Allow return types of initializers

SE-2305 allows returning `Self` and `Self?` from initializers so that the return type can be annotated with `_borrow(a)` etc.

So we need to allow return clauses on initializers.

rdar://123905900

Author: ahoppen

Sources/SwiftParser/Declarations.swift

@@ -968,7 +968,7 @@ extension Parser {
     }
 
     // Parse the signature.
-    let signature = self.parseFunctionSignature(allowOutput: false)
+    let signature = self.parseFunctionSignature()
 
     let whereClause: RawGenericWhereClauseSyntax?
     if self.at(.keyword(.where)) {
@@ -1131,7 +1131,7 @@ extension Parser {
     )
   }
 
-  mutating func parseFunctionSignature(allowOutput: Bool = true) -> RawFunctionSignatureSyntax {
+  mutating func parseFunctionSignature() -> RawFunctionSignatureSyntax {
     let parameterClause = self.parseParameterClause(RawFunctionParameterClauseSyntax.self) { parser in
       parser.parseFunctionParameter()
     }
@@ -1148,19 +1148,10 @@ extension Parser {
       returnClause = nil
     }
 
-    var unexpectedAfterReturnClause: RawUnexpectedNodesSyntax?
-    if !allowOutput,
-      let unexpectedOutput = returnClause
-    {
-      returnClause = nil
-      unexpectedAfterReturnClause = RawUnexpectedNodesSyntax([unexpectedOutput], arena: self.arena)
-    }
-
     return RawFunctionSignatureSyntax(
       parameterClause: parameterClause,
       effectSpecifiers: effectSpecifiers,
       returnClause: returnClause,
-      unexpectedAfterReturnClause,
       arena: self.arena
     )
   }

Tests/SwiftParserTest/DeclarationTests.swift

@@ -3156,4 +3156,14 @@ final class DeclarationTests: ParserTestCase {
       XCTAssertEqual(decl.description, input, line: line)
     }
   }
+
+  func testInitializerWithReturnType() {
+    assertParse(
+      "init(_ ptr: UnsafeRawBufferPointer, _ a: borrowing Array<Int>) -> _borrow(a) Self",
+      experimentalFeatures: .nonescapableTypes
+    )
+
+    // Not actually valid, needs to be diagnosed during type checking
+    assertParse("public init() -> Int")
+  }
 }