carbon-lang/toolchain/parse/state.def

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
// This is an X-macro header. It does not use `#include` guards, and instead is
// designed to be `#include`ed after the x-macro is defined in order for its
// inclusion to expand to the desired output. Macro definitions are cleaned up
// at the end of this file.
//
// Supported x-macros are:
// - CARBON_PARSE_STATE(Name)
//   Defines a parser state.
//
// Parser states may be clustered when there are multiple related variants,
// named `StateAsVariant`. When there are variants, they share a common helper
// function for most logic.
//
// The comments before each state describe the portion of the grammar that the
// state is implementing, by giving an example of each kind of token sequence
// that this state handles. In these examples, `...` indicates a sequence of
// tokens handled by some other state, and `???` indicates a sequence of invalid
// tokens. A trailing `??? ;` indicates an attempt to skip to the end of the
// declaration, which may or may not actually find a `;` token.
//
// The position in the token stream before the state is indicated by the caret
// `^` on the line below the example, and all tokens consumed by the state are
// underlined by the caret and following `~`s. If no tokens are consumed, the
// caret will point between tokens. Therefore, the position in the token stream
// after the state is the first token in the example after the underlined
// region.
//
// Following each set of examples, the output states for that situation are
// listed. States are numbered in the order they'll be executed; in other
// words, `1` is the top of the state stack. The comment `(state done)`
// indicates that no new states are added to the stack.

#ifndef CARBON_PARSE_STATE
#error "Must define the x-macro to use this file."
#define CARBON_PARSE_STATE(Name)
#endif

// Use CARBON_PARSE_STATE_VARIANTSN(Kind, Variant1, Variant2, ...) to generate
// KindAsVariant1, KindAsVariant2, ... states.
#define CARBON_PARSE_STATE_VARIANT(Kind, Variant) \
  CARBON_PARSE_STATE(Kind##As##Variant)
#define CARBON_PARSE_STATE_VARIANTS2(Kind, Variant1, Variant2) \
  CARBON_PARSE_STATE_VARIANT(Kind, Variant1)                   \
  CARBON_PARSE_STATE_VARIANT(Kind, Variant2)
#define CARBON_PARSE_STATE_VARIANTS3(Kind, Variant1, Variant2, Variant3) \
  CARBON_PARSE_STATE_VARIANT(Kind, Variant1)                             \
  CARBON_PARSE_STATE_VARIANTS2(Kind, Variant2, Variant3)
#define CARBON_PARSE_STATE_VARIANTS4(Kind, Variant1, Variant2, Variant3, \
                                     Variant4)                           \
  CARBON_PARSE_STATE_VARIANT(Kind, Variant1)                             \
  CARBON_PARSE_STATE_VARIANTS3(Kind, Variant2, Variant3, Variant4)

// Used as a default for State initialization in some cases. Should
// not be put on the state stack.
CARBON_PARSE_STATE(Invalid)

// Handles an index expression:
//
// a[0]
//  ^
//   1. Expr
//   2. IndexExprFinish
CARBON_PARSE_STATE(IndexExpr)

// Handles finishing the index expression.
//
// a[0]
//    ^
//   (state done)
CARBON_PARSE_STATE(IndexExprFinish)

// Handles the array keyword and opening paren in an array expression.
//
// array(T, N)
// ^~~~~~
//   1. Expr
//   2. ArrayExprComma
CARBON_PARSE_STATE(ArrayExpr)

// Handles ',' in an array expression.
//
// array(T, N)
//        ^
//   1. Expr
//   2. ArrayExprFinish
CARBON_PARSE_STATE(ArrayExprComma)

// Handles finishing the array expression.
//
// array(T, N)
//           ^
//   (state done)
CARBON_PARSE_STATE(ArrayExprFinish)

// Handles the `{` of a brace expression.
//
// {}
// ^
//   1. BraceExprFinishAsUnknown
//
// { ... }
// ^
//   1. BraceExprParamAsUnknown
//   2. BraceExprFinishAsUnknown
CARBON_PARSE_STATE(BraceExpr)

// Handles a brace expression parameter. Note this will always start as unknown,
// but should be known after the first valid parameter. All later inconsistent
// parameters are invalid.
//
// { .foo ... }
//  ^
//   1. PeriodAsStruct
//   2. BraceExprParamAfterDesignatorAs(Type|Value|Unknown)
//
// { ???
//  ^
//   1. BraceExprParamFinishAs(Type|Value|Unknown)
CARBON_PARSE_STATE_VARIANTS3(BraceExprParam, Type, Value, Unknown)

// Handles a brace expression parameter after the initial designator. This
// should be at a `:` or `=`, depending on whether it's a type or value literal.
//
// { .foo = ref bar ... }
//        ^~~~~
//   1. Expr
//   2. RefTagFinishAsRegular
//   3. BraceExprParamFinishAsValue
//
// { .foo = bar ... }
//        ^
//   1. Expr
//   2. BraceExprParamFinishAsValue
//
// { .foo: bar ... }
//       ^
//   1. Expr
//   2. BraceExprParamFinishAsType
//
// { .foo ???
//       ^
//   1. BraceExprParamFinishAs(Type|Value|Unknown)
CARBON_PARSE_STATE_VARIANTS3(BraceExprParamAfterDesignator, Type, Value,
                             Unknown)

// Handles the end of a brace expression parameter.
//
// { ... }
//      ^
//   (state done)
//
// { .foo = bar, ... }
//             ^
//   1. BraceExprParamAsValue
//
// { .foo: bar, ... }
//            ^
//   1. BraceExprParamAsType
//
// { ??? , ... }
//       ^
//   1. BraceExprParamAsUnknown
CARBON_PARSE_STATE_VARIANTS3(BraceExprParamFinish, Type, Value, Unknown)

