Merge pull request swiftlang#217 from allevato/async-keyword

Add build-only support for the `async` keyword.

Author: allevato

Sources/SwiftFormatPrettyPrint/TokenStreamCreator.swift

@@ -1401,6 +1401,7 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
   override func visit(_ node: FunctionTypeSyntax) -> SyntaxVisitorContinueKind {
     after(node.leftParen, tokens: .break(.open, size: 0), .open)
     before(node.rightParen, tokens: .break(.close, size: 0), .close)
+    before(node.asyncKeyword, tokens: .break)
     before(node.throwsOrRethrowsKeyword, tokens: .break)
     before(node.arrow, tokens: .break)
     before(node.returnType.firstToken, tokens: .break)
@@ -1603,6 +1604,7 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
   }
 
   override func visit(_ node: FunctionSignatureSyntax) -> SyntaxVisitorContinueKind {
+    before(node.asyncKeyword, tokens: .break)
     before(node.throwsOrRethrowsKeyword, tokens: .break)
     before(node.output?.firstToken, tokens: .break)
     return .visitChildren

Sources/SwiftFormatRules/UseShorthandTypeNames.swift

@@ -414,6 +414,7 @@ public final class UseShorthandTypeNames: SyntaxFormatRule {
         leftParen: functionType.leftParen,
         argumentTypes: functionType.arguments,
         rightParen: functionType.rightParen,
+        asyncKeyword: functionType.asyncKeyword,
         throwsOrRethrowsKeyword: functionType.throwsOrRethrowsKeyword,
         arrow: functionType.arrow,
         returnType: functionType.returnType
@@ -455,6 +456,7 @@ public final class UseShorthandTypeNames: SyntaxFormatRule {
     leftParen: TokenSyntax,
     argumentTypes: TupleTypeElementListSyntax,
     rightParen: TokenSyntax,
+    asyncKeyword: TokenSyntax?,
     throwsOrRethrowsKeyword: TokenSyntax?,
     arrow: TokenSyntax,
     returnType: TypeSyntax
@@ -471,6 +473,7 @@ public final class UseShorthandTypeNames: SyntaxFormatRule {
       elementList: argumentTypeExprs,
       rightParen: rightParen)
     let arrowExpr = SyntaxFactory.makeArrowExpr(
+      asyncKeyword: asyncKeyword,
       throwsToken: throwsOrRethrowsKeyword,
       arrowToken: arrow)