From d2ee1cec93ab59c53dd4377b5c2f601ede1ba298 Mon Sep 17 00:00:00 2001
From: David Smith <david_smith@apple.com>
Date: Sat, 25 Jan 2025 01:54:49 -0800
Subject: [PATCH] Attempt to transcode into smol strings

---
 stdlib/public/core/String.swift       |  4 ++++
 stdlib/public/core/StringCreate.swift | 34 +++++++++++++++++++++++++--
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/stdlib/public/core/String.swift b/stdlib/public/core/String.swift
index aca5fde7dbe86..b94bcd59d4f11 100644
--- a/stdlib/public/core/String.swift
+++ b/stdlib/public/core/String.swift
@@ -445,6 +445,10 @@ extension String {
   public init<C: Collection, Encoding: Unicode.Encoding>(
     decoding codeUnits: C, as sourceEncoding: Encoding.Type
   ) where C.Iterator.Element == Encoding.CodeUnit {
+    guard codeUnits.count > 0 else {
+      self = ""
+      return
+    }
     guard _fastPath(sourceEncoding == UTF8.self) else {
       self = String._fromCodeUnits(
         codeUnits, encoding: sourceEncoding, repair: true)!.0
diff --git a/stdlib/public/core/StringCreate.swift b/stdlib/public/core/StringCreate.swift
index 2aaf097629ebc..8c3a2f670dce0 100644
--- a/stdlib/public/core/StringCreate.swift
+++ b/stdlib/public/core/StringCreate.swift
@@ -203,7 +203,6 @@ extension String {
     return contents.withUnsafeBufferPointer { String._uncheckedFromUTF8($0) }
   }
 
-  @inline(never) // slow path
   private static func _slowFromCodeUnits<
     Input: Collection,
     Encoding: Unicode.Encoding
@@ -213,7 +212,34 @@ extension String {
     repair: Bool
   ) -> (String, repairsMade: Bool)?
   where Input.Element == Encoding.CodeUnit {
-    // TODO(String Performance): Attempt to form smol strings
+    if input.count < _SmallString.capacity {
+      var repaired = false
+      var overflow = false
+      let result = _SmallString(initializingUTF8With: { buffer in
+        var bytesUsed = 0
+        repaired = transcode(
+          input.makeIterator(),
+          from: encoding,
+          to: UTF8.self,
+          stoppingOnError: false,
+          into: {
+            if bytesUsed < buffer.count {
+              buffer[bytesUsed] = $0
+            }
+            bytesUsed &+= 1
+          }
+        )
+        guard bytesUsed <= buffer.count else {
+          overflow = true
+          return 0
+        }
+        return bytesUsed
+      })
+      if !overflow {
+        return repair || !repaired
+          ? (String(_StringGuts(result)), repairsMade: repaired) : nil
+      }
+    }
 
     // TODO(String performance): Skip intermediary array, transcode directly
     // into a StringStorage space.
@@ -236,6 +262,10 @@ extension String {
     where Input == UnsafeBufferPointer<UInt8>, Encoding == Unicode.ASCII)
   @_specialize(
     where Input == Array<UInt8>, Encoding == Unicode.ASCII)
+  @_specialize(
+    where Input == UnsafeBufferPointer<UInt16>, Encoding == UTF16)
+  @_specialize(
+    where Input == Array<UInt16>, Encoding == UTF16)
   internal static func _fromCodeUnits<
     Input: Collection,
     Encoding: Unicode.Encoding