// Handles the `}` of a brace expression.
//
// { ... }
//       ^
//   (state done)
CARBON_PARSE_STATE_VARIANTS3(BraceExprFinish, Type, Value, Unknown)

// Handles the end of an argument expression tagged with `ref`. The
// `AfterOpenParen` variant diagnoses if the expression is not followed by `,`.
//
// ref ...
//        ^
//   (state done)
CARBON_PARSE_STATE_VARIANTS2(RefTagFinish, Regular, AfterOpenParen)

// Handles a call expression `(...)`, including the initial `ref`, if any.
//
// F()
//  ^
//   1. CallExprFinish
//
// F(ref ...
//  ^~~~
//
// 1. Expr
// 2. RefTagFinishAsRegular
// 2. TupleLiteralElementFinish
// 3. CallExprFinish
//
// F( ...
//  ^
//   1. Expr
//   2. TupleLiteralElementFinish
//   3. CallExprFinish
CARBON_PARSE_STATE(CallExpr)

// Handles finishing the call expression.
//
// F(a, b)
//       ^
//   (state done)
CARBON_PARSE_STATE(CallExprFinish)

// Handles processing at the `{` on a typical code block.
//
// if (cond) {
//           ^
//   1. StatementScopeLoop
//   2. CodeBlockFinish
//
// if (cond) ???
//          ^
//   1. Statement
//   2. CodeBlockFinish
CARBON_PARSE_STATE(CodeBlock)

// Handles processing at the `}` on a typical code block, after a statement
// scope is done.
//
// if (cond) { ... }
//                 ^
//   (state done)
CARBON_PARSE_STATE(CodeBlockFinish)

// Handles a declaration name and parameters, such as `Foo[...](...)`.
//
// name . ...
// ^~~~~~
//   1. DeclNameAndParams
//
// name [ ... ]
// ^~~~
//   1. PatternListAsImplicit
//   2. DeclNameAndParamsAfterImplicit
//
// name ( ... )
// ^~~~
//   1. PatternListAsExplicit
//   2. DeclNameAndParamsAfterParams
//
// name ...
// ^~~~
//   (state done)
//
//  ???
// ^
//   (state done)
CARBON_PARSE_STATE(DeclNameAndParams)

// Inside a declaration, conditionally starts explicit parameter handling. Only
// used after implicit parameters.
//
// name [ ... ] ( ... )
//             ^
//   1. PatternListAsExplicit
//   2. DeclNameAndParamsAfterParams
//
// name [ ... ] ???
//             ^
//   (state done)
CARBON_PARSE_STATE(DeclNameAndParamsAfterImplicit)

// Inside a declaration, conditionally starts another qualified name. Only used
// after regular parameters.
//
// name [ ... ] ( ... ) .
//                      ^
//   1. DeclNameAndParams
//
// name [ ... ] ( ... ) ...
//                     ^
//   (state done)
CARBON_PARSE_STATE(DeclNameAndParamsAfterParams)

// Handles processing of a declaration. Things like fn, class, interface, and so
// on.
//
// abstract
// ^~~~~~~~
// base class
// ^~~~
// default
// ^~~~~~~
// export import
// ^~~~~~
// extend base
// ^~~~~~
// final
// ^~~~~
// override fn
// ^~~~~~~~
// private
// ^~~~~~~
// protected
// ^~~~~~~~~
// virtual
// ^~~~~~~
//   1. Decl
//
// adapt ...
// ^~~~~
//   1. AdaptAfterIntroducer
//
// alias ...
// ^~~~~
//   1. Alias
//
// base : ...
// ^~~~
//   1. BaseAfterIntroducer
//
// choice ...
// ^~~~~~
//   1. ChoiceIntroducer
//
// class ...
// ^~~~~
//   1. TypeAfterIntroducerAsClass
//
// constraint ...
// ^~~~~~~~~~
//   1. TypeAfterIntroducerAsNamedConstraint
//
// export <name> ...
// ^~~~~~
//   1. ExportName
//
// fn ...
// ^~
//   1. FunctionIntroducer
//
// impl ...
// ^~~~
//   1. ImplAfterIntroducer
//
// import ...    (in packaging directives)
// ^~~~~~
//   1. Import
//
// interface ...
// ^~~~~~~~~
//   1. TypeAfterIntroducerAsInterface
//
// let ...        (variant is Regular)
// ^~~
//   1. Let
//
// let ...        (variant is Interface)
// ^~~
//   1. AssociatedConstant
//
// library ...    (in packaging directives)
// ^~~~~~~
//   1. Library
//
// namespace ...
// ^~~~~~~~~
//   1. Namespace
//
// package ...    (in packaging directives)
// ^~~~~~~
//   1. Package
//
// require ...
// ^~~~~~~
//   1. RequireAfterIntroducer
//
// var ...        (variant is Regular)
// ^~~
//   1. VarAsRegular
//
// var ...        (variant is Class)
// ^~~
//   1. FieldDecl
//
// ;
// ^
//   (state done)
//
// ??? ;
// ^~~~~
//   (state done)
CARBON_PARSE_STATE_VARIANTS3(Decl, Class, Interface, Regular)

// Handles processing of a declaration scope, which contains a sequence of
// declarations.
//
//  }
// ^
//  <eof>
// ^
//   (state done)
//
//  ...
// ^
//   1. DeclAs(Class|Interface|Regular)
//   2. DeclScopeLoopAs(Class|Interface|Regular)
//
CARBON_PARSE_STATE_VARIANTS3(DeclScopeLoop, Class, Interface, Regular)

