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+A struct with the representation hint `repr(transparent)` had two or more fields
+that were not guaranteed to be zero-sized.
+
+Erroneous code example:
+
+```compile_fail,E0690
+#[repr(transparent)]
+struct LengthWithUnit<U> { // error: transparent struct needs at most one
+    value: f32,            //        non-zero-sized field, but has 2
+    unit: U,
+}
+```
+
+Because transparent structs are represented exactly like one of their fields at
+run time, said field must be uniquely determined. If there are multiple fields,
+it is not clear how the struct should be represented.
+Note that fields of zero-sized types (e.g., `PhantomData`) can also exist
+alongside the field that contains the actual data, they do not count for this
+error. When generic types are involved (as in the above example), an error is
+reported because the type parameter could be non-zero-sized.
+
+To combine `repr(transparent)` with type parameters, `PhantomData` may be
+useful:
+
+```
+use std::marker::PhantomData;
+
+#[repr(transparent)]
+struct LengthWithUnit<U> {
+    value: f32,
+    unit: PhantomData<U>,
+}
+```