Parse an extended type as part of a nominal type declaration's name.

These are parser changes necessary for automatic extension synthesis
that would allow users to write

```swift
struct Outer.Inner {}
```

instead of

```swift
extension Outer {
  struct Inner {}
}
```

Author: allevato

CodeGeneration/Sources/SyntaxSupport/DeclNodes.swift

@@ -206,6 +206,17 @@ public let DECL_NODES: [Node] = [
         kind: .token(choices: [.keyword(.actor)]),
         documentation: "The `actor` keyword."
       ),
+      Child(
+        name: "extendedType",
+        kind: .node(kind: .type),
+        documentation: "The `type` in which this actor is nested.",
+        isOptional: true
+      ),
+      Child(
+        name: "period",
+        kind: .token(choices: [.token(.period)]),
+        isOptional: true
+      ),
       Child(
         name: "name",
         kind: .token(choices: [.token(.identifier)]),
@@ -386,6 +397,17 @@ public let DECL_NODES: [Node] = [
         kind: .token(choices: [.keyword(.class)]),
         documentation: "The `class` keyword for this declaration."
       ),
+      Child(
+        name: "extendedType",
+        kind: .node(kind: .type),
+        documentation: "The `type` in which this class is nested.",
+        isOptional: true
+      ),
+      Child(
+        name: "period",
+        kind: .token(choices: [.token(.period)]),
+        isOptional: true
+      ),
       Child(
         name: "name",
         kind: .token(choices: [.token(.identifier)]),
@@ -830,6 +852,17 @@ public let DECL_NODES: [Node] = [
         kind: .token(choices: [.keyword(.enum)]),
         documentation: "The `enum` keyword for this declaration."
       ),
+      Child(
+        name: "extendedType",
+        kind: .node(kind: .type),
+        documentation: "The `type` in which this enum declaration is nested.",
+        isOptional: true
+      ),
+      Child(
+        name: "period",
+        kind: .token(choices: [.token(.period)]),
+        isOptional: true
+      ),
       Child(
         name: "name",
         kind: .token(choices: [.token(.identifier)]),
@@ -2085,6 +2118,17 @@ public let DECL_NODES: [Node] = [
         kind: .token(choices: [.keyword(.protocol)]),
         documentation: "The `protocol` keyword for this declaration."
       ),
+      Child(
+        name: "extendedType",
+        kind: .node(kind: .type),
+        documentation: "The `type` in which this protocol is nested.",
+        isOptional: true
+      ),
+      Child(
+        name: "period",
+        kind: .token(choices: [.token(.period)]),
+        isOptional: true
+      ),
       Child(
         name: "name",
         kind: .token(choices: [.token(.identifier)]),
@@ -2265,6 +2309,17 @@ public let DECL_NODES: [Node] = [
         kind: .token(choices: [.keyword(.struct)]),
         documentation: "The `struct` keyword for this declaration."
       ),
+      Child(
+        name: "extendedType",
+        kind: .node(kind: .type),
+        documentation: "The `type` in which this struct is nested.",
+        isOptional: true
+      ),
+      Child(
+        name: "period",
+        kind: .token(choices: [.token(.period)]),
+        isOptional: true
+      ),
       Child(
         name: "name",
         kind: .token(choices: [.token(.identifier)]),
@@ -2432,6 +2487,17 @@ public let DECL_NODES: [Node] = [
         name: "typealiasKeyword",
         kind: .token(choices: [.keyword(.typealias)])
       ),
+      Child(
+        name: "extendedType",
+        kind: .node(kind: .type),
+        documentation: "The `type` in which this type alias is nested.",
+        isOptional: true
+      ),
+      Child(
+        name: "period",
+        kind: .token(choices: [.token(.period)]),
+        isOptional: true
+      ),
       Child(
         name: "name",
         kind: .token(choices: [.token(.identifier)])

Sources/SwiftParser/Declarations.swift

@@ -1660,6 +1660,15 @@ extension Parser {
     _ handle: RecoveryConsumptionHandle
   ) -> RawTypeAliasDeclSyntax {
     let (unexpectedBeforeTypealiasKeyword, typealiasKeyword) = self.eat(handle)
+
+    var extendedType: RawTypeSyntax? = nil
+    var unexpectedBeforePeriod: RawUnexpectedNodesSyntax? = nil
+    var period: RawTokenSyntax? = nil
+    if self.lookahead().canParseExtendedTypeForNominalTypeDecl() {
+      extendedType = parseSimpleType(parsingContext: .nominalTypeDeclExtendedName)
+      (unexpectedBeforePeriod, period) = self.expect(.period)
+    }
+
     let (unexpectedBeforeName, name) = self.expectIdentifier(keywordRecovery: true)
 
