2 files changed, 28 insertions, 7 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0404.md b/compiler/rustc_error_codes/src/error_codes/E0404.md
index 1360cc99afc..d6fa51e618c 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0404.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0404.md
@@ -8,14 +8,15 @@ struct Foo;
 struct Bar;
 
 impl Foo for Bar {} // error: `Foo` is not a trait
+fn baz<T: Foo>(t: T) {} // error: `Foo` is not a trait
 ```
 
 Another erroneous code example:
 
 ```compile_fail,E0404
-struct Foo;
+type Foo = Iterator<Item=String>;
 
-fn bar<T: Foo>(t: T) {} // error: `Foo` is not a trait
+fn bar<T: Foo>(t: T) {} // error: `Foo` is a type alias
 ```
 
 Please verify that the trait's name was not misspelled or that the right
@@ -30,14 +31,27 @@ struct Bar;
 impl Foo for Bar { // ok!
     // functions implementation
 }
+
+fn baz<T: Foo>(t: T) {} // ok!
 ```
 
-or:
+Alternatively, you could introduce a new trait with your desired restrictions
+as a super trait:
 
 ```
-trait Foo {
-    // some functions
-}
+# trait Foo {}
+# struct Bar;
+# impl Foo for Bar {}
+trait Qux: Foo {} // Anything that implements Qux also needs to implement Foo
+fn baz<T: Qux>(t: T) {} // also ok!
+```
+
+Finally, if you are on nightly and want to use a trait alias
+instead of a type alias, you should use `#![feature(trait_alias)]`:
+
+```
+#![feature(trait_alias)]
+trait Foo = Iterator<Item=String>;
 
 fn bar<T: Foo>(t: T) {} // ok!
 ```
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index 6fae6921fc9..e33c374f562 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -930,7 +930,14 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                     let msg = "you might have meant to use `#![feature(trait_alias)]` instead of a \
                                `type` alias";
                     if let Some(span) = self.def_span(def_id) {
-                        err.span_help(span, msg);
+                        if let Ok(snip) = self.r.session.source_map().span_to_snippet(span) {
+                            // The span contains a type alias so we should be able to
+                            // replace `type` with `trait`.
+                            let snip = snip.replacen("type", "trait", 1);
+                            err.span_suggestion(span, msg, snip, Applicability::MaybeIncorrect);
+                        } else {
+                            err.span_help(span, msg);
+                        }
                     } else {
                         err.help(msg);
                     }