Auto merge of #43901 - GuillaumeGomez:unsized-union-field, r=petrochenkov

udpdate error message for unsized union field

Fixes #36312.

src/test/ui/union-sized-field.rs

@@ -0,0 +1,26 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(untagged_unions)]
+
+union Foo<T: ?Sized> {
+    value: T,
+}
+
+struct Foo2<T: ?Sized> {
+    value: T,
+    t: u32,
+}
+
+enum Foo3<T: ?Sized> {
+    Value(T),
+}
+
+fn main() {}

src/test/ui/union-sized-field.stderr

@@ -0,0 +1,32 @@
+error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
+  --> $DIR/union-sized-field.rs:14:5
+   |
+14 |     value: T,
+   |     ^^^^^^^^ `T` does not have a constant size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: consider adding a `where T: std::marker::Sized` bound
+   = note: no field of a union may have a dynamically sized type
+
+error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
+  --> $DIR/union-sized-field.rs:18:5
+   |
+18 |     value: T,
+   |     ^^^^^^^^ `T` does not have a constant size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: consider adding a `where T: std::marker::Sized` bound
+   = note: only the last field of a struct may have a dynamically sized type
+
+error[E0277]: the trait bound `T: std::marker::Sized` is not satisfied
+  --> $DIR/union-sized-field.rs:23:11
+   |
+23 |     Value(T),
+   |           ^^ `T` does not have a constant size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `T`
+   = help: consider adding a `where T: std::marker::Sized` bound
+   = note: no field of an enum variant may have a dynamically sized type
+
+error: aborting due to 3 previous errors
+