     // Parse a generic parameter list if it is present.
@@ -1700,6 +1709,9 @@ extension Parser {
       modifiers: attrs.modifiers,
       unexpectedBeforeTypealiasKeyword,
       typealiasKeyword: typealiasKeyword,
+      extendedType: extendedType,
+      unexpectedBeforePeriod,
+      period: period,
       unexpectedBeforeName,
       name: name,
       genericParameterClause: generics,

Sources/SwiftParser/Expressions.swift

@@ -1027,7 +1027,7 @@ extension Parser {
     // the token is an operator starts with '.', or the following token is '['.
     let rootType: RawTypeSyntax?
     if !self.at(prefix: ".") {
-      rootType = self.parseSimpleType(allowMemberTypes: false)
+      rootType = self.parseSimpleType(parsingContext: .keyPathRoot)
     } else {
       rootType = nil
     }

Sources/SwiftParser/Names.swift

@@ -271,6 +271,14 @@ extension Parser.Lookahead {
     }
     return lookahead.at(prefix: ".")
   }
+
+  func canParseExtendedTypeForNominalTypeDecl() -> Bool {
+    var lookahead = self.lookahead()
+    guard lookahead.canParseSimpleType(parsingContext: .nominalTypeDeclExtendedName) else {
+      return false
+    }
+    return lookahead.at(prefix: ".")
+  }
 }
 
