Add note when item accessed from module via m.i rather than m::i.

src/librustc_resolve/lib.rs

@@ -179,7 +179,7 @@ pub enum ResolutionError<'a> {
     /// error E0424: `self` is not available in a static method
     SelfNotAvailableInStaticMethod,
     /// error E0425: unresolved name
-    UnresolvedName(&'a str, &'a str),
+    UnresolvedName(&'a str, &'a str, UnresolvedNameContext),
     /// error E0426: use of undeclared label
     UndeclaredLabel(&'a str),
     /// error E0427: cannot use `ref` binding mode with ...
@@ -202,6 +202,12 @@ pub enum ResolutionError<'a> {
     AttemptToUseNonConstantValueInConstant,
 }
 
+#[derive(Clone, PartialEq, Eq, Debug)]
+pub enum UnresolvedNameContext {
+    PathIsMod(ast::NodeId),
+    Other,
+}
+
 fn resolve_error<'b, 'a: 'b, 'tcx: 'a>(resolver: &'b Resolver<'a, 'tcx>,
                                        span: syntax::codemap::Span,
                                        resolution_error: ResolutionError<'b>) {
@@ -402,13 +408,46 @@ fn resolve_error<'b, 'a: 'b, 'tcx: 'a>(resolver: &'b Resolver<'a, 'tcx>,
                       "`self` is not available in a static method. Maybe a `self` argument is \
                        missing?");
         }
-        ResolutionError::UnresolvedName(path, name) => {
+        ResolutionError::UnresolvedName(path, msg, context) => {
             span_err!(resolver.session,
                       span,
                       E0425,
                       "unresolved name `{}`{}",
                       path,
-                      name);
+                      msg);
+
+            match context {
+                UnresolvedNameContext::Other => {} // no help available
+                UnresolvedNameContext::PathIsMod(id) => {
+                    let mut help_msg = String::new();
+                    let parent_id = resolver.ast_map.get_parent_node(id);
+                    if let Some(hir_map::Node::NodeExpr(e)) = resolver.ast_map.find(parent_id) {
+                        match e.node {
+                            ExprField(_, ident) => {
+                                help_msg = format!("To reference an item from the \
+                                                    `{module}` module, use \
+                                                    `{module}::{ident}`",
+                                                   module = &*path,
+                                                   ident = ident.node);
+                            }
+
+                            ExprMethodCall(ident, _, _) => {
+                                help_msg = format!("To call a function from the \
+                                                    `{module}` module, use \
+                                                    `{module}::{ident}(..)`",
+                                                   module = &*path,
+                                                   ident = ident.node);
+                            }
+
+                            _ => {} // no help available
+                        }
+                    }
+
+                    if !help_msg.is_empty() {
+                        resolver.session.fileline_help(span, &help_msg);
+                    }
+                }
+            }
         }
         ResolutionError::UndeclaredLabel(name) => {
             span_err!(resolver.session,
@@ -3509,13 +3548,33 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                                         format!("to call `{}::{}`", path_str, path_name),
                                 };
 
+                                let mut context =  UnresolvedNameContext::Other;
                                 if !msg.is_empty() {
-                                    msg = format!(". Did you mean {}?", msg)
+                                    msg = format!(". Did you mean {}?", msg);
+                                } else {
+                                    // we check if this a module and if so, we display a help
+                                    // message
+                                    let name_path = path.segments.iter()
+                                                        .map(|seg| seg.identifier.name)
+                                                        .collect::<Vec<_>>();
+                                    let current_module = self.current_module.clone();
+
+                                    match self.resolve_module_path(current_module,
+                                                   &name_path[..],
+                                                   UseLexicalScope,
+                                                   expr.span,
+                                                   PathSearch) {
+                                        Success(_) => {
+                                            context = UnresolvedNameContext::PathIsMod(expr.id);
+                                        },
+                                        _ => {},
+                                    };
                                 }
 
                                 resolve_error(self,
                                               expr.span,
-                                              ResolutionError::UnresolvedName(&*path_name, &*msg));
+                                              ResolutionError::UnresolvedName(
+                                                  &*path_name, &*msg, context));
                             }
                         }
                     }

src/test/compile-fail/suggest-path-instead-of-mod-dot-item.rs

@@ -0,0 +1,60 @@
+// Copyright 2015 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.
+
+// Beginners write `mod.item` when they should write `mod::item`.
+// This tests that we suggest the latter when we encounter the former.
+
+pub mod a {
+    pub const I: i32 = 1;
+
+    pub fn f() -> i32 { 2 }
+
+    pub mod b {
+        pub const J: i32 = 3;
+
+        pub fn g() -> i32 { 4 }
+    }
+}
+
+fn h1() -> i32 {
+    a.I
+        //~^ ERROR E0425
+        //~| HELP To reference an item from the `a` module, use `a::I`
+}
+
+fn h2() -> i32 {
+    a.g()
+        //~^ ERROR E0425
+        //~| HELP To call a function from the `a` module, use `a::g(..)`
+}
+
+fn h3() -> i32 {
+    a.b.J
+        //~^ ERROR E0425
+        //~| HELP To reference an item from the `a` module, use `a::b`
+}
+
+fn h4() -> i32 {
+    a::b.J
+        //~^ ERROR E0425
+        //~| HELP To reference an item from the `a::b` module, use `a::b::J`
+}
+
+fn h5() -> i32 {
+    a.b.f()
+        //~^ ERROR E0425
+        //~| HELP To reference an item from the `a` module, use `a::b`
+}
+
+fn h6() -> i32 {
+    a::b.f()
+        //~^ ERROR E0425
+        //~| HELP To call a function from the `a::b` module, use `a::b::f(..)`
+}