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+A `#[repr(..)]` attribute was placed on an unsupported item.
+
+Examples of erroneous code:
+
+```compile_fail,E0517
+#[repr(C)]
+type Foo = u8;
+
+#[repr(packed)]
+enum Foo {Bar, Baz}
+
+#[repr(u8)]
+struct Foo {bar: bool, baz: bool}
+
+#[repr(C)]
+impl Foo {
+    // ...
+}
+```
+
+* The `#[repr(C)]` attribute can only be placed on structs and enums.
+* The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs.
+* The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums.
+
+These attributes do not work on typedefs, since typedefs are just aliases.
+
+Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
+discriminant size for enums with no data fields on any of the variants, e.g.
+`enum Color {Red, Blue, Green}`, effectively setting the size of the enum to
+the size of the provided type. Such an enum can be cast to a value of the same
+type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
+with a constrained set of allowed values.
+
+Only field-less enums can be cast to numerical primitives, so this attribute
+will not apply to structs.
+
+`#[repr(packed)]` reduces padding to make the struct size smaller. The
+representation of enums isn't strictly defined in Rust, and this attribute
+won't work on enums.
+
+`#[repr(simd)]` will give a struct consisting of a homogeneous series of machine
+types (i.e., `u8`, `i32`, etc) a representation that permits vectorization via
+SIMD. This doesn't make much sense for enums since they don't consist of a
+single list of data.