add long error explanation for E0228

1 files changed, 40 insertions, 0 deletions

diff --git a/src/librustc_error_codes/error_codes/E0228.md b/src/librustc_error_codes/error_codes/E0228.md
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+The lifetime bound for this object type cannot be deduced from context and must
+be specified.
+
+Erroneous code example:
+
+```compile_fail,E0228
+trait Trait { }
+
+struct TwoBounds<'a, 'b, T: Sized + 'a + 'b> {
+    x: &'a i32,
+    y: &'b i32,
+    z: T,
+}
+
+type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait>;
+```
+
+When a trait object is used as a type argument of a generic type, Rust will try
+to infer its lifetime if unspecified. However, this isn't possible when the
+containing type has more than one lifetime bound.
+
+The above example can be resolved by either reducing the number of lifetime
+bounds to one or by making the trait object lifetime explicit, like so:
+
+```
+trait Trait { }
+
+struct TwoBounds<'a, 'b, T: Sized + 'a + 'b> {
+    x: &'a i32,
+    y: &'b i32,
+    z: T,
+}
+
+type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait + 'b>;
+```
+
+For more information, see [RFC 599] and its amendment [RFC 1156].
+
+[RFC 599]: https://github.com/rust-lang/rfcs/blob/master/text/0599-default-object-bound.md
+[RFC 1156]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md