Merge pull request #34879 from Lukasa/cb-simpler-buffer-initializer

[stdlib] Simplify buffer pointer initialization.

Author: atrick

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -499,7 +499,8 @@ extension Unsafe${Mutable}BufferPointer {
   @inlinable // unsafe-performance
   public init(rebasing slice: Slice<UnsafeBufferPointer<Element>>) {
     let base = slice.base.baseAddress?.advanced(by: slice.startIndex)
-    self.init(start: base, count: slice.count)
+    let count = slice.endIndex &- slice.startIndex
+    self.init(start: base, count: count)
   }
 
 %  end
@@ -526,7 +527,8 @@ extension Unsafe${Mutable}BufferPointer {
   @inlinable // unsafe-performance
   public init(rebasing slice: Slice<UnsafeMutableBufferPointer<Element>>) {
     let base = slice.base.baseAddress?.advanced(by: slice.startIndex)
-    self.init(start: base, count: slice.count)
+    let count = slice.endIndex &- slice.startIndex
+    self.init(start: base, count: count)
   }
 
   /// Deallocates the memory block previously allocated at this buffer pointer’s 

stdlib/public/core/UnsafeRawBufferPointer.swift.gyb

@@ -512,7 +512,8 @@ extension Unsafe${Mutable}RawBufferPointer {
   @inlinable
   public init(rebasing slice: Slice<UnsafeRawBufferPointer>) {
     let base = slice.base.baseAddress?.advanced(by: slice.startIndex)
-    self.init(start: base, count: slice.count)
+    let count = slice.endIndex &- slice.startIndex
+    self.init(start: base, count: count)
   }
 %  end # !mutable
 
@@ -539,7 +540,8 @@ extension Unsafe${Mutable}RawBufferPointer {
   @inlinable
   public init(rebasing slice: Slice<UnsafeMutableRawBufferPointer>) {
     let base = slice.base.baseAddress?.advanced(by: slice.startIndex)
-    self.init(start: base, count: slice.count)
+    let count = slice.endIndex &- slice.startIndex
+    self.init(start: base, count: count)
   }
 
   /// A pointer to the first byte of the buffer.