// Handles periods. Only does one `.<expression>` segment; the source is
// responsible for handling chaining.
//
// The forms of this are:
// - Member access expressions.
// - Designated names in structs.
//
// Expressions have member accesses such as `x.y`, while structs have
// designators such as `.z`.
//
// . name
// ^~~~~~
// -> name
// ^~~~~~~
// . base    (variant is not Decl)
// ^~~~~~
// -> base   (variant is not Decl)
// ^~~~~~~
// . ???     (??? consumed if it is a keyword)
// ^
// -> ???    (??? consumed if it is a keyword)
// ^~
//   (state done)
//
// expr . ( ... )
//      ^
//   1. OnlyParenExpr
//   2. CompoundMemberAccess
//
// expr -> ( ... )
//      ^~
//   1. OnlyParenExpr
//   2. CompoundPointerMemberAccess
CARBON_PARSE_STATE_VARIANTS2(Period, Expr, Struct)

// Handles a compound member access after we parse the name expression.
//
// expr . ( expr )
//                ^
CARBON_PARSE_STATE(CompoundMemberAccess)

// Handles a compound pointer member access after we parse the name expression.
//
// expr -> ( expr )
//                 ^
CARBON_PARSE_STATE(CompoundPointerMemberAccess)

// Handles `->name` expressions. Identical to PeriodAsExpr except for the
// leading token.
//
// -> name
// ^~~~~~~
//   (state done)
//
// -> ???    (??? consumed if it is a keyword)
// ^~
//   (state done)
CARBON_PARSE_STATE(ArrowExpr)

// Handles processing of an expression.
//
// if ...
// ^~
//   1. Expr
//   2. IfExprCondition
//   3. IfExprFinish
//
// <prefix operator> ...
// ^~~~~~~~~~~~~~~~~
//   1. Expr
//   2. ExprLoopForPrefix
//
//  ...
// ^
//   1. ExprInPostfix
//   2. ExprLoop
CARBON_PARSE_STATE(Expr)

// Handles the initial part of postfix expressions, such as an identifier or
// literal value, then proceeds to the loop.
//
// identifier
// ^~~~~~~~~~
// literal
// ^~~~~~~
// self
// ^~~~
// Self
// ^~~~
//   1. ExprInPostfixLoop
//
//  {
// ^
//   1. BraceExpr
//   2. ExprInPostfixLoop
//
//  (
// ^
//   1. ParenExpr
//   2. ExprInPostfixLoop
//
//  [
// ^
//   1. ArrayExpr
//   2. ExprInPostfixLoop
//
//  ???
// ^
//   (state done)
CARBON_PARSE_STATE(ExprInPostfix)

// Handles looping through elements following the initial postfix expression,
// such as designators or parenthesized parameters.
//
// expr . ...
//     ^
//   1. PeriodAsExpr
//   2. ExprInPostfixLoop
//
// expr -> ...
//     ^
//   1. ArrowExpr
//   2. ExprInPostfixLoop
//
// expr ( ... )
//     ^
//   1. CallExpr
//   2. ExprInPostfixLoop
//
// expr [ ... ]
//     ^
//   1. IndexExprStart
//   2. ExprInPostfixLoop
//
//  ...
// ^
//   (state done)
CARBON_PARSE_STATE(ExprInPostfixLoop)

// Handles processing of an expression.
//
// expr <infix operator> ...
//      ^~~~~~~~~~~~~~~~
//   1. Expr
//   2. ExprLoopForBinary
//
// expr <postfix operator>
//      ^~~~~~~~~~~~~~~~~~
//   1. ExprLoop
//
// expr <short circuit operator> ...
//      ^~~~~~~~~~~~~~~~~~~~~~~~
//   1. Expr
//   2. ExprLoopForShortCircuitOperator
//
// expr ...
//     ^
//   (state done)
CARBON_PARSE_STATE(ExprLoop)

// Completes an ExprLoop pass by adding an infix operator, then goes back
// to ExprLoop.
//
// expr <infix operator> expr ...
//                           ^
//   1. ExprLoop
CARBON_PARSE_STATE(ExprLoopForInfixOperator)

// Completes an ExprLoop pass by adding a prefix operator, then goes back
// to ExprLoop.
//
// <prefix operator> expr ...
//                       ^
//   1. ExprLoop
CARBON_PARSE_STATE(ExprLoopForPrefixOperator)

// Completes an ExprLoop pass by adding a short circuit operator, then goes back
// to ExprLoop.
//
// expr <short circuit operator> expr ...
//                                   ^
//   1. ExprLoop
CARBON_PARSE_STATE_VARIANTS2(ExprLoopForShortCircuitOperator, And, Or)

// Completes the condition of an `if` expression and handles the `then` token.
//
// if expr then ...
//         ^~~~
//   1. Expr
//   2. IfExprFinishThen
//
// if expr ???
//        ^
//  (state done)
CARBON_PARSE_STATE(IfExprFinishCondition)

// Completes the first alternative in an `if` expression and handles the `else`
// token.
//
// if expr then expr else ...
//                   ^~~~
//   1. Expr
//   2. IfExprFinishElse
//
// if expr then expr ???
//                  ^
//  (state done)
CARBON_PARSE_STATE(IfExprFinishThen)

// Completes the second alternative in an `if` expression.
//
// if expr then expr else expr
//                            ^
//  (state done)
CARBON_PARSE_STATE(IfExprFinishElse)

// Completes an IfExpr.
//
// if expr then expr else expr
//                            ^
// if ???
//       ^
//   (state done)
CARBON_PARSE_STATE(IfExprFinish)

