Attempt to safely bitcast for printing of parameter packs (#369)

Author: stephencelis

Sources/Parsing/Builders/ParserBuilder.swift

@@ -272,13 +272,23 @@ extension ParserBuilder.Take2 {
 extension ParserBuilder.Take2.Map: ParserPrinter
 where P0: ParserPrinter, P1: ParserPrinter {
   public func print(_ output: NewOutput, into input: inout P0.Input) throws {
-    guard let tuple = output as? (P0.Output, P1.Output) else {
+    guard canBitCast(NewOutput.self, to: (P0.Output, P1.Output).self)
+    else {
       throw ParsingError.failed(
         summary: "Could not convert output to required tuple type",
         from: output,
         to: input
       )
     }
-    try upstream.print(tuple, into: &input)
+    try upstream.print(
+      unsafeBitCast(output, to: (P0.Output, P1.Output).self),
+      into: &input
+    )
   }
 }
+
+private func canBitCast<T, U>(_ type: T.Type, to otherType: U.Type) -> Bool {
+  MemoryLayout<T>.size == MemoryLayout<U>.size
+    && MemoryLayout<T>.alignment == MemoryLayout<U>.alignment
+    && MemoryLayout<T>.stride == MemoryLayout<U>.stride
+}

Tests/ParsingTests/ParserBuilderTests.swift

@@ -160,4 +160,46 @@ final class ParserBuilderTests: XCTestCase {
     }
     XCTAssertEqual(input, " Blob"[...])
   }
+
+  func testNestedPrint() throws {
+    let p1 = ParsePrint(input: Substring.self) {
+      Digits()
+      ","
+      Digits()
+    }
+    let p2 = ParsePrint(input: Substring.self) {
+      Digits()
+      ","
+      Digits()
+    }
+    let p3 = ParsePrint {
+      p1
+      ","
+      p2
+    }
+    var input = ""[...]
+    try p3.print((1, 2, (3, 4)), into: &input)
+    XCTAssertEqual(input, "1,2,3,4")
+  }
+
+  func testNestedPrint_differentLayouts() throws {
+    let p1 = ParsePrint(input: Substring.self) {
+      Int32.parser()
+      ","
+      Int8.parser()
+    }
+    let p2 = ParsePrint(input: Substring.self) {
+      Int8.parser()
+      ","
+      Int32.parser()
+    }
+    let p3 = ParsePrint {
+      p1
+      ","
+      p2
+    }
+    var input = ""[...]
+    try p3.print((1, 2, (3, 4)), into: &input)
+    XCTAssertEqual(input, "1,2,3,4")
+  }
 }