Fix error codes explanation' code examples

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0445.md

@@ -12,6 +12,8 @@ trait Foo {
 pub trait Bar : Foo {} // error: private trait in public interface
 pub struct Bar2<T: Foo>(pub T); // same error
 pub fn foo<T: Foo> (t: T) {} // same error
+
+fn main() {}
 ```
 
 To solve this error, please ensure that the trait is also public. The trait
@@ -26,4 +28,6 @@ pub trait Foo { // we set the Foo trait public
 pub trait Bar : Foo {} // ok!
 pub struct Bar2<T: Foo>(pub T); // ok!
 pub fn foo<T: Foo> (t: T) {} // ok!
+
+fn main() {}
 ```

src/librustc_error_codes/error_codes/E0446.md

@@ -12,6 +12,8 @@ mod Foo {
         Bar(0)
     }
 }
+
+fn main() {}
 ```
 
 To solve this error, please ensure that the type is also public. The type
@@ -27,4 +29,6 @@ mod Foo {
         Bar(0)
     }
 }
+
+fn main() {}
 ```

src/librustc_error_codes/error_codes/E0491.md

@@ -3,30 +3,34 @@ A reference has a longer lifetime than the data it references.
 Erroneous code example:
 
 ```compile_fail,E0491
-trait SomeTrait<'a> {
-    type Output;
+struct Foo<'a> {
+    x: fn(&'a i32),
 }
 
-impl<'a, T> SomeTrait<'a> for T {
-    type Output = &'a T; // compile error E0491
+trait Trait<'a, 'b> {
+    type Out;
+}
+
+impl<'a, 'b> Trait<'a, 'b> for usize {
+    type Out = &'a Foo<'b>; // error!
 }
 ```
 
-Here, the problem is that a reference type like `&'a T` is only valid
-if all the data in T outlives the lifetime `'a`. But this impl as written
-is applicable to any lifetime `'a` and any type `T` -- we have no guarantee
-that `T` outlives `'a`. To fix this, you can add a where clause like
-`where T: 'a`.
+Here, the problem is that the compiler cannot be sure that the `'b` lifetime
+will live longer than `'a`, which should be mandatory in order to be sure that
+`Trait::Out` will always have a reference pointing to an existing type. So in
+this case, we just need to tell the compiler than `'b` must outlive `'a`:
 
 ```
-trait SomeTrait<'a> {
-    type Output;
+struct Foo<'a> {
+    x: fn(&'a i32),
+}
+
+trait Trait<'a, 'b> {
+    type Out;
 }
 
-impl<'a, T> SomeTrait<'a> for T
-where
-    T: 'a,
-{
-    type Output = &'a T; // compile error E0491
+impl<'a, 'b: 'a> Trait<'a, 'b> for usize { // we added the lifetime enforcement
+    type Out = &'a Foo<'b>; // it now works!
 }
 ```