add a suggestion about undeclared alloc module

compiler/rustc_resolve/src/diagnostics.rs

@@ -420,6 +420,10 @@ impl<'a> Resolver<'a> {
                 err.span_label(span, label);
 
                 if let Some((suggestions, msg, applicability)) = suggestion {
+                    if suggestions.is_empty() {
+                        err.help(&msg);
+                        return err;
+                    }
                     err.multipart_suggestion(&msg, suggestions, applicability);
                 }
 

compiler/rustc_resolve/src/lib.rs

@@ -2523,6 +2523,15 @@ impl<'a> Resolver<'a> {
                         } else {
                             (
                                 format!("use of undeclared crate or module `{}`", ident),
+                                if ident.name == sym::alloc {
+                                    Some((
+                                        vec![],
+                                        String::from(
+                                            "add `extern crate alloc` to use the builtin `alloc` module",
+                                        ),
+                                        Applicability::MaybeIncorrect,
+                                    ))
+                                } else {
                                     self.find_similarly_named_module_or_crate(
                                         ident.name,
                                         &parent_scope.module,
@@ -2535,7 +2544,8 @@ impl<'a> Resolver<'a> {
                                             ),
                                             Applicability::MaybeIncorrect,
                                         )
-                                }),
+                                    })
+                                },
                             )
                         }
                     } else {

src/test/ui/suggestions/undeclared-module-alloc.rs

@@ -0,0 +1,5 @@
+// edition:2018
+
+use alloc::rc::Rc; //~ ERROR failed to resolve: use of undeclared crate or module `alloc`
+
+fn main() {}

src/test/ui/suggestions/undeclared-module-alloc.stderr

@@ -0,0 +1,11 @@
+error[E0433]: failed to resolve: use of undeclared crate or module `alloc`
+  --> $DIR/undeclared-module-alloc.rs:3:5
+   |
+LL | use alloc::rc::Rc;
+   |     ^^^^^ use of undeclared crate or module `alloc`
+   |
+   = help: add `extern crate alloc` to use the builtin `alloc` module
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0433`.