Introduce an experimental feature for typed throws

Author: DougGregor

Sources/SwiftParser/Specifiers.swift

@@ -654,7 +654,7 @@ extension Parser {
       throwsKeyword = throwsKw
     }
 
-    if throwsKeyword != nil && self.at(.leftParen) {
+    if throwsKeyword != nil && self.at(.leftParen) && experimentalFeatures.contains(.typedThrows) {
       thrownType = parseThrownType()
     }
 

Sources/SwiftParser/generated/ExperimentalFeatures.swift

@@ -29,4 +29,7 @@ extension Parser.ExperimentalFeatures {
 
   /// Whether to enable the parsing of 'then' statements.
   public static let thenStatements = Self(rawValue: 1 << 1)
+
+  /// Whether to enable the parsing of typed throws.
+  public static let typedThrows = Self(rawValue: 1 << 2)
 }

Tests/SwiftParserTest/DeclarationTests.swift

@@ -893,15 +893,17 @@ final class DeclarationTests: ParserTestCase {
 
   func testTypedThrows() {
     assertParse(
-      "func test() throws(any Error) -> Int { }"
+      "func test() throws(any Error) -> Int { }",
+      experimentalFeatures: [.typedThrows]
     )
 
     assertParse(
       """
       struct X {
         init() throws(any Error) { }
       }
-      """
+      """,
+      experimentalFeatures: [.typedThrows]
     )
   }
 

Tests/SwiftParserTest/TypeTests.swift

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_spi(RawSyntax) import SwiftParser
+@_spi(RawSyntax)  @_spi(ExperimentalLanguageFeatures) import SwiftParser
 @_spi(RawSyntax) import SwiftSyntax
 import XCTest
 
@@ -309,11 +309,18 @@ final class TypeTests: ParserTestCase {
     assertParse(
       """
       { () throws(PosixError) -> Void in }
-      """
+      """,
+      experimentalFeatures: [.typedThrows]
     )
 
-    assertParse("typealias T = () throws(PosixError) -> Void")
+    assertParse(
+      "typealias T = () throws(PosixError) -> Void",
+      experimentalFeatures: [.typedThrows]
+    )
 
-    assertParse("[() throws(PosixError) -> Void]()")
+    assertParse(
+      "[() throws(PosixError) -> Void]()",
+      experimentalFeatures: [.typedThrows]
+    )
   }
 }