Create interpolation strings with appropriate initial capacity

Should avoid relying so heavily on the optimizer realizing it can remove branches.

This had somewhat complicated performance impacts in local benchmarking; we’ll see what it does on CI.

Author: beccadax

stdlib/public/core/StringGuts.swift

@@ -910,23 +910,31 @@ extension _StringGuts {
     _invariantCheck()
   }
 
-  @inlinable // @testable
-  mutating func reserveCapacity(_ capacity: Int) {
-    // Small strings can accomodate small capacities
+  @inlinable internal
+  init(_initialCapacity capacity: Int) {
     if capacity <= _SmallUTF8String.capacity {
-      return
+      self.init()
+    } else {
+      let storage = _SwiftStringStorage<UInt8>.create(
+        capacity: capacity,
+        count: 0)
+      self.init(_large: storage)
     }
-    reserveCapacitySlow(capacity)
   }
 
-  @usableFromInline
-  mutating func reserveCapacitySlow(_ capacity: Int) {
+  @usableFromInline // @testable
+  mutating func reserveCapacity(_ capacity: Int) {
     if _fastPath(_isUniqueNative()) {
       if _fastPath(_object.nativeRawStorage.capacity >= capacity) {
         return
       }
     }
 
+    // Small strings can accomodate small capacities
+    if capacity <= _SmallUTF8String.capacity {
+      return
+    }
+
     let selfCount = self.count
     if isASCII {
       let storage = _copyToNativeStorage(

stdlib/public/core/StringInterpolation.swift

@@ -63,7 +63,7 @@
 public struct DefaultStringInterpolation: StringInterpolationProtocol {
   /// The string contents accumulated by this instance.
   @usableFromInline
-  internal var _storage: String = ""
+  internal var _storage: String
 
   /// Creates a string interpolation with storage pre-sized for a literal
   /// with the indicated attributes.
@@ -75,7 +75,7 @@ public struct DefaultStringInterpolation: StringInterpolationProtocol {
     let capacityPerInterpolation = 2
     let initialCapacity = literalCapacity +
       interpolationCount * capacityPerInterpolation
-    _storage.reserveCapacity(initialCapacity)
+    _storage = String(_StringGuts(_initialCapacity: initialCapacity))
   }
 
   /// Appends a literal segment of a string interpolation.

stdlib/public/core/StringRangeReplaceableCollection.swift

@@ -125,7 +125,6 @@ extension String {
   ///   to allocate.
   ///
   /// - Complexity: O(*n*)
-  @inlinable
   public mutating func reserveCapacity(_ n: Int) {
     _guts.reserveCapacity(n)
   }