 extension Parser.Lookahead {

Sources/SwiftParser/Nominals.swift

@@ -24,6 +24,9 @@ protocol NominalTypeDeclarationTrait {
     modifiers: RawDeclModifierListSyntax,
     _ unexpectedBeforeIntroducerKeyword: RawUnexpectedNodesSyntax?,
     introducerKeyword: RawTokenSyntax,
+    extendedType: RawTypeSyntax?,
+    _ unexpectedBeforePeriod: RawUnexpectedNodesSyntax?,
+    period: RawTokenSyntax?,
     _ unexpectedBeforeIdentifier: RawUnexpectedNodesSyntax?,
     name: RawTokenSyntax,
     primaryOrGenerics: PrimaryOrGenerics?,
@@ -42,6 +45,9 @@ extension RawProtocolDeclSyntax: NominalTypeDeclarationTrait {
     modifiers: RawDeclModifierListSyntax,
     _ unexpectedBeforeIntroducerKeyword: RawUnexpectedNodesSyntax?,
     introducerKeyword: RawTokenSyntax,
+    extendedType: RawTypeSyntax?,
+    _ unexpectedBeforePeriod: RawUnexpectedNodesSyntax?,
+    period: RawTokenSyntax?,
     _ unexpectedBeforeIdentifier: RawUnexpectedNodesSyntax?,
     name: RawTokenSyntax,
     primaryOrGenerics: RawPrimaryAssociatedTypeClauseSyntax?,
@@ -55,6 +61,9 @@ extension RawProtocolDeclSyntax: NominalTypeDeclarationTrait {
       modifiers: modifiers,
       unexpectedBeforeIntroducerKeyword,
       protocolKeyword: introducerKeyword,
+      extendedType: extendedType,
+      unexpectedBeforePeriod,
+      period: period,
       unexpectedBeforeIdentifier,
       name: name,
       primaryAssociatedTypeClause: primaryOrGenerics,
@@ -76,6 +85,9 @@ extension RawClassDeclSyntax: NominalTypeDeclarationTrait {
     modifiers: RawDeclModifierListSyntax,
     _ unexpectedBeforeIntroducerKeyword: RawUnexpectedNodesSyntax?,
     introducerKeyword: RawTokenSyntax,
+    extendedType: RawTypeSyntax?,
+    _ unexpectedBeforePeriod: RawUnexpectedNodesSyntax?,
+    period: RawTokenSyntax?,
     _ unexpectedBeforeIdentifier: RawUnexpectedNodesSyntax?,
     name: RawTokenSyntax,
     primaryOrGenerics: RawGenericParameterClauseSyntax?,
@@ -89,6 +101,9 @@ extension RawClassDeclSyntax: NominalTypeDeclarationTrait {
       modifiers: modifiers,
       unexpectedBeforeIntroducerKeyword,
       classKeyword: introducerKeyword,
+      extendedType: extendedType,
+      unexpectedBeforePeriod,
+      period: period,
       unexpectedBeforeIdentifier,
       name: name,
       genericParameterClause: primaryOrGenerics,
@@ -110,6 +125,9 @@ extension RawActorDeclSyntax: NominalTypeDeclarationTrait {
     modifiers: RawDeclModifierListSyntax,
     _ unexpectedBeforeIntroducerKeyword: RawUnexpectedNodesSyntax?,
     introducerKeyword: RawTokenSyntax,
+    extendedType: RawTypeSyntax?,
+    _ unexpectedBeforePeriod: RawUnexpectedNodesSyntax?,
+    period: RawTokenSyntax?,
     _ unexpectedBeforeIdentifier: RawUnexpectedNodesSyntax?,
     name: RawTokenSyntax,
     primaryOrGenerics: RawGenericParameterClauseSyntax?,
@@ -123,6 +141,9 @@ extension RawActorDeclSyntax: NominalTypeDeclarationTrait {
       modifiers: modifiers,
       unexpectedBeforeIntroducerKeyword,
       actorKeyword: introducerKeyword,
+      extendedType: extendedType,
+      unexpectedBeforePeriod,
+      period: period,
       unexpectedBeforeIdentifier,
       name: name,
       genericParameterClause: primaryOrGenerics,
@@ -144,6 +165,9 @@ extension RawStructDeclSyntax: NominalTypeDeclarationTrait {
     modifiers: RawDeclModifierListSyntax,
     _ unexpectedBeforeIntroducerKeyword: RawUnexpectedNodesSyntax?,
     introducerKeyword: RawTokenSyntax,
+    extendedType: RawTypeSyntax?,
+    _ unexpectedBeforePeriod: RawUnexpectedNodesSyntax?,
+    period: RawTokenSyntax?,
     _ unexpectedBeforeIdentifier: RawUnexpectedNodesSyntax?,
     name: RawTokenSyntax,
     primaryOrGenerics: RawGenericParameterClauseSyntax?,
@@ -157,6 +181,9 @@ extension RawStructDeclSyntax: NominalTypeDeclarationTrait {
       modifiers: modifiers,
       unexpectedBeforeIntroducerKeyword,
       structKeyword: introducerKeyword,
+      extendedType: extendedType,
+      unexpectedBeforePeriod,
+      period: period,
       unexpectedBeforeIdentifier,
       name: name,
       genericParameterClause: primaryOrGenerics,
@@ -178,6 +205,9 @@ extension RawEnumDeclSyntax: NominalTypeDeclarationTrait {
     modifiers: RawDeclModifierListSyntax,
     _ unexpectedBeforeIntroducerKeyword: RawUnexpectedNodesSyntax?,
     introducerKeyword: RawTokenSyntax,
+    extendedType: RawTypeSyntax?,
+    _ unexpectedBeforePeriod: RawUnexpectedNodesSyntax?,
+    period: RawTokenSyntax?,
     _ unexpectedBeforeIdentifier: RawUnexpectedNodesSyntax?,
     name: RawTokenSyntax,
     primaryOrGenerics: RawGenericParameterClauseSyntax?,
@@ -191,6 +221,9 @@ extension RawEnumDeclSyntax: NominalTypeDeclarationTrait {
       modifiers: modifiers,
       unexpectedBeforeIntroducerKeyword,
       enumKeyword: introducerKeyword,
+      extendedType: extendedType,
+      unexpectedBeforePeriod,
+      period: period,
       unexpectedBeforeIdentifier,
       name: name,
       genericParameterClause: primaryOrGenerics,
@@ -214,13 +247,25 @@ extension Parser {
     introucerHandle: RecoveryConsumptionHandle
   ) -> T where T: NominalTypeDeclarationTrait {
     let (unexpectedBeforeIntroducerKeyword, introducerKeyword) = self.eat(introucerHandle)
+
+    var extendedType: RawTypeSyntax? = nil
+    var unexpectedBeforePeriod: RawUnexpectedNodesSyntax? = nil
+    var period: RawTokenSyntax? = nil
+    if self.lookahead().canParseExtendedTypeForNominalTypeDecl() {
+      extendedType = parseSimpleType(parsingContext: .nominalTypeDeclExtendedName)
+      (unexpectedBeforePeriod, period) = self.expect(.period)
+    }
+
     let (unexpectedBeforeName, name) = self.expectIdentifier(keywordRecovery: true)
     if unexpectedBeforeName == nil && name.isMissing && self.atStartOfLine {
       return T.init(
         attributes: attrs.attributes,
         modifiers: attrs.modifiers,
         unexpectedBeforeIntroducerKeyword,
         introducerKeyword: introducerKeyword,
+        extendedType: extendedType,
+        unexpectedBeforePeriod,
+        period: period,
         unexpectedBeforeName,
         name: name,
         primaryOrGenerics: nil,
@@ -264,6 +309,9 @@ extension Parser {
       modifiers: attrs.modifiers,
       unexpectedBeforeIntroducerKeyword,
       introducerKeyword: introducerKeyword,
+      extendedType: extendedType,
+      unexpectedBeforePeriod,
+      period: period,
       unexpectedBeforeName,
       name: name,
       primaryOrGenerics: primaryOrGenerics,

