diff --git a/Package.swift b/Package.swift
index 3d5543627e..80e340d73d 100644
--- a/Package.swift
+++ b/Package.swift
@@ -25,11 +25,18 @@ let historicalNIOPosixDependencyRequired: [Platform] = [.macOS, .iOS, .tvOS, .wa
 
 let strictConcurrencyDevelopment = false
 
-let strictConcurrencySettings: [SwiftSetting] = {
+// Standard Swift settings we apply to all Swift targets
+let standardSwiftSettings: [SwiftSetting] = {
     var initialSettings: [SwiftSetting] = []
     initialSettings.append(contentsOf: [
         .enableUpcomingFeature("StrictConcurrency"),
         .enableUpcomingFeature("InferSendableFromCaptures"),
+
+        // The Language Steering Group has promised that they won't break the APIs that currently exist under
+        // this "experimental" feature flag without two subsequent releases. We assume they will respect that
+        // promise, so we enable this here. For more, see:
+        // https://forums.swift.org/t/experimental-support-for-lifetime-dependencies-in-swift-6-2-and-beyond/78638
+        .enableExperimentalFeature("Lifetimes"),
     ])
 
     if strictConcurrencyDevelopment {
@@ -83,15 +90,15 @@ let package = Package(
                 swiftCollections,
                 swiftAtomics,
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "_NIODataStructures",
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "_NIOBase64",
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "NIOEmbedded",
@@ -102,7 +109,7 @@ let package = Package(
                 swiftAtomics,
                 swiftCollections,
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "NIOPosix",
@@ -118,7 +125,7 @@ let package = Package(
             ],
             exclude: includePrivacyManifest ? [] : ["PrivacyInfo.xcprivacy"],
             resources: includePrivacyManifest ? [.copy("PrivacyInfo.xcprivacy")] : [],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "NIO",
@@ -127,7 +134,7 @@ let package = Package(
                 "NIOEmbedded",
                 "NIOPosix",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "_NIOConcurrency",
@@ -135,7 +142,7 @@ let package = Package(
                 .target(name: "NIO", condition: .when(platforms: historicalNIOPosixDependencyRequired)),
                 "NIOCore",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "NIOFoundationCompat",
@@ -143,7 +150,7 @@ let package = Package(
                 .target(name: "NIO", condition: .when(platforms: historicalNIOPosixDependencyRequired)),
                 "NIOCore",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "CNIOAtomics",
@@ -190,7 +197,7 @@ let package = Package(
             dependencies: [
                 "CNIOAtomics"
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "NIOHTTP1",
@@ -201,7 +208,7 @@ let package = Package(
                 "CNIOLLHTTP",
                 swiftCollections,
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "NIOWebSocket",
@@ -212,7 +219,7 @@ let package = Package(
                 "CNIOSHA1",
                 "_NIOBase64",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "CNIOLLHTTP",
@@ -228,7 +235,7 @@ let package = Package(
                 "NIOCore",
                 swiftCollections,
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "NIOTestUtils",
@@ -239,7 +246,7 @@ let package = Package(
                 "NIOHTTP1",
                 swiftAtomics,
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .target(
             name: "NIOFileSystem",
@@ -255,7 +262,7 @@ let package = Package(
             path: "Sources/NIOFileSystem",
             exclude: includePrivacyManifest ? [] : ["PrivacyInfo.xcprivacy"],
             resources: includePrivacyManifest ? [.copy("PrivacyInfo.xcprivacy")] : [],
-            swiftSettings: strictConcurrencySettings + [
+            swiftSettings: standardSwiftSettings + [
                 .define("ENABLE_MOCKING", .when(configuration: .debug))
             ]
         ),
@@ -266,7 +273,7 @@ let package = Package(
                 "NIOFoundationCompat",
             ],
             path: "Sources/NIOFileSystemFoundationCompat",
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
 
         .target(
@@ -283,7 +290,7 @@ let package = Package(
             path: "Sources/_NIOFileSystem",
             exclude: includePrivacyManifest ? [] : ["PrivacyInfo.xcprivacy"],
             resources: includePrivacyManifest ? [.copy("PrivacyInfo.xcprivacy")] : [],
-            swiftSettings: strictConcurrencySettings + [
+            swiftSettings: standardSwiftSettings + [
                 .define("ENABLE_MOCKING", .when(configuration: .debug))
             ]
         ),
@@ -294,7 +301,7 @@ let package = Package(
                 "NIOFoundationCompat",
             ],
             path: "Sources/_NIOFileSystemFoundationCompat",
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
 
         // MARK: - Examples
@@ -306,7 +313,7 @@ let package = Package(