// Handles the beginning of a requirement expression after a `where` operator in
// an expression.
// TODO: Also a `require` declaration?
//
// expr where .designator = ...
//            ^~~~~~~~~~~~~
//   1. Expr
//   2. RequirementOperatorFinish
// expr where ...
//           ^
//   1. Expr
//   2. RequirementOperator
CARBON_PARSE_STATE(RequirementBegin)

// Handles a requirement operator in a `where` expression.
//
// expr where expr impls ...
//                 ^~~~
// expr where expr = ...
//                 ^
// expr where expr == ...
//                 ^~
//   1. Expr
//   2. RequirementOperatorFinish
CARBON_PARSE_STATE(RequirementOperator)

// Finishes a requirement operator in a `where` expression.
//
// expr where expr impls expr
//                           ^
// expr where expr = expr
//                       ^
// expr where expr == expr
//                        ^
//   (state done)
// expr where expr impls expr and
//                            ^~~
// expr where expr = expr and
//                        ^~~
// expr where expr == expr and
//                         ^~~
//   1. RequirementBegin
CARBON_PARSE_STATE(RequirementOperatorFinish)

// Finishes an `where` expression.
//
// expr where requirement
//                       ^
//   (state done)
CARBON_PARSE_STATE(WhereFinish)

// Handles the `;` for an expression statement, which is different from most
// keyword statements.
//
// expr ;
//      ^
// expr ??? ;
//      ^~~~~
//   (state done)
CARBON_PARSE_STATE(ExprStatementFinish)

// Handles a function's introducer.
//
// fn ...
//   ^
//   1. DeclNameAndParams
//   2. FunctionAfterParams
CARBON_PARSE_STATE(FunctionIntroducer)

// Handles processing of a function's syntax after `)`, primarily the
// possibility a `->` return type is there. Always enqueues signature finish
// handling.
//
// fn F(...) -> ...
//           ^~
//   1. Expr
//   2. FunctionReturnTypeFinish
//   3. FunctionSignatureFinish
//
// fn F(...) ...
//          ^
//   1. FunctionSignatureFinish
CARBON_PARSE_STATE(FunctionAfterParams)

// Finishes a function return type.
//
// fn F(...) -> expr ...
//                  ^
//   (state done)
CARBON_PARSE_STATE(FunctionReturnTypeFinish)

// Finishes a function signature. If it's a declaration, the function is done;
// otherwise, this also starts definition processing.
//
// fn ... ;
//        ^
//   (state done)
//
// fn ... {
//        ^
//   1. StatementScopeLoop
//   2. FunctionDefinitionFinish
//
// fn ... ??? ;
//        ^~~~~
//   (state done)
CARBON_PARSE_STATE(FunctionSignatureFinish)

// Finishes a function definition.
//
// fn ... }
//         ^
// fn ... ;
//         ^
//   (state done)
CARBON_PARSE_STATE(FunctionDefinitionFinish)

// Handles `export <name>`.
//
// export Name;
//       ^
//   1. DeclNameAndParams
//   2. ExportFinish
CARBON_PARSE_STATE(ExportName)

// Finishes an `export <name>`.
//
// export Name;
//            ^
//   (state done)
CARBON_PARSE_STATE(ExportNameFinish)

// Handles `import`.
//
// import pkgname [library "libname"] ;
//        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// import library "libname" ;
//        ^~~~~~~~~~~~~~~~~~~
// import ??? ;
//        ^~~~~
//   (state done)
CARBON_PARSE_STATE(Import)

// Handles `library` in declaration form.
//
// library "libname" ;
//         ^~~~~~~~~~~
// library ??? ;
//         ^~~~~
//   (state done)
CARBON_PARSE_STATE(Library)

// Handles `namespace`.
//
// namespace ...
//          ^
//   1. DeclNameAndParams
//   2. NamespaceFinish
CARBON_PARSE_STATE(Namespace)

// Handles `namespace` after the name.
//
// namespace ... ;
//               ^
// namespace ... ??? ;
//               ^~~~~
//   (state done)
CARBON_PARSE_STATE(NamespaceFinish)

// Handles `package`.
//
// package pkgname [library "libname"] ;
//         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// package ??? ;
//         ^~~~~
//   (state done)
CARBON_PARSE_STATE(Package)

// Handles `alias` after the introducer.
//
// alias ...
//      ^
//   1. DeclNameAndParams
//   2. AliasInitializer
CARBON_PARSE_STATE(Alias)

// Handles `alias` after the name.
//
// alias ... = ... ;
//           ^
//   1. Expr
//   2. AliasFinish
//
// alias ???
//       ^
//   (state done)
CARBON_PARSE_STATE(AliasAfterName)

// Handles `alias` at the end.
//
// alias ... ;
//           ^
// alias ???
//       ^
//   (state done)
CARBON_PARSE_STATE(AliasFinish)

// Starts parsing a pattern in a comma-separated list. The variants mark
// whether it is part of an implicit parameter list or a tuple pattern.
//
//  ...
// ^
//   1. Pattern
//   2. PatternListElementFinishAs(Tuple|Explicit|Implicit)
CARBON_PARSE_STATE_VARIANTS3(PatternListElement, Tuple, Explicit, Implicit)

// Finishes parsing a pattern in a comma-separated list, including the
// optional trailing `,`. If there are more patterns, enqueues another
// pattern parsing state.
//
// ... , )  (variant is Tuple)
//     ^
//   (state done)
//
// ... , ]  (variant is Implicit)
//     ^
//   (state done)
//
// ... , ...
//     ^
//   1. PatternListElementAs(Tuple|Explicit|Implicit)
//
// ...
//    ^
//   (state done)
CARBON_PARSE_STATE_VARIANTS3(PatternListElementFinish, Tuple, Explicit,
                             Implicit)

