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+A value was moved whose size was not known at compile time.
+
+Erroneous code example:
+
+```compile_fail,E0161
+#![feature(box_syntax)]
+trait Bar {
+    fn f(self);
+}
+
+impl Bar for i32 {
+    fn f(self) {}
+}
+
+fn main() {
+    let b: Box<dyn Bar> = box (0 as i32);
+    b.f();
+    // error: cannot move a value of type dyn Bar: the size of dyn Bar cannot
+    //        be statically determined
+}
+```
+
+In Rust, you can only move a value when its size is known at compile time.
+
+To work around this restriction, consider "hiding" the value behind a reference:
+either `&x` or `&mut x`. Since a reference has a fixed size, this lets you move
+it around as usual. Example:
+
+```
+#![feature(box_syntax)]
+
+trait Bar {
+    fn f(&self);
+}
+
+impl Bar for i32 {
+    fn f(&self) {}
+}
+
+fn main() {
+    let b: Box<dyn Bar> = box (0 as i32);
+    b.f();
+    // ok!
+}
+```