Modified format logic so that braces are not allowed in custom format…

… specifiers (#23062)

* Modified format logic so that braces are not allowed in custom format specifiers.

* Disabled failing test temporarily and fixed code style issue.

PR needed to pass test: dotnet/corefx#35820

* Slight changes to logic to better align the diff.

* Fixed counting error.

* Fix nullable

* Move exclusion near related test

* Add skip to new issue file

* Delete CoreFX.issues.json

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

src/System.Private.CoreLib/shared/System/Text/StringBuilder.cs

@@ -1538,7 +1538,6 @@ internal StringBuilder AppendFormatHelper(IFormatProvider? provider, string form
  int pos = 0;
  int len = format.Length;
  char ch = '\x0';
- StringBuilder? unescapedItemFormat = null;
 
  ICustomFormatter? cf = null;
  if (provider != null)
@@ -1665,7 +1664,7 @@ internal StringBuilder AppendFormatHelper(IFormatProvider? provider, string form
  // Start of parsing of optional formatting parameter.
  //
  object? arg = args[index];
- string? itemFormat = null;
+
  ReadOnlySpan<char> itemFormatSpan = default; // used if itemFormat is null
  // Is current character a colon? which indicates start of formatting parameter.
  if (ch == ':')
@@ -1678,67 +1677,40 @@ internal StringBuilder AppendFormatHelper(IFormatProvider? provider, string form
  // If reached end of text then error. (Unexpected end of text)
  if (pos == len) FormatError();
  ch = format[pos];
- pos++;
 
- // Is character a opening or closing brace?
- if (ch == '}' || ch == '{')
+ if (ch == '}')
  {
- if (ch == '{')
- {
- // Yes, is next character also a opening brace, then treat as escaped. eg {{
- if (pos < len && format[pos] == '{')
- pos++;
- else
- // Error Argument Holes can not be nested.
- FormatError();
- }
- else
- {
- // Yes, is next character also a closing brace, then treat as escaped. eg }}
- if (pos < len && format[pos] == '}')
- pos++;
- else
- {
- // No, then treat it as the closing brace of an Arg Hole.
- pos--;
- break;
- }
- }
-
- // Reaching here means the brace has been escaped
- // so we need to build up the format string in segments
- if (unescapedItemFormat == null)
- {
- unescapedItemFormat = new StringBuilder();
- }
- unescapedItemFormat.Append(format, startPos, pos - startPos - 1);
- startPos = pos;
+ // Argument hole closed
+ break;
  }
- }
-
- if (unescapedItemFormat == null || unescapedItemFormat.Length == 0)
- {
- if (startPos != pos)
+ else if (ch == '{')
  {
- // There was no brace escaping, extract the item format as a single string
- itemFormatSpan = format.AsSpan(startPos, pos - startPos);
+ // Braces inside the argument hole are not supported
+ FormatError();
  }
+
+ pos++;
  }
- else
+
+ if (pos > startPos)
  {
- unescapedItemFormat.Append(format, startPos, pos - startPos);
- itemFormatSpan = itemFormat = unescapedItemFormat.ToString();
- unescapedItemFormat.Clear();
+ itemFormatSpan = format.AsSpan(startPos, pos - startPos);
  }
  }
- // If current character is not a closing brace then error. (Unexpected Character)
- if (ch != '}') FormatError();
+ else if (ch != '}')
+ {
+ // Unexpected character
+ FormatError();
+ }
+
  // Construct the output for this arg hole.
  pos++;
  string? s = null;
+ string? itemFormat = null;
+
  if (cf != null)
  {
- if (itemFormatSpan.Length != 0 && itemFormat == null)
+ if (itemFormatSpan.Length != 0)
  {
  itemFormat = new string(itemFormatSpan);
  }