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-                      r6a8c773
+                      r41fb996
 \lstnewenvironment{rust}[1][]{\lstset{language=Rust,escapechar=\$,moredelim=**[is][\color{red}]{@}{@},}\lstset{#1}}{}
+Enumerations
+Rust provides a scoped enumeration based on variant types.
+% An enumeration, also referred to as an enum, is a simultaneous definition of a nominal enumerated type as well as a set of constructors, that can be used to create or pattern-match values of the corresponding enumerated type.
+An enumeration without constructors is called field-less.
 \begin{rust}
+        Syntax
+        Enumeration :
+           enum IDENTIFIER  GenericParams? WhereClause? { EnumItems? }
+        EnumItems :
+           EnumItem ( , EnumItem )* ,?
+        EnumItem :
+           OuterAttribute* Visibility?
+           IDENTIFIER ( EnumItemTuple | EnumItemStruct )? EnumItemDiscriminant?
+        EnumItemTuple :
+           ( TupleFields? )
+        EnumItemStruct :
+           { StructFields? }
+        EnumItemDiscriminant :
+           = Expression
+enum Week { Mon, Tues, Wed, Thu, Fri, Sat, Sun@,@ }
+let mut week: Week = Week::Mon;
+week = Week::Fri;
 \end{rust}
+An enumeration, also referred to as an enum, is a simultaneous definition of a nominal enumerated type as well as a set of constructors, that can be used to create or pattern-match values of the corresponding enumerated type.
+Enumerations are declared with the keyword enum.
+An example of an enum item and its use:
+A field-less enumeration with only unit variants is called unit-only.
 \begin{rust}
+enum Animal {
+        Dog,
+        Cat,
+}
+let mut a: Animal = Animal::Dog;
+a = Animal::Cat;
+enum Week { Mon = 0, Tues = 1, Wed = 2, Thu = 3, Fri = 4, Sat = 5, Sun = 6 }
 \end{rust}
 Enum constructors can have either named or unnamed fields:
 \begin{rust}
 enum Animal {
+        Dog(String, f64),
+        Cat { name: String, weight: f64 },
+}
+        Dog( String, f64 ),
+        Cat{ name: String, weight: f64 },
+}
 let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2);
 a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 };
 \end{rust}
+In this example, Cat is a struct-like enum variant, whereas Dog is simply called an enum variant.
+An enum where no constructors contain fields are called a field-less enum. For example, this is a fieldless enum:
+Here, @Dog@ is an @enum@ variant, whereas @Cat@ is a struct-like variant.
+Each @enum@ type has an implicit integer tag (discriminant), with a unique value for each variant type.
+Like a C enumeration, the tag values for the variant types start at 0 with auto incrementing.
+The tag is re-purposed for enumeration by allowing it to be explicitly set, and auto incrmenting continues from that value.
+\begin{cquote}
+\sf\setlength{\tabcolsep}{3pt}
+\begin{tabular}{rcccccccr}
+@enum@ Week \{  & Mon,  & Tue,  & Wed = 2,      & Thu = 10,     & Fri,  & Sat = 5,      & Sun   & \};   \\
+\rm tags                & 0             & 1             & 2                     & 10            & 11    & 5                     & 6             &               \\
+\end{tabular}
+\end{cquote}
+In general, the tag can only be read as an opaque reference for comparison.
 \begin{rust}
+enum Fieldless {
+        Tuple(),
+        Struct{},
+        Unit,
+}
+if mem::discriminant(&week) == mem::discriminant(&Week::Mon) ...
 \end{rust}
 If a field-less enum only contains unit variants, the enum is called an unit-only enum. For example:
+If the enumeration is unit-only, or field-less with no explicit discriminants and where only unit variants are explicit, then the discriminant is accessible with a numeric cast.
 \begin{rust}
+enum Enum {
+        Foo = 3,
+        Bar = 2,
+        Baz = 1,
+}
+\end{rust}
+\subsection{Discriminants}
+Each enum instance has a discriminant: an integer logically associated to it that is used to determine which variant it holds.
+Under the default representation, the discriminant is interpreted as an isize value. However, the compiler is allowed to use a smaller type (or another means of distinguishing variants) in its actual memory layout.
+\subsection{Assigning discriminant values}
+\subsection{Explicit discriminants}
+In two circumstances, the discriminant of a variant may be explicitly set by following the variant name with = and a constant expression:
+        if the enumeration is "unit-only".
+        if a primitive representation is used. For example:
+\begin{rust}
+        #[repr(u8)]
+        enum Enum {
+                Unit = 3,
+                Tuple(u16),
+                Struct {
+                        a: u8,
+                        b: u16,
+                } = 1,
+        }
+\end{rust}
+\subsection{Implicit discriminants}
+If a discriminant for a variant is not specified, then it is set to one higher than the discriminant of the previous variant in the declaration. If the discriminant of the first variant in the declaration is unspecified, then it is set to zero.
