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diff --git a/compiler/rustc_error_codes/src/error_codes/E0620.md b/compiler/rustc_error_codes/src/error_codes/E0620.md
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+A cast to an unsized type was attempted.
+
+Erroneous code example:
+
+```compile_fail,E0620
+let x = &[1_usize, 2] as [usize]; // error: cast to unsized type: `&[usize; 2]`
+                                  //        as `[usize]`
+```
+
+In Rust, some types don't have a known size at compile-time. For example, in a
+slice type like `[u32]`, the number of elements is not known at compile-time and
+hence the overall size cannot be computed. As a result, such types can only be
+manipulated through a reference (e.g., `&T` or `&mut T`) or other pointer-type
+(e.g., `Box` or `Rc`). Try casting to a reference instead:
+
+```
+let x = &[1_usize, 2] as &[usize]; // ok!
+```