                 "NIOCore",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOTCPEchoClient",
@@ -315,7 +322,7 @@ let package = Package(
                 "NIOCore",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOEchoServer",
@@ -325,7 +332,7 @@ let package = Package(
                 "NIOConcurrencyHelpers",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOEchoClient",
@@ -335,7 +342,7 @@ let package = Package(
                 "NIOConcurrencyHelpers",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOHTTP1Server",
@@ -346,7 +353,7 @@ let package = Package(
                 "NIOConcurrencyHelpers",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOHTTP1Client",
@@ -357,7 +364,7 @@ let package = Package(
                 "NIOConcurrencyHelpers",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOChatServer",
@@ -367,7 +374,7 @@ let package = Package(
                 "NIOConcurrencyHelpers",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOChatClient",
@@ -377,7 +384,7 @@ let package = Package(
                 "NIOConcurrencyHelpers",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOWebSocketServer",
@@ -388,7 +395,7 @@ let package = Package(
                 "NIOWebSocket",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOWebSocketClient",
@@ -399,7 +406,7 @@ let package = Package(
                 "NIOWebSocket",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOMulticastChat",
@@ -407,7 +414,7 @@ let package = Package(
                 "NIOPosix",
                 "NIOCore",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOUDPEchoServer",
@@ -416,7 +423,7 @@ let package = Package(
                 "NIOCore",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOUDPEchoClient",
@@ -425,7 +432,7 @@ let package = Package(
                 "NIOCore",
             ],
             exclude: ["README.md"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOAsyncAwaitDemo",
@@ -434,7 +441,7 @@ let package = Package(
                 "NIOCore",
                 "NIOHTTP1",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
 
         // MARK: - Tests
@@ -449,7 +456,7 @@ let package = Package(
                 "NIOFoundationCompat",
                 "NIOWebSocket",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .executableTarget(
             name: "NIOCrashTester",
@@ -461,7 +468,7 @@ let package = Package(
                 "NIOWebSocket",
                 "NIOFoundationCompat",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOCoreTests",
@@ -473,7 +480,7 @@ let package = Package(
                 "NIOTestUtils",
                 swiftAtomics,
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOEmbeddedTests",
@@ -482,7 +489,7 @@ let package = Package(
                 "NIOCore",
                 "NIOEmbedded",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOPosixTests",
@@ -497,7 +504,7 @@ let package = Package(
                 "CNIODarwin",
                 "NIOTLS",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOConcurrencyHelpersTests",
@@ -505,17 +512,17 @@ let package = Package(
                 "NIOConcurrencyHelpers",
                 "NIOCore",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIODataStructuresTests",
             dependencies: ["_NIODataStructures"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOBase64Tests",
             dependencies: ["_NIOBase64"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOHTTP1Tests",
@@ -527,7 +534,7 @@ let package = Package(
                 "NIOFoundationCompat",
                 "NIOTestUtils",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOTLSTests",
@@ -538,7 +545,7 @@ let package = Package(
                 "NIOFoundationCompat",
                 "NIOTestUtils",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOWebSocketTests",
@@ -547,7 +554,7 @@ let package = Package(
                 "NIOEmbedded",
                 "NIOWebSocket",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOTestUtilsTests",
@@ -557,7 +564,7 @@ let package = Package(
                 "NIOEmbedded",
                 "NIOPosix",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOFoundationCompatTests",
@@ -565,17 +572,17 @@ let package = Package(
                 "NIOCore",
                 "NIOFoundationCompat",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOTests",
             dependencies: ["NIO"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOSingletonsTests",
             dependencies: ["NIOCore", "NIOPosix"],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOFileSystemTests",
@@ -586,7 +593,7 @@ let package = Package(
                 swiftCollections,
                 swiftSystem,
             ],
-            swiftSettings: strictConcurrencySettings + [
+            swiftSettings: standardSwiftSettings + [
                 .define("ENABLE_MOCKING", .when(configuration: .debug))
             ]
         ),
@@ -604,7 +611,7 @@ let package = Package(
                 // the build.
