     self = result._guts
   }
 
-  @inline(__always) // Always-specialize
   internal mutating func replaceSubrange<C>(
     _ bounds: Range<Index>,
     with newElements: C

        return
      }
    }

    append(_StringGutsSlice(other))
  }

  internal mutating func append(_ slicedOther: _StringGutsSlice) {
    defer { self._invariantCheck() }

    if self.isSmall && slicedOther._guts.isSmall {
      // TODO: In-register slicing
      let smolSelf = self.asSmall
      if let smol = slicedOther.withFastUTF8({ otherUTF8 in
        return _SmallString(smolSelf, appending: _SmallString(otherUTF8)!)
      }) {
        self = _StringGuts(smol)
        return
      }
    }

    prepareForAppendInPlace(otherUTF8Count: slicedOther.utf8Count)

    if slicedOther.isFastUTF8 {
      let otherIsASCII = slicedOther.isASCII


  @inline(never) // slow-path
  private mutating func _foreignGrow(_ n: Int) {
    let newString = String(uninitializedCapacity: n) { buffer in
      guard let count = _foreignCopyUTF8(into: buffer) else {
       fatalError("String capacity was smaller than required")
      }
      return count
    }
    self = newString._guts
  }

  // Ensure unique native storage with sufficient capacity for the following
  // append.
  private mutating func prepareForAppendInPlace(
    totalCount: Int,
    otherUTF8Count otherCount: Int
  ) {
    defer {