// Handles processing of a tuple pattern (parentheses) or implicit parameter
// list (square brackets).
//
// ( )        (variant is Tuple or Explicit)
// ^
// [ ]        (variant is Implicit)
// ^
//   1. PatternListFinishAs(Tuple|Explicit|Implicit)
//
// ( ... )    (variant is Tuple or Explicit)
// ^
// [ ... ]    (variant is Implicit)
// ^
//   1. PatternListElementAs(Tuple|Explicit|Implicit)
//   2. PatternListFinishAs(Tuple|Explicit|Implicit)
CARBON_PARSE_STATE_VARIANTS3(PatternList, Tuple, Explicit, Implicit)

// Handles processing of a parameter list `]` or `)`.
//
// ( ... )    (variant is Tuple or Explicit)
//       ^
// [ ... ]    (variant is Implicit)
//       ^
//   (state done)
CARBON_PARSE_STATE_VARIANTS3(PatternListFinish, Tuple, Explicit, Implicit)

// Handles the processing of a `(condition)` up through the expression.
//
// if/while/match {    (invalid)
//               ^
//   1. ParenConditionAs(If|While|Match)Finish
//
// if/while/match ( ... )
//                ^
// if/while/match ???
//               ^
//   1. Expr
//   2. ParenConditionAs(If|While|Match)Finish
CARBON_PARSE_STATE_VARIANTS3(ParenCondition, If, While, Match)

// Finishes the processing of a `(condition)` after the expression.
//
// if/while/match ( expr )
//                       ^
// if/while/match {
//               ^
// if/while/match ??? {
//                   ^
//   (state done)
CARBON_PARSE_STATE_VARIANTS3(ParenConditionFinish, If, While, Match)

// Handles compound member access `(` after `->` or `.`
//
// ( ... )
// ^
//   1. Expr
//   2. OnlyParenExprFinish
CARBON_PARSE_STATE(OnlyParenExpr)

// Handles the `)` after `->(`... or `.(`...
//
// ( ... )
//       ^
CARBON_PARSE_STATE(OnlyParenExprFinish)

// Handles the `(` of an expression that's presumed to be a parenthesized
// single expression, but may later be reclassified as a tuple literal.
// This also handles a `ref` following the `(`, if present. That's only
// valid in tuple literals, but doesn't cause the expression to be a tuple
// literal.
//
// ( )
// ^
//   1. TupleLiteralFinish
//
// (ref ... )
// ^~~~
//
//   1. Expr
//   2. RefTagFinishAsAfterOpenParen
//   3. ExprAfterOpenParenFinish
//   4. ParenExprFinish             (SPECIAL: may be replaced)
//
// ( ... )
// ^
//   1. Expr
//   2. ExprAfterOpenParenFinish
//   3. ParenExprFinish             (SPECIAL: may be replaced)
CARBON_PARSE_STATE(ParenExpr)

// Handles the `)` of a tuple literal.
//
// ( ... )
//       ^
//   (state done)
CARBON_PARSE_STATE(TupleLiteralFinish)

// Handles the end of an expression following an open parenthesis.
//
// ( ... , )
//       ^
//   (state done)
//   SPECIAL: parent becomes TupleLiteralFinish
//
// ( ... , ref ... )
//       ^~~~~
//   1. Expr
//   2. RefTagFinishAsRegular
//   3. TupleLiteralElementFinish
//   SPECIAL: parent becomes TupleLiteralFinish
//
// ( ... , ... )
//       ^
//   1. Expr
//   2. TupleLiteralElementFinish
//   SPECIAL: parent becomes TupleLiteralFinish
//
// ( ... )
//      ^
//   (state done)
CARBON_PARSE_STATE(ExprAfterOpenParenFinish)

// Handles the end of an expression that is known to be an element of a tuple
// literal expression.
//
// ( ... , )
//       ^
//   (state done)
//
// ( ... , ref ... )
//       ^~~~~
//   1. Expr
//   2. RefTagFinishAsRegular
//   3. TupleLiteralElementFinish
//
// ( ... , ... )
//       ^
//   1. Expr
//   2. TupleLiteralElementFinish
//
// ( ... )
//      ^
//   (state done)
CARBON_PARSE_STATE(TupleLiteralElementFinish)

// Handles the `)` of a parenthesized single expression.
//
// ( ... )
//       ^
//   (state done)
CARBON_PARSE_STATE(ParenExprFinish)

// Handles processing of a pattern.
//
//  ( ... )
// ^
//   1. PatternListAsTuple
//
//  var ...
// ^
//
//   1. VariablePattern
//
//  ...
// ^
//   1. BindingPattern
CARBON_PARSE_STATE(Pattern)

// Handles the initial part of a binding pattern, enqueuing type expression
// processing.
//
// addr        (variant is not Variable)
// ^~~~
//   3. BindingPatternAddr
//
// THEN
//
// [ref] name: ...
// ^~~~~
// [ref] self: ...
// ^~~~~
// [ref] _: ...
// ^~
//   1. Expr
//   2. BindingPatternFinishAsRegular
//
// [template] name:! ...
// ^~~~~~~~~~~~~~~~~
// [template] self:! ...
// ^~~~~~~~~~~~~~~~~
// [template] _:! ...
// ^~~~~~~~~~~~~~
//   1. Expr
//   2. BindingPatternFinishAsGeneric
//
//  ???
// ^
//   1. BindingPatternFinishAsRegular
CARBON_PARSE_STATE(BindingPattern)

