diff --git a/stdlib/public/core/Span/MutableRawSpan.swift b/stdlib/public/core/Span/MutableRawSpan.swift
index 47c4aa7d43308..151d34fad7483 100644
--- a/stdlib/public/core/Span/MutableRawSpan.swift
+++ b/stdlib/public/core/Span/MutableRawSpan.swift
@@ -27,6 +27,7 @@ public struct MutableRawSpan: ~Copyable & ~Escapable {
   @usableFromInline
   internal let _count: Int
 
+  @unsafe
   @_alwaysEmitIntoClient
   internal func _start() -> UnsafeMutableRawPointer {
     unsafe _pointer._unsafelyUnwrappedUnchecked
@@ -171,8 +172,8 @@ extension RawSpan {
   @_alwaysEmitIntoClient
   @lifetime(borrow mutableRawSpan)
   public init(_mutableRawSpan mutableRawSpan: borrowing MutableRawSpan) {
-    let (start, count) = (mutableRawSpan._start(), mutableRawSpan._count)
-    let span = unsafe RawSpan(_unsafeStart: start, byteCount: count)
+    let (start, count) = unsafe (mutableRawSpan._pointer, mutableRawSpan._count)
+    let span = unsafe RawSpan(_unchecked: start, byteCount: count)
     self = unsafe _overrideLifetime(span, borrowing: mutableRawSpan)
   }
 }
@@ -432,6 +433,10 @@ extension MutableRawSpan {
   public mutating func update(
     fromContentsOf source: RawSpan
   ) -> Int {
+    _precondition(
+      source.byteCount <= self.byteCount,
+      "destination span cannot contain every byte from source."
+    )
     if source.byteCount == 0 { return 0 }
     unsafe source.withUnsafeBytes {
       unsafe _start().copyMemory(from: $0.baseAddress!, byteCount: $0.count)
@@ -560,7 +565,7 @@ extension MutableRawSpan {
   @_alwaysEmitIntoClient
   @lifetime(&self)
   mutating public func extracting(_: UnboundedRange) -> Self {
-    let newSpan = unsafe Self(_unchecked: _start(), byteCount: _count)
+    let newSpan = unsafe Self(_unchecked: _pointer, byteCount: _count)
     return unsafe _overrideLifetime(newSpan, mutating: &self)
   }
 }
diff --git a/stdlib/public/core/Span/MutableSpan.swift b/stdlib/public/core/Span/MutableSpan.swift
index 7cbbcf439b3cb..c097ef0111d77 100644
--- a/stdlib/public/core/Span/MutableSpan.swift
+++ b/stdlib/public/core/Span/MutableSpan.swift
@@ -28,6 +28,7 @@ public struct MutableSpan<Element: ~Copyable>
   @usableFromInline
   internal let _count: Int
 
+  @unsafe
   @_alwaysEmitIntoClient
   internal func _start() -> UnsafeMutableRawPointer {
     unsafe _pointer._unsafelyUnwrappedUnchecked
@@ -447,7 +448,7 @@ extension MutableSpan where Element: BitwiseCopyable {
     _ body: (_ buffer: UnsafeMutableRawBufferPointer) throws(E) -> Result
   ) throws(E) -> Result {
     let bytes = unsafe UnsafeMutableRawBufferPointer(
-      start: (_count == 0) ? nil : _start(),
+      start: (_count == 0) ? nil : _pointer,
       count: _count &* MemoryLayout<Element>.stride
     )
     return try unsafe body(bytes)
@@ -462,6 +463,7 @@ extension MutableSpan {
   @_alwaysEmitIntoClient
   @lifetime(self: copy self)
   public mutating func update(repeating repeatedValue: consuming Element) {
+    guard !isEmpty else { return }
     unsafe _start().withMemoryRebound(to: Element.self, capacity: count) {
       unsafe $0.update(repeating: repeatedValue, count: count)
     }
@@ -599,7 +601,7 @@ extension MutableSpan where Element: BitwiseCopyable {
   ) where Element: BitwiseCopyable {
     guard count > 0 else { return }
     // rebind _start manually in order to avoid assumptions about alignment.
-    let rp = _start()._rawValue
+    let rp = unsafe _start()._rawValue
     let binding = Builtin.bindMemory(rp, count._builtinWordValue, Element.self)
     let rebound = unsafe UnsafeMutablePointer<Element>(rp)
     unsafe rebound.update(repeating: repeatedValue, count: count)
@@ -796,7 +798,7 @@ extension MutableSpan where Element: ~Copyable {
   @_alwaysEmitIntoClient
   @lifetime(&self)
   mutating public func extracting(_: UnboundedRange) -> Self {
-    let newSpan = unsafe Self(_unchecked: _start(), count: _count)
+    let newSpan = unsafe Self(_unchecked: _pointer, count: _count)
     return unsafe _overrideLifetime(newSpan, mutating: &self)
   }
 }