Sources/SwiftParser/Types.swift

@@ -16,6 +16,13 @@
 @_spi(RawSyntax) @_spi(ExperimentalLanguageFeatures) import SwiftSyntax
 #endif
 
+enum SimpleTypeParsingContext {
+  case `default`
+  case keyPathRoot
+  case attributeName
+  case nominalTypeDeclExtendedName
+}
+
 extension Parser {
   /// Parse a type.
   mutating func parseType(misplacedSpecifiers: [RawTokenSyntax] = []) -> RawTypeSyntax {
@@ -187,12 +194,11 @@ extension Parser {
 
   /// Parse the subset of types that we allow in attribute names.
   mutating func parseAttributeName() -> RawTypeSyntax {
-    return parseSimpleType(forAttributeName: true)
+    return parseSimpleType(parsingContext: .attributeName)
   }
 
   mutating func parseSimpleType(
-    allowMemberTypes: Bool = true,
-    forAttributeName: Bool = false
+    parsingContext: SimpleTypeParsingContext = .default
   ) -> RawTypeSyntax {
     enum TypeBaseStart: TokenSpecSet {
       case `Self`
@@ -257,9 +263,25 @@ extension Parser {
       return wrapInTilde(RawTypeSyntax(RawMissingTypeSyntax(arena: self.arena)))
     }
 
+    func shouldContinueAfterPeriod() -> Bool {
+      if self.peek(isAt: .keyword(.Type), .keyword(.Protocol)) {
+        return true
+      }
+      switch parsingContext {
+      case .keyPathRoot:
+        return false
+      case .nominalTypeDeclExtendedName:
+        var lookahead = self.lookahead()
+        lookahead.eat(.period)
+        return lookahead.canParseExtendedTypeForNominalTypeDecl()
+      default:
+        return true
+      }
+    }
+
     var loopProgress = LoopProgressCondition()
     while self.hasProgressed(&loopProgress) {
-      if self.at(.period) && (allowMemberTypes || self.peek(isAt: .keyword(.Type), .keyword(.Protocol))) {
+      if self.at(.period) && shouldContinueAfterPeriod() {
         let (unexpectedPeriod, period, skipMemberName) = self.consumeMemberPeriod(previousNode: base)
         if skipMemberName {
           let missingIdentifier = missingToken(.identifier)
@@ -315,7 +337,7 @@ extension Parser {
       }
 
       // Do not allow ? or ! suffixes when parsing attribute names.
-      if forAttributeName {
+      if parsingContext == .attributeName {
         break
       }
 
@@ -691,7 +713,7 @@ extension Parser.Lookahead {
     return true
   }
 
-  mutating func canParseSimpleType() -> Bool {
+  mutating func canParseSimpleType(parsingContext: SimpleTypeParsingContext = .default) -> Bool {
     switch self.currentToken {
     case TokenSpec(.Any):
       self.consumeAnyToken()
@@ -736,6 +758,10 @@ extension Parser.Lookahead {
     var loopProgress = LoopProgressCondition()
     while self.hasProgressed(&loopProgress) {
       if self.at(.period) {
+        if parsingContext == .nominalTypeDeclExtendedName {
+          return true
+        }
+
         self.consumeAnyToken()
         if self.at(.keyword(.Type)) || self.at(.keyword(.Protocol)) {
           self.consumeAnyToken()