Point to immutable arg/fields when trying to use as &mut

src/librustc/hir/map/blocks.rs

@@ -38,7 +38,7 @@ use syntax_pos::Span;
 ///   - The default implementation for a trait method.
 ///
 /// To construct one, use the `Code::from_node` function.
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Debug)]
 pub struct FnLikeNode<'a> { node: map::Node<'a> }
 
 /// MaybeFnLike wraps a method that indicates if an object

src/librustc/middle/mem_categorization.rs

@@ -193,6 +193,63 @@ pub struct cmt_<'tcx> {
 
 pub type cmt<'tcx> = Rc<cmt_<'tcx>>;
 
+impl<'tcx> cmt_<'tcx> {
+    pub fn get_field(&self, name: ast::Name) -> Option<DefId> {
+        match self.cat {
+            Categorization::Deref(ref cmt, ..) |
+            Categorization::Interior(ref cmt, _) |
+            Categorization::Downcast(ref cmt, _) => {
+                if let Categorization::Local(_) = cmt.cat {
+                    if let ty::TyAdt(def, _) = self.ty.sty {
+                        return def.struct_variant().find_field_named(name).map(|x| x.did);
+                    }
+                    None
+                } else {
+                    cmt.get_field(name)
+                }
+            }
+            _ => None
+        }
+    }
+
+    pub fn get_field_name(&self) -> Option<ast::Name> {
+        match self.cat {
+            Categorization::Interior(_, ref ik) => {
+                if let InteriorKind::InteriorField(FieldName::NamedField(name)) = *ik {
+                    Some(name)
+                } else {
+                    None
+                }
+            }
+            Categorization::Deref(ref cmt, ..) |
+            Categorization::Downcast(ref cmt, _) => {
+                cmt.get_field_name()
+            }
+            _ => None,
+        }
+    }
+
+    pub fn get_arg_if_immutable(&self, map: &hir_map::Map) -> 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 {
+                    if let ty::TyAdt(_, _) = self.ty.sty {
+                        if let ty::TyRef(_, ty::TypeAndMut{mutbl: MutImmutable, ..}) = cmt.ty.sty {
+                            return Some(nid);
+                        }
+                    }
+                    None
+                } else {
+                    cmt.get_arg_if_immutable(map)
+                }
+            }
+            _ => None
+        }
+    }
+}
+
 pub trait ast_node {
     fn id(&self) -> ast::NodeId;
     fn span(&self) -> Span;

src/librustc/ty/mod.rs

@@ -1325,6 +1325,7 @@ bitflags! {
     }
 }
 
+#[derive(Debug)]
 pub struct VariantDef {
     /// The variant's DefId. If this is a tuple-like struct,
     /// this is the DefId of the struct's ctor.
@@ -1335,6 +1336,7 @@ pub struct VariantDef {
     pub ctor_kind: CtorKind,
 }
 
+#[derive(Debug)]
 pub struct FieldDef {
     pub did: DefId,
     pub name: Name,

src/librustc/ty/sty.rs

@@ -134,7 +134,7 @@ pub enum TypeVariants<'tcx> {
     TyRawPtr(TypeAndMut<'tcx>),
 
     /// A reference; a pointer with an associated lifetime. Written as
-    /// `&a mut T` or `&'a T`.
+    /// `&'a mut T` or `&'a T`.
     TyRef(&'tcx Region, TypeAndMut<'tcx>),
 
     /// The anonymous type of a function declaration/definition. Each

src/librustc_borrowck/borrowck/gather_loans/mod.rs

@@ -195,15 +195,17 @@ fn check_aliasability<'a, 'tcx>(bccx: &BorrowckCtxt<'a, 'tcx>,
             bccx.report_aliasability_violation(
                         borrow_span,
                         loan_cause,
-                        mc::AliasableReason::UnaliasableImmutable);
+                        mc::AliasableReason::UnaliasableImmutable,
+                        cmt);
             Err(())
         }
         (mc::Aliasability::FreelyAliasable(alias_cause), ty::UniqueImmBorrow) |
         (mc::Aliasability::FreelyAliasable(alias_cause), ty::MutBorrow) => {
             bccx.report_aliasability_violation(
                         borrow_span,
                         loan_cause,
-                        alias_cause);
+                        alias_cause,
+                        cmt);
             Err(())
         }
         (..) => {

src/librustc_borrowck/borrowck/mod.rs

@@ -540,7 +540,7 @@ pub fn opt_loan_path<'tcx>(cmt: &mc::cmt<'tcx>) -> Option<Rc<LoanPath<'tcx>>> {
 // Errors
 
 // Errors that can occur
-#[derive(PartialEq)]
+#[derive(Debug, PartialEq)]
 pub enum bckerr_code<'tcx> {
     err_mutbl,
     /// superscope, subscope, loan cause
@@ -550,7 +550,7 @@ pub enum bckerr_code<'tcx> {
 
