Merge pull request #428 from ehuss/grammar-enum-expr

Grammar: Add enum expression.

Author: Havvy

src/expressions/enum-variant-expr.md

@@ -1,7 +1,30 @@
 # Enumeration Variant expressions
 
-Enumeration variants can be constructed similarly to structs, using a path to
-an enum variant instead of to a struct:
+> **<sup>Syntax</sup>**\
+> _EnumerationVariantExpression_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; _EnumExprStruct_\
+> &nbsp;&nbsp; | _EnumExprTuple_\
+> &nbsp;&nbsp; | _EnumExprFieldless_
+>
+> _EnumExprStruct_ :\
+> &nbsp;&nbsp; [_PathInExpression_] `{` _EnumExprFields_<sup>?</sup> `}`
+>
+> _EnumExprFields_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; _EnumExprField_ (`,` _EnumExprField_)<sup>\*</sup> `,`<sup>?</sup>
+>
+> _EnumExprField_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; [IDENTIFIER]\
+> &nbsp;&nbsp; | ([IDENTIFIER] | [TUPLE_INDEX]) `:` [_Expression_]
+>
+> _EnumExprTuple_ :\
+> &nbsp;&nbsp; [_PathInExpression_] `(`\
+> &nbsp;&nbsp; &nbsp;&nbsp; ( [_Expression_] (`,` [_Expression_])<sup>\*</sup> `,`<sup>?</sup> )<sup>?</sup>\
+> &nbsp;&nbsp; `)`
+>
+> _EnumExprFieldless_ : [_PathInExpression_]
+
+Enumeration variants can be constructed similarly to [structs], using a path to an enum
+variant instead of to a struct:
 
 ```rust
 # enum Message {
@@ -13,3 +36,12 @@ let q = Message::Quit;
 let w = Message::WriteString("Some string".to_string());
 let m = Message::Move { x: 50, y: 200 };
 ```
+
+Enum variant expressions have the same syntax, behavior, and restrictions as [struct
+expressions][structs], except they do not support base update with the `..` syntax.
+
+[IDENTIFIER]: identifiers.html
+[TUPLE_INDEX]: tokens.html#integer-literals
+[_Expression_]: expressions.html
+[_PathInExpression_]: paths.html#paths-in-expressions
+[structs]: expressions/struct-expr.html