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diff --git a/compiler/rustc_error_codes/src/error_codes/E0731.md b/compiler/rustc_error_codes/src/error_codes/E0731.md
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+An enum with the representation hint `repr(transparent)` had zero or more than
+one variants.
+
+Erroneous code example:
+
+```compile_fail,E0731
+#[repr(transparent)]
+enum Status { // error: transparent enum needs exactly one variant, but has 2
+    Errno(u32),
+    Ok,
+}
+```
+
+Because transparent enums are represented exactly like one of their variants at
+run time, said variant must be uniquely determined. If there is no variant, or
+if there are multiple variants, it is not clear how the enum should be
+represented.