                 "Test Data"
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
         .testTarget(
             name: "NIOFileSystemFoundationCompatTests",
@@ -612,7 +619,7 @@ let package = Package(
                 "NIOFileSystem",
                 "NIOFileSystemFoundationCompat",
             ],
-            swiftSettings: strictConcurrencySettings
+            swiftSettings: standardSwiftSettings
         ),
     ]
 )
diff --git a/Sources/NIOCore/ByteBuffer-aux.swift b/Sources/NIOCore/ByteBuffer-aux.swift
index e95ab8f37d..792b48a273 100644
--- a/Sources/NIOCore/ByteBuffer-aux.swift
+++ b/Sources/NIOCore/ByteBuffer-aux.swift
@@ -785,6 +785,36 @@ extension ByteBuffer {
         self = ByteBufferAllocator().buffer(dispatchData: dispatchData)
     }
     #endif
+
+    #if compiler(>=6.2)
+    /// Create a fresh ``ByteBuffer`` with a minimum size, and initializing it safely via an `OutputSpan`.
+    ///
+    /// This will allocate a new ``ByteBuffer`` with at least `capacity` bytes of storage, and then calls
+    /// `initializer` with an `OutputSpan` over the entire allocated storage. This is a convenient method
+    /// to initialize a buffer directly and safely in a single allocation, including from C code.
+    ///
+    /// Once this call returns, the buffer will have its ``ByteBuffer/writerIndex`` appropriately advanced to encompass
+    /// any memory initialized by the `initializer`. Uninitialized memory will be after the ``ByteBuffer/writerIndex``,
+    /// available for subsequent use.
+    ///
+    /// - info: If you have access to a `Channel`, `ChannelHandlerContext`, or `ByteBufferAllocator` we
+    ///         recommend using `channel.allocator.buffer(capacity:initializingWith:)`. Or if you want to write multiple items into
+    ///         the buffer use `channel.allocator.buffer(capacity: ...)` to allocate a `ByteBuffer` of the right
+    ///         size followed by a `write(minimumWritableBytes:initializingWith:)` instead of using this method. This allows SwiftNIO to do
+    ///         accounting and optimisations of resources acquired for operations on a given `Channel` in the future.
+    ///
+    /// - parameters:
+    ///     - capacity: The minimum initial space to allocate for the buffer.
+    ///     - initializer: The initializer that will be invoked to initialize the allocated memory.
+    @inlinable
+    @available(macOS 10.14.4, iOS 12.2, watchOS 5.2, tvOS 12.2, visionOS 1.0, *)
+    public init(
+        initialCapacity capacity: Int,
+        initializingWith initializer: (_ span: inout OutputRawSpan) throws -> Void
+    ) rethrows {
+        self = try ByteBufferAllocator().buffer(capacity: capacity, initializingWith: initializer)
+    }
+    #endif
 }
 
 extension ByteBuffer: Codable {
@@ -924,6 +954,32 @@ extension ByteBufferAllocator {
         return buffer
     }
     #endif
+
+    #if compiler(>=6.2)
+    /// Create a fresh ``ByteBuffer`` with a minimum size, and initializing it safely via an `OutputSpan`.
+    ///
+    /// This will allocate a new ``ByteBuffer`` with at least `capacity` bytes of storage, and then calls
+    /// `initializer` with an `OutputSpan` over the entire allocated storage. This is a convenient method
+    /// to initialize a buffer directly and safely in a single allocation, including from C code.
+    ///
+    /// Once this call returns, the buffer will have its ``ByteBuffer/writerIndex`` appropriately advanced to encompass
+    /// any memory initialized by the `initializer`. Uninitialized memory will be after the ``ByteBuffer/writerIndex``,
+    /// available for subsequent use.
