Accept self in place of mod in use items

[breaking-change]

`mod` is still accepted, but gives a deprecated warning

Author: nrc

src/librustc_resolve/build_reduced_graph.rs

@@ -681,9 +681,10 @@ impl<'a, 'b:'a, 'tcx:'b> GraphBuilder<'a, 'b, 'tcx> {
                     ViewPathSimple(binding, ref full_path, id) => {
                         let source_name =
                             full_path.segments.last().unwrap().identifier.name;
-                        if token::get_name(source_name).get() == "mod" {
+                        if token::get_name(source_name).get() == "mod" ||
+                           token::get_name(source_name).get() == "self" {
                             self.resolve_error(view_path.span,
-                                "`mod` imports are only allowed within a { } list");
+                                "`self` imports are only allowed within a { } list");
                         }
 
                         let subclass = SingleImport(binding.name,
@@ -704,10 +705,10 @@ impl<'a, 'b:'a, 'tcx:'b> GraphBuilder<'a, 'b, 'tcx> {
                         }).collect::<Vec<Span>>();
                         if mod_spans.len() > 1 {
                             self.resolve_error(mod_spans[0],
-                                "`mod` import can only appear once in the list");
+                                "`self` import can only appear once in the list");
                             for other_span in mod_spans.iter().skip(1) {
                                 self.session.span_note(*other_span,
-                                    "another `mod` import appears here");
+                                    "another `self` import appears here");
                             }
                         }
 
@@ -720,7 +721,7 @@ impl<'a, 'b:'a, 'tcx:'b> GraphBuilder<'a, 'b, 'tcx> {
                                         Some(name) => *name,
                                         None => {
                                             self.resolve_error(source_item.span,
-                                                "`mod` import can only appear in an import list \
+                                                "`self` import can only appear in an import list \
                                                  with a non-empty prefix");
                                             continue;
                                         }

src/librustc_resolve/lib.rs

@@ -971,6 +971,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         }
     }
 
+
     // Import resolution
     //
     // This is a fixed-point algorithm. We resolve imports until our efforts

src/libsyntax/parse/parser.rs

@@ -546,6 +546,10 @@ impl<'a> Parser<'a> {
     pub fn parse_path_list_item(&mut self) -> ast::PathListItem {
         let lo = self.span.lo;
         let node = if self.eat_keyword(keywords::Mod) {
+            let span = self.last_span;
+            self.span_warn(span, "deprecated syntax; use the `self` keyword now");
+            ast::PathListMod { id: ast::DUMMY_NODE_ID }
+        } else if self.eat_keyword(keywords::Self) {
             ast::PathListMod { id: ast::DUMMY_NODE_ID }
         } else {
             let ident = self.parse_ident();

src/libsyntax/print/pprust.rs

@@ -2540,7 +2540,7 @@ impl<'a> State<'a> {
                             s.print_ident(name)
                         },
                         ast::PathListMod { .. } => {
-                            word(&mut s.s, "mod")
+                            word(&mut s.s, "self")
                         }
                     }
                 }));