Merge pull request #85544 from tbkka/tbkka-Issue85539

Properly initialize buffer when called via legacy ABI

Author: tbkka

stdlib/public/core/FloatingPointToString.swift

@@ -157,6 +157,9 @@ public func _float16ToStringImpl(
   var buffer = unsafe MutableSpan<UTF8.CodeUnit>(
     _unchecked: textBuffer,
     count: Int(bufferLength))
+  for i in 0..<Int(bufferLength) {
+      buffer[unchecked: i] = 0x30
+  }
   let textRange = _Float16ToASCII(value: value, buffer: &buffer)
   let textLength = textRange.upperBound - textRange.lowerBound
 
@@ -446,6 +449,9 @@ internal func _float32ToStringImpl(
   var buffer = unsafe MutableSpan<UTF8.CodeUnit>(
     _unchecked: textBuffer,
     count: Int(bufferLength))
+  for i in 0..<Int(bufferLength) {
+      buffer[unchecked: i] = 0x30
+  }
   let textRange = _Float32ToASCII(value: value, buffer: &buffer)
   let textLength = textRange.upperBound - textRange.lowerBound
 
@@ -694,6 +700,9 @@ internal func _float64ToStringImpl(
   var buffer = unsafe MutableSpan<UTF8.CodeUnit>(
     _unchecked: textBuffer,
     count: Int(bufferLength))
+  for i in 0..<Int(bufferLength) {
+      buffer[unchecked: i] = 0x30
+  }
   let textRange = _Float64ToASCII(value: value, buffer: &buffer)
   let textLength = textRange.upperBound - textRange.lowerBound
 
@@ -1197,6 +1206,9 @@ internal func _float80ToStringImpl(
   var buffer = unsafe MutableSpan<UTF8.CodeUnit>(
     _unchecked: textBuffer,
     count: Int(bufferLength))
+  for i in 0..<Int(bufferLength) {
+      buffer[unchecked: i] = 0x30
+  }
   let textRange = _Float80ToASCII(value: value, buffer: &buffer)
   let textLength = textRange.upperBound - textRange.lowerBound