[string] Move wCSIA check higher than _HasContiguousBytes

Author: milseman

stdlib/public/core/String.swift

@@ -423,6 +423,24 @@ extension String {
       return
     }
 
+    // Fast path for user-defined Collections and typed contiguous collections.
+    //
+    // Note: this comes first, as the optimizer nearly always has insight into
+    // wCSIA, but cannot prove that a type does not have conformance to
+    // _HasContiguousBytes.
+    if let str = codeUnits.withContiguousStorageIfAvailable({
+      (buffer: UnsafeBufferPointer<C.Element>) -> String in
+      Builtin.onFastPath() // encourage SIL Optimizer to inline this closure :-(
+      let rawBufPtr = UnsafeRawBufferPointer(buffer)
+      return String._fromUTF8Repairing(
+        UnsafeBufferPointer(
+          start: rawBufPtr.baseAddress?.assumingMemoryBound(to: UInt8.self),
+          count: rawBufPtr.count)).0
+    }) {
+      self = str
+      return
+    }
+
     // Fast path for untyped raw storage and known stdlib types
     if let contigBytes = codeUnits as? _HasContiguousBytes,
       contigBytes._providesContiguousBytesNoCopy
@@ -436,16 +454,6 @@ extension String {
       return
     }
 
-    // Fast path for user-defined Collections
-    if let str = codeUnits.withContiguousStorageIfAvailable({
-      (buffer: UnsafeBufferPointer<C.Element>) -> String in
-      return String._fromUTF8Repairing(
-        UnsafeRawBufferPointer(buffer).bindMemory(to: UInt8.self)).0
-    }) {
-      self = str
-      return
-    }
-
     self = String._fromNonContiguousUnsafeBitcastUTF8Repairing(codeUnits).0
   }