also fix the Fixed code

[rust.git] / src / librustc / diagnostics.rs

diff --git a/src/librustc/diagnostics.rs b/src/librustc/diagnostics.rs
index b3a904f2f5fec520598714aee8882b5642f33c12..2fd875c3447677b4d841b320535abbfe543b6917 100644 (file)
--- a/src/librustc/diagnostics.rs
+++ b/src/librustc/diagnostics.rs
@@ -2074,6 +2074,58 @@ struct Foo {
 transparent wrapper around a float. This can make a difference for the ABI.
 "##,
 
+E0909: r##"
+The `impl Trait` return type captures lifetime parameters that do not
+appear within the `impl Trait` itself.
+
+Erroneous code example:
+
+```compile-fail,E0909
+#![feature(conservative_impl_trait)]
+
+use std::cell::Cell;
+
+trait Trait<'a> { }
+
+impl<'a, 'b> Trait<'b> for Cell<&'a u32> { }
+
+fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y>
+where 'x: 'y
+{
+    x
+}
+```
+
+Here, the function `foo` returns a value of type `Cell<&'x u32>`,
+which references the lifetime `'x`. However, the return type is
+declared as `impl Trait<'y>` -- this indicates that `foo` returns
+"some type that implements `Trait<'y>`", but it also indicates that
+the return type **only captures data referencing the lifetime `'y`**.
+In this case, though, we are referencing data with lifetime `'x`, so
+this function is in error.
+
+To fix this, you must reference the lifetime `'x` from the return
+type. For example, changing the return type to `impl Trait<'y> + 'x`
+would work:
+
+```
+#![feature(conservative_impl_trait)]
+
+use std::cell::Cell;
+
+trait Trait<'a> { }
+
+impl<'a,'b> Trait<'b> for Cell<&'a u32> { }
+
+fn foo<'x, 'y>(x: Cell<&'x u32>) -> impl Trait<'y> + 'x
+where 'x: 'y
+{
+    x
+}
+```
+"##,
+
+
 }