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Fix error codes explanation' code examples

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GuillaumeGomez committed Jan 4, 2020
1 parent 9876f6b commit 5b7d64e9473216dc8650ca5f910100389ab91726
@@ -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() {}
```
@@ -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() {}
```
@@ -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!
}
```

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