opt_ty_info: Some(ty_span),
-                    ..
-                }))) => ty_span,
-
-                Some(ClearCrossCrate::Clear) => bug!("saw cleared local state"),
+                Some(ty_span) => ty_span,
 
                 // otherwise, just highlight the span associated with
                 // the (MIR) LocalDecl.
-                _ => local_decl.source_info.span,
+                None => local_decl.source_info.span,
             };
 
             let ty_mut = local_decl.ty.builtin_deref(true).unwrap();
             assert_eq!(ty_mut.mutbl, hir::MutImmutable);
-            return (highlight_span, format!("&mut {}", ty_mut.ty));
+            (highlight_span, format!("&mut {}", ty_mut.ty))
+        }
+
+        fn suggest_ref_mut<'cx, 'gcx, 'tcx>(
+            tcx: TyCtxt<'cx, 'gcx, 'tcx>,
+            local_decl: &mir::LocalDecl<'tcx>,
+        ) -> (Span, String) {
+            let hi_span = local_decl.source_info.span;
+            let hi_src = tcx.sess.codemap().span_to_snippet(hi_span).unwrap();
+            assert!(hi_src.starts_with("ref"));
+            assert!(hi_src["ref".len()..].starts_with(Pattern_White_Space));
+            let suggestion = format!("ref mut{}", &hi_src["ref".len()..]);
+            (hi_span, suggestion)
         }
     }
 

src/librustc_mir/lib.rs

@@ -33,6 +33,7 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![feature(never_type)]
 #![feature(specialization)]
 #![feature(try_trait)]
+#![feature(unicode_internals)]
 
 #![recursion_limit="256"]
 
@@ -56,6 +57,7 @@ extern crate rustc_target;
 extern crate log_settings;
 extern crate rustc_apfloat;
 extern crate byteorder;
+extern crate core;
 
 mod diagnostics;
 

src/test/ui/did_you_mean/issue-38147-1.nll.stderr

@@ -2,7 +2,7 @@ error[E0596]: cannot borrow immutable item `*self.s` as mutable
   --> $DIR/issue-38147-1.rs:27:9
    |
 LL |     fn f(&self) {
-   |          ----- help: consider changing this to be a mutable reference: `&mut Foo<'_>`
+   |          ----- help: consider changing this to be a mutable reference: `&mut self`
 LL |         self.s.push('x'); //~ ERROR cannot borrow data mutably
    |         ^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 

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

@@ -10,15 +10,15 @@ error[E0596]: cannot borrow immutable item `self.x` as mutable
   --> $DIR/issue-39544.rs:26:17
    |
 LL |     fn foo<'z>(&'z self) {
-   |                -------- help: consider changing this to be a mutable reference: `&mut Z`
+   |                -------- help: consider changing this to be a mutable reference: `&mut self`
 LL |         let _ = &mut self.x; //~ ERROR cannot borrow
    |                 ^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
 error[E0596]: cannot borrow immutable item `self.x` as mutable
   --> $DIR/issue-39544.rs:30:17
    |
 LL |     fn foo1(&self, other: &Z) {
-   |             ----- help: consider changing this to be a mutable reference: `&mut Z`
+   |             ----- help: consider changing this to be a mutable reference: `&mut self`
 LL |         let _ = &mut self.x; //~ ERROR cannot borrow
    |                 ^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
@@ -35,7 +35,7 @@ error[E0596]: cannot borrow immutable item `self.x` as mutable
   --> $DIR/issue-39544.rs:35:17
    |
 LL |     fn foo2<'a>(&'a self, other: &Z) {
-   |                 -------- help: consider changing this to be a mutable reference: `&mut Z`
+   |                 -------- help: consider changing this to be a mutable reference: `&mut self`
 LL |         let _ = &mut self.x; //~ ERROR cannot borrow
    |                 ^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 

src/test/ui/suggestions/suggest-ref-mut.stderr

@@ -1,20 +1,28 @@
 error[E0594]: cannot assign to `*foo` which is behind a `&` reference
   --> $DIR/suggest-ref-mut.rs:17:5
    |
+LL |     let ref foo = 16;
+   |         ------- help: consider changing this to be a mutable reference: `ref mut foo`
+...
 LL |     *foo = 32;
-   |     ^^^^^^^^^ cannot assign
+   |     ^^^^^^^^^ `foo` is a `&` reference, so the data it refers to cannot be written
 
 error[E0594]: cannot assign to `*bar` which is behind a `&` reference
   --> $DIR/suggest-ref-mut.rs:22:9
    |
+LL |     if let Some(ref bar) = Some(16) {
+   |                 ------- help: consider changing this to be a mutable reference: `ref mut bar`
+...
 LL |         *bar = 32;
-   |         ^^^^^^^^^ cannot assign
+   |         ^^^^^^^^^ `bar` is a `&` reference, so the data it refers to cannot be written
 
 error[E0594]: cannot assign to `*quo` which is behind a `&` reference
   --> $DIR/suggest-ref-mut.rs:26:22
    |
 LL |         ref quo => { *quo = 32; },
-   |                      ^^^^^^^^^ cannot assign
+   |         -------      ^^^^^^^^^ `quo` is a `&` reference, so the data it refers to cannot be written
+   |         |
+   |         help: consider changing this to be a mutable reference: `ref mut quo`
 
 error: aborting due to 3 previous errors