// Handles `addr` in a binding pattern.
//
// addr name: type
//                ^
//   (state done)
CARBON_PARSE_STATE(BindingPatternAddr)

// Finishes binding pattern processing.
//
// name: type
//           ^
//   (state done)
CARBON_PARSE_STATE_VARIANTS2(BindingPatternFinish, Generic, Regular)

// Handles `var` in a pattern context.
//
// var ...
// ^~~
//   1. Pattern
//   2. FinishVariablePattern
CARBON_PARSE_STATE(VariablePattern)

// Finishes `var` in a pattern context.
//
// var ...
//        ^
//   (state done)
CARBON_PARSE_STATE(FinishVariablePattern)

// Handles a single statement. While typically within a statement block, this
// can also be used for error recovery where we expect a statement block and
// are missing braces.
//
// break ...
// ^~~~~
//   1. StatementBreakFinish
//
// continue ...
// ^~~~~~~~
//   1. StatementContinueFinish
//
// for ...
// ^~~
//   1. StatementForHeader
//   2. StatementForFinish
//
//  if ...
// ^
//   1. StatementIf
//
//  return ...
// ^
//   1. StatementReturn
//
//  returned ...
// ^
//   1. VarAsReturned
//
//  while ...
// ^
//   1. StatementWhile
//
//  ...
// ^
//   1. Expr
//   2. ExprStatementFinish
//
//  match ...
// ^
//   1. MatchIntroducer
//
//  <decl introducer or modifier>
// ^
//   1. Decl
CARBON_PARSE_STATE(Statement)

// Handles `break` processing at the `;`.
//
// break ;
//       ^
//   (state done)
CARBON_PARSE_STATE(StatementBreakFinish)

// Handles `continue` processing at the `;`.
//
// continue ;
//          ^
//   (state done)
CARBON_PARSE_STATE(StatementContinueFinish)

// Handles `for` processing of `(`, proceeding to a pattern before continuing.
//
// for ( ... )
//     ^
// for ???
//    ^
//   1. Pattern
//   2. StatementForHeaderIn
CARBON_PARSE_STATE(StatementForHeader)

// Handles `for` processing of `in`, proceeding to an expression before
// continuing.
//
// for ( ... in ... )
//           ^~
// for ( ... : ... )
//           ^
// for ( ... ??? )
//          ^
//   1. Expr
//   2. StatementForHeaderFinish
CARBON_PARSE_STATE(StatementForHeaderIn)

// Handles `for` processing of `)`, proceeding to the statement block.
//
// for ( ... ) ...
//           ^
//   1. CodeBlock
CARBON_PARSE_STATE(StatementForHeaderFinish)

// Handles `for` processing after the final `}`.
//
// for ( ... ) { ... }
//                    ^
//   (state done)
CARBON_PARSE_STATE(StatementForFinish)

// Handles `if` processing at the start.
//
// if ...
// ^~
//   1. ParenConditionAsIf
//   2. StatementIfConditionFinish
CARBON_PARSE_STATE(StatementIf)

// Handles `if` processing between the condition and start of the first code
// block.
//
// if ( ... ) ...
//           ^
//   1. CodeBlock
//   2. StatementIfThenBlockFinish
CARBON_PARSE_STATE(StatementIfConditionFinish)

// Handles `if` processing after the end of the first code block, with the
// optional `else`.
//
// if ( ... ) { ... } else if ...
//                    ^~~~
//   1. StatementIf
//   2. StatementIfElseBlockFinish
//
// if ( ... ) { ... } else ...
//                    ^~~~
//   1. CodeBlock
//   2. StatementIfElseBlockFinish
//
// if ( ... ) { ... } ...
//   (state done)
CARBON_PARSE_STATE(StatementIfThenBlockFinish)

// Handles `if` processing after a provided `else` code block.
//
// if ( ... ) { ... } else { ... }
//                                ^
//   (state done)
CARBON_PARSE_STATE(StatementIfElseBlockFinish)

// Handles `return` processing.
//
// return ;
// ^~~~~~
//   1. StatementReturnFinish
//
// return var ...
// ^~~~~~~~~~
//   1. StatementReturnFinish
//
// return ...
// ^~~~~~
//   1. Expr
//   2. StatementReturnFinish
CARBON_PARSE_STATE(StatementReturn)

// Handles `return` processing at the `;`.
//
// return ... ;
//            ^
//   (state done)
CARBON_PARSE_STATE(StatementReturnFinish)

// Handles processing of statements within a scope.
//
// { ... }
//      ^
//   (state done)
//
// { ... ... }
//      ^
//   1. Statement
//   2. StatementScopeLoop
CARBON_PARSE_STATE(StatementScopeLoop)

// Handles `while` processing.
//
// while ...
// ^~~~~
//   1. ParenConditionAsWhile
//   2. StatementWhileConditionFinish
CARBON_PARSE_STATE(StatementWhile)

// Handles `while` processing between the condition and start of the code block.
//
// while ( ... ) ...
//              ^
//   1. CodeBlock
//   2. StatementWhileBlockFinish
CARBON_PARSE_STATE(StatementWhileConditionFinish)

// Handles `while` processing after the end of the code block.
//
// while ( ... ) { ... }
//                      ^
//   (state done)
CARBON_PARSE_STATE(StatementWhileBlockFinish)

// Handles parsing after the declaration scope of a type.
//
// class/impl/interface/constraint ... { ... }
//                                           ^
//   (state done)
CARBON_PARSE_STATE_VARIANTS4(DeclDefinitionFinish, Class, Impl, Interface,
                             NamedConstraint)