 // Combination of an error code and the categorization of the expression
 // that caused it
-#[derive(PartialEq)]
+#[derive(Debug, PartialEq)]
 pub struct BckError<'tcx> {
     span: Span,
     cause: AliasableViolationKind,
@@ -601,12 +601,8 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
             _ => { }
         }
 
-        // General fallback.
-        let span = err.span.clone();
-        let mut db = self.struct_span_err(
-            err.span,
-            &self.bckerr_to_string(&err));
-        self.note_and_explain_bckerr(&mut db, err, span);
+        let mut db = self.bckerr_to_diag(&err);
+        self.note_and_explain_bckerr(&mut db, err);
         db.emit();
     }
 
@@ -771,8 +767,11 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
         self.tcx.sess.span_err_with_code(s, msg, code);
     }
 
-    pub fn bckerr_to_string(&self, err: &BckError<'tcx>) -> String {
-        match err.code {
+    pub fn bckerr_to_diag(&self, err: &BckError<'tcx>) -> DiagnosticBuilder<'a> {
+        let span = err.span.clone();
+        let mut immutable_field = None;
+
+        let msg = &match err.code {
             err_mutbl => {
                 let descr = match err.cmt.note {
                     mc::NoteClosureEnv(_) | mc::NoteUpvarRef(_) => {
@@ -783,6 +782,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                             format!("{} {}",
                                     err.cmt.mutbl.to_user_str(),
                                     self.cmt_to_string(&err.cmt))
+
                         }
                         Some(lp) => {
                             format!("{} {} `{}`",
@@ -807,6 +807,19 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     BorrowViolation(euv::AutoUnsafe) |
                     BorrowViolation(euv::ForLoop) |
                     BorrowViolation(euv::MatchDiscriminant) => {
+                        // Check for this field's definition to see if it is an immutable reference
+                        // and suggest making it mutable if that is the case.
+                        immutable_field = err.cmt.get_field_name()
+                            .and_then(|name| err.cmt.get_field(name))
+                            .and_then(|did| self.tcx.hir.as_local_node_id(did))
+                            .and_then(|nid| {
+                                if let hir_map::Node::NodeField(ref field) = self.tcx.hir.get(nid) {
+                                    return self.suggest_mut_for_immutable(&field.ty)
+                                        .map(|msg| (self.tcx.hir.span(nid), msg));
+                                }
+                                None
+                            });
+
                         format!("cannot borrow {} as mutable", descr)
                     }
                     BorrowViolation(euv::ClosureInvocation) => {
@@ -830,13 +843,20 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                          its contents can be safely reborrowed",
                         descr)
             }
+        };
+
+        let mut db = self.struct_span_err(span, msg);
+        if let Some((span, msg)) = immutable_field {
+            db.span_label(span, &msg);
         }
+        db
     }
 
     pub fn report_aliasability_violation(&self,
                                          span: Span,
                                          kind: AliasableViolationKind,
-                                         cause: mc::AliasableReason) {
+                                         cause: mc::AliasableReason,
+                                         cmt: mc::cmt<'tcx>) {
         let mut is_closure = false;
         let prefix = match kind {
             MutabilityViolation => {
@@ -903,6 +923,9 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     self.tcx.sess, span, E0389,
                     "{} in a `&` reference", prefix);
                 e.span_label(span, &"assignment into an immutable reference");
+                if let Some(nid) = cmt.get_arg_if_immutable(&self.tcx.hir) {
+                    self.immutable_argument_should_be_mut(nid, &mut e);
+                }
                 e
             }
         };
@@ -913,6 +936,55 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
         err.emit();
     }
 
+    /// Given a type, if it is an immutable reference, return a suggestion to make it mutable
+    fn suggest_mut_for_immutable(&self, pty: &hir::Ty) -> Option<String> {
+        // Check wether the argument is an immutable reference
+        if let hir::TyRptr(opt_lifetime, hir::MutTy {
+            mutbl: hir::Mutability::MutImmutable,
+            ref ty
+        }) = pty.node {
+            // Account for existing lifetimes when generating the message
+            if let Some(lifetime) = opt_lifetime {
+                if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(ty.span) {
+                    if let Ok(lifetime_snippet) = self.tcx.sess.codemap()
+                        .span_to_snippet(lifetime.span) {
+                            return Some(format!("use `&{} mut {}` here to make mutable",
+                                                lifetime_snippet,
+                                                snippet));
+                    }
+                }
+            } else if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(pty.span) {
+                if snippet.starts_with("&") {
+                    return Some(format!("use `{}` here to make mutable",
+                                        snippet.replace("&", "&mut ")));
+                }
+            } else {
+                bug!("couldn't find a snippet for span: {:?}", pty.span);
+            }
+        }
+        None
+    }
+
+    fn immutable_argument_should_be_mut(&self, nid: ast::NodeId, db: &mut DiagnosticBuilder) {
+        let parent = self.tcx.hir.get_parent_node(nid);
+        let parent_node = self.tcx.hir.get(parent);
+
+        // The parent node is like a fn
+        if let Some(fn_like) = FnLikeNode::from_node(parent_node) {
+            // `nid`'s parent's `Body`
+            let fn_body = self.tcx.hir.body(fn_like.body());
+            // Get the position of `nid` in the arguments list
+            let arg_pos = fn_body.arguments.iter().position(|arg| arg.pat.id == nid);
+            if let Some(i) = arg_pos {
+                // The argument's `Ty`
+                let arg_ty = &fn_like.decl().inputs[i];
+                if let Some(msg) = self.suggest_mut_for_immutable(&arg_ty) {
+                    db.span_label(arg_ty.span, &msg);
+                }
+            }
+        }
+    }
+
     fn report_out_of_scope_escaping_closure_capture(&self,
                                                     err: &BckError<'tcx>,
                                                     capture_span: Span)
@@ -961,8 +1033,8 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
         }
     }
 