+    ///
+    /// - parameters:
+    ///     - capacity: The minimum initial space to allocate for the buffer.
+    ///     - initializer: The initializer that will be invoked to initialize the allocated memory.
+    @inlinable
+    @available(macOS 10.14.4, iOS 12.2, watchOS 5.2, tvOS 12.2, visionOS 1.0, *)
+    public func buffer(
+        capacity: Int,
+        initializingWith initializer: (_ span: inout OutputRawSpan) throws -> Void
+    ) rethrows -> ByteBuffer {
+        var buffer = self.buffer(capacity: capacity)
+        try buffer.writeWithOutputRawSpan(minimumWritableBytes: capacity, initializingWith: initializer)
+        return buffer
+    }
+    #endif
 }
 
 extension Optional where Wrapped == ByteBuffer {
diff --git a/Sources/NIOCore/ByteBuffer-core.swift b/Sources/NIOCore/ByteBuffer-core.swift
index add0340b77..e0287fb733 100644
--- a/Sources/NIOCore/ByteBuffer-core.swift
+++ b/Sources/NIOCore/ByteBuffer-core.swift
@@ -691,6 +691,52 @@ public struct ByteBuffer {
         return try body(.init(rebasing: self._slicedStorageBuffer[range]))
     }
 
+    #if compiler(>=6.2)
+    /// Provides safe high-performance read-only access to the readable bytes of this buffer.
+    @inlinable
+    @available(macOS 10.14.4, iOS 12.2, watchOS 5.2, tvOS 12.2, visionOS 1.0, *)
+    public var readableBytesSpan: RawSpan {
+        @_lifetime(borrow self)
+        borrowing get {
+            let range = Range<Int>(uncheckedBounds: (lower: self.readerIndex, upper: self.writerIndex))
+            return _overrideLifetime(RawSpan(_unsafeBytes: self._slicedStorageBuffer[range]), borrowing: self)
+        }
+    }
+
+    /// Provides mutable access to the readable bytes of this buffer.
+    @inlinable
+    @available(macOS 10.14.4, iOS 12.2, watchOS 5.2, tvOS 12.2, visionOS 1.0, *)
+    public var mutableReadableBytesSpan: MutableRawSpan {
+        @_lifetime(&self)
+        mutating get {
+            self._copyStorageAndRebaseIfNeeded()
+            let range = Range<Int>(uncheckedBounds: (lower: self.readerIndex, upper: self.writerIndex))
+            return _overrideLifetime(MutableRawSpan(_unsafeBytes: self._slicedStorageBuffer[range]), mutating: &self)
+        }
+    }
+
+    /// Enables high-performance low-level appending into the writable section of this buffer.
+    ///
+    /// The writer index will be advanced by the number of bytes written into the
+    /// `OutputRawSpan`.
+    ///
+    /// - parameters:
+    ///     - minimumWritableBytes: The minimum initial space to allocate for the buffer.
+    ///     - initializer: The initializer that will be invoked to initialize the allocated memory.
+    @inlinable
+    @available(macOS 10.14.4, iOS 12.2, watchOS 5.2, tvOS 12.2, visionOS 1.0, *)
+    public mutating func writeWithOutputRawSpan(
+        minimumWritableBytes: Int,
+        initializingWith initializer: (_ span: inout OutputRawSpan) throws -> Void
+    ) rethrows {
+        try self.writeWithUnsafeMutableBytes(minimumWritableBytes: minimumWritableBytes) { ptr in
+            var span = OutputRawSpan(buffer: ptr, initializedCount: 0)
+            try initializer(&span)
+            return span.byteCount
+        }
+    }
+    #endif
+
     /// Yields the bytes currently writable (`bytesWritable` = `capacity` - `writerIndex`). Before reading those bytes you must first
     /// write to them otherwise you will trigger undefined behaviour. The writer index will remain unchanged.
