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diff --git a/compiler/rustc_error_codes/src/error_codes/E0732.md b/compiler/rustc_error_codes/src/error_codes/E0732.md
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+An `enum` with a discriminant must specify a `#[repr(inttype)]`.
+
+Erroneous code example:
+
+```compile_fail,E0732
+#![feature(arbitrary_enum_discriminant)]
+
+enum Enum { // error!
+    Unit = 1,
+    Tuple() = 2,
+    Struct{} = 3,
+}
+# fn main() {}
+```
+
+A `#[repr(inttype)]` must be provided on an `enum` if it has a non-unit
+variant with a discriminant, or where there are both unit variants with
+discriminants and non-unit variants. This restriction ensures that there
+is a well-defined way to extract a variant's discriminant from a value;
+for instance:
+
+```
+#![feature(arbitrary_enum_discriminant)]
+
+#[repr(u8)]
+enum Enum {
+    Unit = 3,
+    Tuple(u16) = 2,
+    Struct {
+        a: u8,
+        b: u16,
+    } = 1,
+}
+
+fn discriminant(v : &Enum) -> u8 {
+    unsafe { *(v as *const Enum as *const u8) }
+}
+
+fn main() {
+    assert_eq!(3, discriminant(&Enum::Unit));
+    assert_eq!(2, discriminant(&Enum::Tuple(5)));
+    assert_eq!(1, discriminant(&Enum::Struct{a: 7, b: 11}));
+}
+```