[Wasm] Improve string constructor in stdlib

libraries/stdlib/wasm/builtins/kotlin/Char.kt

@@ -93,7 +93,7 @@ public class Char private constructor(public val value: Char) : Comparable<Char>
         this.toInt().toDouble()
 
     override fun toString(): String =
-        String(charArrayOf(this))
+        String.unsafeFromCharArray(charArrayOf(this))
 
     override fun hashCode(): Int =
         this.toInt().hashCode()

libraries/stdlib/wasm/builtins/kotlin/String.kt

@@ -12,8 +12,11 @@ import kotlin.math.min
  * The `String` class represents character strings. All string literals in Kotlin programs, such as `"abc"`, are
  * implemented as instances of this class.
  */
-public class String internal constructor(internal val chars: CharArray) : Comparable<String>, CharSequence {
-    public companion object;
+public class String private constructor(internal val chars: CharArray) : Comparable<String>, CharSequence {
+    public companion object {
+        // Note: doesn't copy the array, use with care.
+        internal fun unsafeFromCharArray(chars: CharArray) = String(chars)
+    }
 
     /**
      * Returns a string obtained by concatenating this string with the string representation of the given [other] object.
@@ -68,4 +71,4 @@ public class String internal constructor(internal val chars: CharArray) : Compar
     }
 }
 
-internal fun stringLiteral(startAddr: Int, length: Int) = String(unsafeRawMemoryToCharArray(startAddr, length))
+internal fun stringLiteral(startAddr: Int, length: Int) = String.unsafeFromCharArray(unsafeRawMemoryToCharArray(startAddr, length))

libraries/stdlib/wasm/internal/kotlin/wasm/internal/ExternalWrapper.kt

@@ -147,7 +147,7 @@ internal fun convertJsStringToKotlinString(x: ExternalInterfaceType): String {
     val length = stringLength(x)
     val addr = unsafeGetScratchRawMemory(INT_SIZE_BYTES + length * CHAR_SIZE_BYTES)
     jsWriteStringIntoMemory(x, addr)
-    return kotlin.String(unsafeRawMemoryToCharArray(addr, length))
+    return String.unsafeFromCharArray(unsafeRawMemoryToCharArray(addr, length))
 }
 
 

libraries/stdlib/wasm/internal/kotlin/wasm/internal/Number2String.kt

@@ -64,7 +64,7 @@ internal fun itoa32(inputValue: Int, radix: Int): String {
     if (sign == 1)
         buf[0] = CharCodes.MINUS.code.toChar()
 
-    return kotlin.String(buf)
+    return String.unsafeFromCharArray(buf)
 }
 
 private fun utoaDecSimple(buffer: CharArray, numInput: Int, offsetInput: Int) {
@@ -144,7 +144,7 @@ internal fun itoa64(inputValue: Long, radix: Int): String {
     if (sign == 1)
         buf[0] = CharCodes.MINUS.code.toChar()
 
-    return kotlin.String(buf)
+    return String.unsafeFromCharArray(buf)
 }
 
 // Count number of decimals for u64 values
@@ -178,7 +178,7 @@ internal fun dtoa(value: Double): String {
     val size = dtoaCore(buf, value)
     val ret = CharArray(size)
     buf.copyInto(ret, 0, 0, size)
-    return kotlin.String(ret)
+    return String.unsafeFromCharArray(ret)
 }
 
 private fun dtoaCore(buffer: CharArray, valueInp: Double): Int {

libraries/stdlib/wasm/src/kotlin/Text.kt

@@ -252,7 +252,15 @@ public actual fun Char.isWhitespace(): Boolean = TODO("Wasm stdlib: Text")
  * Converts the characters in the specified array to a string.
  */
 @SinceKotlin("1.2")
-public actual fun String(chars: CharArray): String = TODO("Wasm stdlib: Text")
+@Deprecated("Use CharArray.concatToString() instead", ReplaceWith("chars.concatToString()"))
+@DeprecatedSinceKotlin(warningSince = "1.4", errorSince = "1.5")
+public actual fun String(chars: CharArray): String {
+    var result = ""
+    for (char in chars) {
+        result += char
+    }
+    return result
+}
 
 /**
  * Converts the characters from a portion of the specified array to a string.
@@ -261,7 +269,17 @@ public actual fun String(chars: CharArray): String = TODO("Wasm stdlib: Text")
  * or `offset + length` is out of [chars] array bounds.
  */
 @SinceKotlin("1.2")
-public actual fun String(chars: CharArray, offset: Int, length: Int): String = TODO("Wasm stdlib: Text")
+@Deprecated("Use CharArray.concatToString(startIndex, endIndex) instead", ReplaceWith("chars.concatToString(offset, offset + length)"))
+@DeprecatedSinceKotlin(warningSince = "1.4", errorSince = "1.5")
+public actual fun String(chars: CharArray, offset: Int, length: Int): String {
+    if (offset < 0 || length < 0 || chars.size - offset < length)
+        throw IndexOutOfBoundsException("size: ${chars.size}; offset: $offset; length: $length")
+    var result = ""
+    for (index in offset until offset + length) {
+        result += chars[index]
+    }
+    return result
+}
 
 /**
  * Concatenates characters in this [CharArray] into a String.

libraries/stdlib/wasm/src/kotlin/text/StringBuilder.kt

@@ -15,7 +15,7 @@ internal fun insertString(array: CharArray, distIndex: Int, value: String, sourc
 }
 
 internal fun unsafeStringFromCharArray(array: CharArray, start: Int, size: Int): String =
-    kotlin.String(array.copyOfRange(start, start + size))
+    String.unsafeFromCharArray(array.copyOfRange(start, start + size))
 
 internal fun insertInt(array: CharArray, start: Int, value: Int): Int {
     val valueString = value.toString()