     ///
diff --git a/Tests/NIOCoreTests/ByteBufferSpanTests.swift b/Tests/NIOCoreTests/ByteBufferSpanTests.swift
new file mode 100644
index 0000000000..90a61c4cb3
--- /dev/null
+++ b/Tests/NIOCoreTests/ByteBufferSpanTests.swift
@@ -0,0 +1,248 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftNIO open source project
+//
+// Copyright (c) 2025 Apple Inc. and the SwiftNIO project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import XCTest
+
+import NIOCore
+
+#if compiler(>=6.2)
+@available(macOS 10.14.4, iOS 12.2, watchOS 5.2, tvOS 12.2, visionOS 1.0, *)
+final class ByteBufferSpanTests: XCTestCase {
+    func testReadableBytesSpanOfEmptyByteBuffer() {
+        let bb = ByteBuffer()
+        XCTAssertEqual(bb.readableBytesSpan.byteCount, 0)
+    }
+
+    func testReadableBytesSpanOfSimpleBuffer() {
+        let bb = ByteBuffer(string: "Hello, world!")
+        XCTAssertEqual(bb.readableBytesSpan.byteCount, 13)
+        XCTAssertTrue(bb.readableBytesSpan.elementsEqual("Hello, world!".utf8))
+    }
+
+    func testReadableBytesSpanNotAtTheStart() {
+        var bb = ByteBuffer(string: "Hello, world!")
+        bb.moveReaderIndex(forwardBy: 5)
+        XCTAssertEqual(bb.readableBytesSpan.byteCount, 8)
+        XCTAssertTrue(bb.readableBytesSpan.elementsEqual(", world!".utf8))
+    }
+
+    func testReadableBytesSpanOfSlice() {
+        let first = ByteBuffer(string: "Hello, world!")
+        let bb = first.getSlice(at: 5, length: 5)!
+        XCTAssertEqual(bb.readableBytesSpan.byteCount, 5)
+        XCTAssertTrue(bb.readableBytesSpan.elementsEqual(", wor".utf8))
+    }
+
+    func testMutableReadableBytesSpanOfEmptyByteBuffer() {
+        var bb = ByteBuffer()
+        XCTAssertEqual(bb.mutableReadableBytesSpan.byteCount, 0)
+    }
+
+    func testMutableReadableBytesSpanOfSimpleBuffer() {
+        var bb = ByteBuffer(string: "Hello, world!")
+        XCTAssertEqual(bb.mutableReadableBytesSpan.byteCount, 13)
+        XCTAssertTrue(bb.mutableReadableBytesSpan.elementsEqual("Hello, world!".utf8))
+
+        var readableBytes = bb.mutableReadableBytesSpan
+        readableBytes.storeBytes(of: UInt8(ascii: "o"), toByteOffset: 5, as: UInt8.self)
+
+        XCTAssertEqual(String(buffer: bb), "Helloo world!")
+    }
+
+    func testMutableReadableBytesSpanNotAtTheStart() {
+        var bb = ByteBuffer(string: "Hello, world!")
+        bb.moveReaderIndex(forwardBy: 5)
+        XCTAssertEqual(bb.mutableReadableBytesSpan.byteCount, 8)
+        XCTAssertTrue(bb.mutableReadableBytesSpan.elementsEqual(", world!".utf8))
+
+        var readableBytes = bb.mutableReadableBytesSpan
+        readableBytes.storeBytes(of: UInt8(ascii: "o"), toByteOffset: 5, as: UInt8.self)
+
+        XCTAssertEqual(String(buffer: bb), ", worod!")
+    }
+
+    func testMutableReadableBytesSpanOfSlice() {
+        let first = ByteBuffer(string: "Hello, world!")
+        var bb = first.getSlice(at: 5, length: 5)!
+        XCTAssertEqual(bb.mutableReadableBytesSpan.byteCount, 5)
+        XCTAssertTrue(bb.mutableReadableBytesSpan.elementsEqual(", wor".utf8))
+
+        var readableBytes = bb.mutableReadableBytesSpan
+        readableBytes.storeBytes(of: UInt8(ascii: "o"), toByteOffset: 4, as: UInt8.self)
+
+        XCTAssertEqual(String(buffer: bb), ", woo")
+        XCTAssertEqual(String(buffer: first), "Hello, world!")
