Auto merge of #83729 - JohnTitor:issue-43913, r=estebank

Add a suggestion when using a type alias instead of trait alias

Fixes #43913

r? `@estebank`

1 files changed, 20 insertions, 6 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!
 ```