+\begin{rust}
+enum Foo {
+        Bar,                    // 0
+        Baz = 123,        // 123
+        Quux,              // 124
+}
+let baz_discriminant = Foo::Baz as u32;
+assert_eq!(baz_discriminant, 123);
+\end{rust}
+\subsection{Restrictions}
+It is an error when two variants share the same discriminant.
+\begin{rust}
+enum SharedDiscriminantError {
+        SharedA = 1,
+        SharedB = 1
+}
+enum SharedDiscriminantError2 {
+        Zero,      // 0
+        One,            // 1
+        OneToo = 1  // 1 (collision with previous!)
+}
+\end{rust}
+It is also an error to have an unspecified discriminant where the previous discriminant is the maximum value for the size of the discriminant.
+\begin{rust}
+#[repr(u8)]
+enum OverflowingDiscriminantError {
+        Max = 255,
+        MaxPlusOne // Would be 256, but that overflows the enum.
+}
+#[repr(u8)]
+enum OverflowingDiscriminantError2 {
+        MaxMinusOne = 254, // 254
+        Max,                       // 255
+        MaxPlusOne               // Would be 256, but that overflows the enum.
+}
+\end{rust}
+\subsection{Accessing discriminant}
+\begin{rust}
+Via mem::discriminant
+\end{rust}
+@mem::discriminant@ returns an opaque reference to the discriminant of an enum value which can be compared. This cannot be used to get the value of the discriminant.
+\subsection{Casting}
+If an enumeration is unit-only (with no tuple and struct variants), then its discriminant can be directly accessed with a numeric cast; e.g.:
+\begin{rust}
+enum Enum {
+        Foo,
+        Bar,
+        Baz,
+}
+assert_eq!(0, Enum::Foo as isize);
+assert_eq!(1, Enum::Bar as isize);
+assert_eq!(2, Enum::Baz as isize);
+\end{rust}
+Field-less enums can be casted if they do not have explicit discriminants, or where only unit variants are explicit.
+\begin{rust}
+enum Fieldless {
+        Tuple(),
+        Struct{},
+        Unit,
+}
+assert_eq!(0, Fieldless::Tuple() as isize);
+assert_eq!(1, Fieldless::Struct{} as isize);
+assert_eq!(2, Fieldless::Unit as isize);
+\end{rust}
+\begin{rust}
+#[repr(u8)]
+enum FieldlessWithDiscrimants {
+        First = 10,
+        Tuple(),
+        Second = 20,
+        Struct{},
+        Unit,
+}
+assert_eq!(10, FieldlessWithDiscrimants::First as u8);
+assert_eq!(11, FieldlessWithDiscrimants::Tuple() as u8);
+assert_eq!(20, FieldlessWithDiscrimants::Second as u8);
+assert_eq!(21, FieldlessWithDiscrimants::Struct{} as u8);
+assert_eq!(22, FieldlessWithDiscrimants::Unit as u8);
+\end{rust}
+\subsection{Pointer casting}
+If the enumeration specifies a primitive representation, then the discriminant may be reliably accessed via unsafe pointer casting:
+\begin{rust}
+#[repr(u8)]
+enum Enum {
+        Unit,
+        Tuple(bool),
+        Struct{a: bool},
+}
+impl Enum {
+        fn discriminant(&self) -> u8 {
+                unsafe { *(self as *const Self as *const u8) }
+        }
+}
+let unit_like = Enum::Unit;
+let tuple_like = Enum::Tuple(true);
+let struct_like = Enum::Struct{a: false};
+assert_eq!(0, unit_like.discriminant());
+assert_eq!(1, tuple_like.discriminant());
+assert_eq!(2, struct_like.discriminant());
+\end{rust}
+\subsection{Zero-variant enums}
+Enums with zero variants are known as zero-variant enums. As they have no valid values, they cannot be instantiated.
+\begin{rust}
+enum ZeroVariants {}
+\end{rust}
+Zero-variant enums are equivalent to the never type, but they cannot be coerced into other types.
+\begin{rust}
+let x: ZeroVariants = panic!();
+let y: u32 = x; // mismatched type error
+\end{rust}
+\subsection{Variant visibility}
+Enum variants syntactically allow a Visibility annotation, but this is rejected when the enum is validated. This allows items to be parsed with a unified syntax across different contexts where they are used.
+\begin{rust}
+macro_rules! mac_variant {
+        ($vis:vis $name:ident) => {
+                enum $name {
+                        $vis Unit,
+                        $vis Tuple(u8, u16),
+                        $vis Struct { f: u8 },
+                }
+        }
+}
+// Empty `vis` is allowed.
+mac_variant! { E }
+// This is allowed, since it is removed before being validated.
+#[cfg(FALSE)]
+enum E {
+        pub U,
+        pub(crate) T(u8),
+        pub(super) T { f: String }
+}
+if week as isize == Week::Mon as isize ...
 \end{rust}

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