+    }
+
+    func testEvenCreatingMutableSpanTriggersCoW() {
+        let first = ByteBuffer(string: "Hello, world!")
+        var second = first
+
+        let firstBackingPtr = first.withVeryUnsafeBytes { $0 }.baseAddress
+        let secondBackingPtr = second.withVeryUnsafeBytes { $0 }.baseAddress
+        XCTAssertEqual(firstBackingPtr, secondBackingPtr)
+
+        let readableBytes = second.mutableReadableBytesSpan
+        _ = consume readableBytes
+        let firstNewBackingPtr = first.withVeryUnsafeBytes { $0 }.baseAddress
+        let secondNewBackingPtr = second.withVeryUnsafeBytes { $0 }.baseAddress
+        XCTAssertNotEqual(firstNewBackingPtr, secondNewBackingPtr)
+        XCTAssertEqual(firstBackingPtr, firstNewBackingPtr)
+    }
+
+    func testAppendingToEmptyBufferViaOutputSpan() {
+        var bb = ByteBuffer()
+        bb.writeWithOutputRawSpan(minimumWritableBytes: 15) { span in
+            XCTAssertEqual(span.byteCount, 0)
+            XCTAssertGreaterThanOrEqual(span.capacity, 15)
+            XCTAssertGreaterThanOrEqual(span.freeCapacity, 15)
+            XCTAssertTrue(span.initializedElementsEqual([]))
+
+            span.append(contentsOf: "Hello, world!".utf8)
+
+            XCTAssertEqual(span.byteCount, 13)
+            XCTAssertGreaterThanOrEqual(span.capacity, 2)
+            XCTAssertGreaterThanOrEqual(span.freeCapacity, 2)
+            XCTAssertTrue(span.initializedElementsEqual("Hello, world!".utf8))
+        }
+        XCTAssertEqual(bb.readableBytes, 13)
+        XCTAssertEqual(String(buffer: bb), "Hello, world!")
+    }
+
+    func testAppendingToNonEmptyBufferViaOutputSpanDoesNotExposeInitialBytes() {
+        var bb = ByteBuffer()
+        bb.writeString("Hello")
+        bb.writeWithOutputRawSpan(minimumWritableBytes: 8) { span in
+            XCTAssertEqual(span.byteCount, 0)
+            XCTAssertGreaterThanOrEqual(span.capacity, 8)
+            XCTAssertGreaterThanOrEqual(span.freeCapacity, 8)
+            XCTAssertTrue(span.initializedElementsEqual([]))
+
+            span.append(contentsOf: ", world!".utf8)
+
+            XCTAssertEqual(span.byteCount, 8)
+            XCTAssertGreaterThanOrEqual(span.capacity, 0)
+            XCTAssertGreaterThanOrEqual(span.freeCapacity, 0)
+            XCTAssertTrue(span.initializedElementsEqual(", world!".utf8))
+        }
+        XCTAssertEqual(bb.readableBytes, 13)
+        XCTAssertEqual(String(buffer: bb), "Hello, world!")
+    }
+
+    func testAppendingToASliceViaOutputSpan() {
+        let first = ByteBuffer(string: "Hello, world!")
+        var bb = first.getSlice(at: 5, length: 5)!
+        XCTAssertEqual(bb.mutableReadableBytesSpan.byteCount, 5)
+        XCTAssertTrue(bb.mutableReadableBytesSpan.elementsEqual(", wor".utf8))
+
+        bb.writeWithOutputRawSpan(minimumWritableBytes: 5) { span in
+            span.append(contentsOf: "olleh".utf8)
+        }
+
+        XCTAssertEqual(String(buffer: bb), ", worolleh")
+        XCTAssertEqual(String(buffer: first), "Hello, world!")
+    }
+
+    func testEvenCreatingAnOutputSpanTriggersCoW() {
+        let first = ByteBuffer(string: "Hello, world!")
