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diff --git a/compiler/rustc_error_codes/src/error_codes/E0512.md b/compiler/rustc_error_codes/src/error_codes/E0512.md
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+Transmute with two differently sized types was attempted.
+
+Erroneous code example:
+
+```compile_fail,E0512
+fn takes_u8(_: u8) {}
+
+fn main() {
+    unsafe { takes_u8(::std::mem::transmute(0u16)); }
+    // error: cannot transmute between types of different sizes,
+    //        or dependently-sized types
+}
+```
+
+Please use types with same size or use the expected type directly. Example:
+
+```
+fn takes_u8(_: u8) {}
+
+fn main() {
+    unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
+    // or:
+    unsafe { takes_u8(0u8); } // ok!
+}
+```