// Handles processing of a type after its introducer.
//
// class/interface/constraint ...
//                           ^
//   1. DeclNameAndParams
//   2. DeclOrDefinitionAs(Class|Interface|NamedConstraint)
CARBON_PARSE_STATE_VARIANTS3(TypeAfterIntroducer, Class, Interface,
                             NamedConstraint)

// Handles processing of a type after its optional parameters.
//
// class/impl/interface/constraint name ( ... ) {
//                                              ^
//   1. DeclScopeLoopAs(Class|Regular)
//   2. DeclDefinitionFinishAs(Class|Impl|Interface|NamedConstraint)
//
// class/impl/interface/constraint name ( ... ) ;
//                                              ^
// class/impl/interface/constraint name ( ... ) ???
//                                              ^
//   (state done)
CARBON_PARSE_STATE_VARIANTS4(DeclOrDefinition, Class, Impl, Interface,
                             NamedConstraint)

// Handles processing of an `adapt T` declaration after the introducer.
//
// adapt T ;
//      ^
//   1. Expr
//   2. AdaptDecl
CARBON_PARSE_STATE(AdaptAfterIntroducer)

// Handles processing of a completed `adapt T` declaration.
//
// adapt T ;
//         ^
// adapt T ??? ;
//         ^~~~~
//   (state done)
CARBON_PARSE_STATE(AdaptDecl)

// Handles processing of a `base: B` after the introducer.
//
// base: B ;
//     ^
//   1. Expr
//   2. BaseDecl
// base ??? ;
//      ^~~
//   (state done)
CARBON_PARSE_STATE(BaseAfterIntroducer)

// Handles processing of a completed `base: B` declaration.
//
// base: B ;
//         ^
// base: B ??? ;
//         ^~~~~
//   (state done)
CARBON_PARSE_STATE(BaseDecl)

// Handles processing of an `impl...as` declaration after the introducer.
//
// impl forall [ ...
//      ^~~~~~
//   1. PatternListAsImplicit
//   2. ImplAfterForall
//   3. DeclOrDefinitionAsImpl
// impl as ...
//      ^~
//   1. Expr
//   2. DeclOrDefinitionAsImpl
// impl TypeExpression as ...
//     ^
//   1. Expr
//   2. ImplBeforeAs
//   3. DeclOrDefinitionAsImpl
CARBON_PARSE_STATE(ImplAfterIntroducer)

// Handles processing of an `impl forall` declaration after the implicit
// parameter list.
//
// impl forall [ ... ] as ...
//                     ^~
//   1. Expr
// impl forall [ ... ] TypeExpression as ...
//                    ^
//   1. Expr
//   2. ImplBeforeAs
CARBON_PARSE_STATE(ImplAfterForall)

// Handles processing of the `as` in an `impl` declaration after the type
// expression.
//
// impl TypeExpression as ...
//                     ^~
//   1. Expr
CARBON_PARSE_STATE(ImplBeforeAs)

// Handles processing of a `require...impls` declaration after the introducer.
//
// require impls ...
//         ^~~~~
//   1. Expr
//   2. RequireDecl
// require TypeExpression impls ...
//        ^
//   1. Expr
//   2. RequireBeforeImpls
//   3. RequireDecl
CARBON_PARSE_STATE(RequireAfterIntroducer)

// Handles processing of the `impls` in a `require` declaration after the type
// expression.
//
// require TypeExpression impls ...
//                        ^~~~~
//   1. Expr
CARBON_PARSE_STATE(RequireBeforeImpls)

// Handles processing of a completed `require` declaration.
//
// require TypeExpression impls FacetTypeExpression ;
//                                                  ^
//   (state done)
CARBON_PARSE_STATE(RequireDecl)

// Handles the start of a `var` or `returned var` in a non-class context.
//
// var ...             (variant is Regular)
//    ^
//   1. Pattern
//   2. VarAfterPatternAsVar
//   3. VarFinishAsRegular
//
// returned var ...    (variant is Returned)
// ^~~~~~~~~~~~
//   1. Pattern
//   2. VarAfterPatternAsVar
//   3. VarFinishAsRegular
//
// returned ??? ;      (variant is Returned)
// ^~~~~~~~~~~~~~
//   (state done)
CARBON_PARSE_STATE_VARIANTS2(Var, Regular, Returned)

// Handles `var` after the pattern, either followed by an initializer or the
// semicolon.
//
// var ... = ...
//         ^
// var ... ??? = ...
//         ^~~~~
//   1. Expr
//
// var ... ...
//        ^
//   (state done)
CARBON_PARSE_STATE_VARIANTS2(VarAfterPattern, Var, Field)

// Handles `var` parsing at the end.
//
// var ... ;
//         ^
// var ... ??? ;
//         ^~~~~
//   (state done)
CARBON_PARSE_STATE_VARIANTS2(VarFinish, Regular, Field)

// Handles the beginning of a field declaration (`var` in a class context).
//
// var name : type
//     ^~~~~~
//   1. Expr
//   2. VarAfterPatternAsField
//   3. VarFinishAsField
//
// var ??? ;
//     ^~~~~
// var name ??? ;
//     ^~~~~~~~~
//   (state done)
CARBON_PARSE_STATE(FieldDecl)

// Handles the start of a `let`.
//
// let ...
//    ^
//   1. Pattern
//   2. LetAfterPattern
//   3. LetFinish
CARBON_PARSE_STATE(Let)