+        var second = first
+
+        let firstBackingPtr = first.withVeryUnsafeBytes { $0 }.baseAddress
+        let secondBackingPtr = second.withVeryUnsafeBytes { $0 }.baseAddress
+        XCTAssertEqual(firstBackingPtr, secondBackingPtr)
+
+        second.writeWithOutputRawSpan(minimumWritableBytes: 5) { _ in}
+        let firstNewBackingPtr = first.withVeryUnsafeBytes { $0 }.baseAddress
+        let secondNewBackingPtr = second.withVeryUnsafeBytes { $0 }.baseAddress
+        XCTAssertNotEqual(firstNewBackingPtr, secondNewBackingPtr)
+        XCTAssertEqual(firstBackingPtr, firstNewBackingPtr)
+    }
+
+    func testCanCreateEmptyBufferDirectly() {
+        let bb = ByteBuffer(initialCapacity: 15) { span in
+            XCTAssertEqual(span.byteCount, 0)
+            XCTAssertGreaterThanOrEqual(span.capacity, 15)
+            XCTAssertGreaterThanOrEqual(span.freeCapacity, 15)
+            XCTAssertTrue(span.initializedElementsEqual([]))
+
+            span.append(contentsOf: "Hello, world!".utf8)
+
+            XCTAssertEqual(span.byteCount, 13)
+            XCTAssertGreaterThanOrEqual(span.capacity, 2)
+            XCTAssertGreaterThanOrEqual(span.freeCapacity, 2)
+            XCTAssertTrue(span.initializedElementsEqual("Hello, world!".utf8))
+        }
+        XCTAssertEqual(bb.readableBytes, 13)
+        XCTAssertEqual(String(buffer: bb), "Hello, world!")
+    }
+
+    func testCanCreateEmptyBufferDirectlyFromAllocator() {
+        let bb = ByteBufferAllocator().buffer(capacity: 15) { span in
+            XCTAssertEqual(span.byteCount, 0)
+            XCTAssertGreaterThanOrEqual(span.capacity, 15)
+            XCTAssertGreaterThanOrEqual(span.freeCapacity, 15)
+            XCTAssertTrue(span.initializedElementsEqual([]))
+
+            span.append(contentsOf: "Hello, world!".utf8)
+
+            XCTAssertEqual(span.byteCount, 13)
+            XCTAssertGreaterThanOrEqual(span.capacity, 2)
+            XCTAssertGreaterThanOrEqual(span.freeCapacity, 2)
+            XCTAssertTrue(span.initializedElementsEqual("Hello, world!".utf8))
+        }
+        XCTAssertEqual(bb.readableBytes, 13)
+        XCTAssertEqual(String(buffer: bb), "Hello, world!")
+    }
+}
+
+@available(macOS 10.14.4, iOS 12.2, watchOS 5.2, tvOS 12.2, visionOS 1.0, *)
+extension RawSpan {
+    func elementsEqual<Other: Collection>(_ other: Other) -> Bool where Other.Element == UInt8 {
+        guard other.count == self.byteCount else { return false }
+
+        var index = other.startIndex
+        var offset = 0
+        while index < other.endIndex {
+            guard other[index] == self.unsafeLoadUnaligned(fromByteOffset: offset, as: UInt8.self) else {
+                return false
+            }
+            other.formIndex(after: &index)
+            offset &+= 1
+        }
+
+        return true
+    }
+}
+
+@available(macOS 10.14.4, iOS 12.2, watchOS 5.2, tvOS 12.2, visionOS 1.0, *)
+extension MutableRawSpan {
+    func elementsEqual<Other: Collection>(_ other: Other) -> Bool where Other.Element == UInt8 {
+        self.bytes.elementsEqual(other)
+    }
+}
+
+@available(macOS 10.14.4, iOS 12.2, watchOS 5.2, tvOS 12.2, visionOS 1.0, *)
+extension OutputRawSpan {
+    func initializedElementsEqual<Other: Collection>(_ other: Other) -> Bool where Other.Element == UInt8 {
+        self.bytes.elementsEqual(other)
+    }
+
+    @_lifetime(self: copy self)
+    mutating func append<Other: Collection>(contentsOf other: Other) where Other.Element == UInt8 {
+        for element in other {
+            self.append(element)
+        }
+    }
+}
+#endif