-    pub fn note_and_explain_bckerr(&self, db: &mut DiagnosticBuilder, err: BckError<'tcx>,
-        error_span: Span) {
+    pub fn note_and_explain_bckerr(&self, db: &mut DiagnosticBuilder, err: BckError<'tcx>) {
+        let error_span = err.span.clone();
         match err.code {
             err_mutbl => self.note_and_explain_mutbl_error(db, &err, &error_span),
             err_out_of_scope(super_scope, sub_scope, cause) => {
@@ -1103,41 +1175,13 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                 }
             }
             _ => {
-                if let Categorization::Deref(ref inner_cmt, ..) = err.cmt.cat {
-                    if let Categorization::Local(local_id) = inner_cmt.cat {
-                        let parent = self.tcx.hir.get_parent_node(local_id);
-
-                        if let Some(fn_like) = FnLikeNode::from_node(self.tcx.hir.get(parent)) {
-                            if let Some(i) = self.tcx.hir.body(fn_like.body()).arguments.iter()
-                                                     .position(|arg| arg.pat.id == local_id) {
-                                let arg_ty = &fn_like.decl().inputs[i];
-                                if let hir::TyRptr(
-                                    opt_lifetime,
-                                    hir::MutTy{mutbl: hir::Mutability::MutImmutable, ref ty}) =
-                                    arg_ty.node {
-                                    if let Some(lifetime) = opt_lifetime {
-                                        if let Ok(snippet) = self.tcx.sess.codemap()
-                                            .span_to_snippet(ty.span) {
-                                            if let Ok(lifetime_snippet) = self.tcx.sess.codemap()
-                                                .span_to_snippet(lifetime.span) {
-                                                    db.span_label(arg_ty.span,
-                                                                  &format!("use `&{} mut {}` \
-                                                                            here to make mutable",
-                                                                            lifetime_snippet,
-                                                                            snippet));
-                                            }
-                                        }
-                                    }
-                                    else if let Ok(snippet) = self.tcx.sess.codemap()
-                                        .span_to_snippet(arg_ty.span) {
-                                        if snippet.starts_with("&") {
-                                            db.span_label(arg_ty.span,
-                                                          &format!("use `{}` here to make mutable",
-                                                                   snippet.replace("&", "&mut ")));
-                                        }
-                                    }
-                                }
-                            }
+                if let Categorization::Deref(..) = err.cmt.cat {
+                    db.span_label(*error_span, &"cannot borrow as mutable");
+                    if let Some(local_id) = err.cmt.get_arg_if_immutable(&self.tcx.hir) {
+                        self.immutable_argument_should_be_mut(local_id, db);
+                    } else if let Categorization::Deref(ref inner_cmt, ..) = err.cmt.cat {
+                        if let Categorization::Local(local_id) = inner_cmt.cat {
+                            self.immutable_argument_should_be_mut(local_id, db);
                         }
                     }
                 } else if let Categorization::Local(local_id) = err.cmt.cat {

src/test/ui/did_you_mean/issue-38147-1.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 Pass<'a> {
+    s: &'a mut String
+}
+
+impl<'a> Pass<'a> {
+    fn f(&mut self) {
+        self.s.push('x');
+    }
+}
+
+struct Foo<'a> {
+    s: &'a mut String
+}
+
+impl<'a> Foo<'a> {
+    fn f(&self) {
+        self.s.push('x');
+    }
+}
+
+fn main() {}

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

@@ -0,0 +1,10 @@
+error[E0389]: cannot borrow data mutably in a `&` reference
+  --> $DIR/issue-38147-1.rs:27:9
+   |
+26 |     fn f(&self) {
+   |          ----- use `&mut self` here to make mutable
+27 |         self.s.push('x');
+   |         ^^^^^^ assignment into an immutable reference
+
+error: aborting due to previous error
+