Use weak let when available (#564)

This suppresses some warnings on newer builds.

Author: Lukasa

Tests/NIOSSLTests/ObjectIdentifierTests.swift

@@ -76,7 +76,12 @@ final class ObjectIdentifierTests: XCTestCase {
 
     func testUnowned() {
         var owner: Optional = OIDMemoryOwner("1.2.3")!
+
+        #if compiler(>=6.3)
+        weak let weakReferenceToOwner = owner
+        #else
         weak var weakReferenceToOwner = owner
+        #endif
 
         var oid: Optional = NIOSSLObjectIdentifier(borrowing: owner!.reference, owner: owner!)
         XCTAssertEqual(oid?.description, "1.2.3")
@@ -95,7 +100,12 @@ final class ObjectIdentifierTests: XCTestCase {
 
     func testCopy() {
         var owner: Optional = OIDMemoryOwner("1.2.3")!
+
+        #if compiler(>=6.3)
+        weak let weakReferenceToOwner = owner
+        #else
         weak var weakReferenceToOwner = owner
+        #endif
 
         let oid: Optional = withExtendedLifetime(owner) {
             NIOSSLObjectIdentifier(copyOf: $0?.reference)

Tests/NIOSSLTests/SSLCertificateExtensionsTests.swift

@@ -67,7 +67,12 @@ final class SSLCertificateExtensionsTests: XCTestCase {
 
     func testUnowned() throws {
         var owner: Optional = try NIOSSLCertificate(bytes: Array(samplePemCert.utf8), format: .pem)
+
+        #if compiler(>=6.3)
+        weak let weakReferenceToOwner = owner
+        #else
         weak var weakReferenceToOwner = owner
+        #endif
 
         var extensions: Optional = owner!._extensions
         XCTAssertEqual(extensions.map { Array($0) }?.count, 3)