// Handles `let` after the pattern, optionally followed by an initializer. The
// initializer is required except in an associated constant declaration, but
// that is enforced by check.
//
// let ... = ...
//         ^
// let ... ??? = ...
//         ^~~~~
//   1. Expr
//
// let ... ;
//        ^
// let ... ??? ;
//         ^~~
//   (state done)
CARBON_PARSE_STATE_VARIANTS2(LetAfterPattern, Regular, AssociatedConstant)

// Handles `let` parsing at the end.
//
// let ... ;
//         ^
// let ... ??? ;
//         ^~~~~
//   (state done)
CARBON_PARSE_STATE_VARIANTS2(LetFinish, Regular, AssociatedConstant)

// Handles the start of an associated constant declaration (`let` in an
// interface context).
//
// let name :! ...
//     ^~~~~~~
//   1. AssociatedConstantFinish
// let ??? ;
//     ^~~~~
// let name ??? ;
//     ^~~~~~~~~
//   (state done)
CARBON_PARSE_STATE(AssociatedConstant)

// Handles a choice's introducer.
//
// choice ...
// ^~~~~~
//   1. DeclNameAndParams
//   2. ChoiceDefinitionStart
//   3. ChoiceDefinitionFinish
CARBON_PARSE_STATE(ChoiceIntroducer)

// Handles processing of a choice after its optional parameters.
//
// choice name ... {}
//                 ^
//   (state done)
//
// choice name ... { ... }
//                 ^
//   1. ChoiceAlternative
//
// choice name ... ???
//                 ^
//   (state done)
CARBON_PARSE_STATE(ChoiceDefinitionStart)

// Starts alternative parsing.
//
//  name( ... )
//  ^~~~
//   1. ParamListAsRegular
//   2. ChoiceAlternativeFinish
//  name ...
//  ^~~~
//   1. ChoiceAlternativeFinish
CARBON_PARSE_STATE(ChoiceAlternative)

// Finishes parsing a choice's alternative, including the optional trailing `,`.
// If there are more alternatives, enqueues another alternative parsing state.
//
// ... , }
//     ^
//   (state done)
//
// ... , ...
//     ^
//   1. ChoiceAlternative
//
// ...
//    ^
//   (state done)
CARBON_PARSE_STATE(ChoiceAlternativeFinish)

// Finishes a choice definition.
//
// choice ... }
//            ^
//   (state done)
CARBON_PARSE_STATE(ChoiceDefinitionFinish)

// Handles `match` introducer.
//
// match ...
// ^~~~~
//   1. ParenConditionAsMatch
//   2. MatchConditionFinish
CARBON_PARSE_STATE(MatchIntroducer)

// Handles `match` cases block start after the condition.
//
// match (...) { ... }
//             ^
//   1. MatchCaseLoop
//   2. MatchStatementFinish
//
// match (...) ???
//             ^
//   (state done)
CARBON_PARSE_STATE(MatchConditionFinish)

// Handles `match` cases.
//
// match (...) { case ...}
//              ^
//   1. MatchCaseIntroducer
//   2. MatchCaseLoop
// match (...) { default ...}
//              ^
//   1. MatchDefaultIntroducer
//   2. MatchCaseLoopAfterDefault
CARBON_PARSE_STATE(MatchCaseLoop)

// Handles `match` `case` introducer.
//
// match (...) { case ...}
//               ^~~~
//   1. Pattern
//   2. MatchCaseAfterPattern
CARBON_PARSE_STATE(MatchCaseIntroducer)

// Handles `match` case after pattern.
//
// match (...) { case ... => ... }
//                       ^
//   1. MatchCaseStart
// match (...) { case ... if (...) }
//                        ^~~~
//   1. Expr
//   2. MatchCaseGuardFinish
//   3. MatchCaseStart
CARBON_PARSE_STATE(MatchCaseAfterPattern)

// Handles `match` case guard closing parenthesis.
//
// match (...) { case ... if (...) => ... }
//                               ^
//   (state done)
CARBON_PARSE_STATE(MatchCaseGuardFinish)

// Handles `match` case `=>` and `{` opening statements block.
//
// match (...) { case ... => {...} }
//                        ^~~~
//   1. StatementScopeLoop
//   2. MatchCaseFinish
//
// match (...) { case ... ??? }
//                        ^
//   (state done)
//
// match (...) { case ... => ??? }
//                           ^
//   (state done)
CARBON_PARSE_STATE(MatchCaseStart)

// Handles `match` case statements block closing `}`.
//
// match (...) { case ... => {...} }
//                               ^
//   (state done)
CARBON_PARSE_STATE(MatchCaseFinish)

// Handles `match` default introducer, `=>` and `{` opening statements block.
//
// match (...) { default => {...} }
//               ^~~~~~~~~~~~
//   1. StatementScopeLoop
//   2. MatchDefaultFinish
//
// match (...) { default ??? }
//                       ^
//   (state done)
//
// match (...) { default => ??? }
//                          ^
//   (state done)
CARBON_PARSE_STATE(MatchDefaultIntroducer)

// Handles `match` default case statements block closing `}`.
//
// match (...) { default => {...} }
//                              ^
//   (state done)
CARBON_PARSE_STATE(MatchDefaultFinish)

// Handles `match` cases after the `default` case.
//
// match (...) { default => {...} case ... }
//                                ^~~~
//   1. MatchCaseLoopAfterDefault
//
// match (...) { default => {...} default ... }
//                                ^~~~~~~
//   1. MatchCaseLoopAfterDefault
//
// match (...) { default => {...} }
//                                ^
//   (state done)
CARBON_PARSE_STATE(MatchCaseLoopAfterDefault)

// Finishes `match` statement.
//
// match (...) {...}
//                 ^
//   (state done)
CARBON_PARSE_STATE(MatchStatementFinish)

#undef CARBON_PARSE_STATE