Pull Request Overview

This pull request removes the legacy BSTR trail byte support from the CoreCLR, migrating it from a sync block-based implementation to a ConditionalWeakTable-based approach in managed code. This change aims to reduce memory overhead and simplify the string object implementation.

Key Changes

• Removed trail byte field and related methods from SyncBlock
• Removed trail byte methods from StringObject (HasTrailByte, GetTrailByte, SetTrailByte)
• Moved trail byte storage to managed code using ConditionalWeakTable in BSTRMarshaler
• Updated BSTR marshaling code to call managed methods instead of native interop

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

//The special string helpers are used as flag bits for weird strings that have bytes
//after the terminating 0.  The only case where we use this right now is the VB BSTR as
//byte array which is described in MakeStringAsByteArrayFromBytes.
#define SPECIAL_STRING_VB_BYTE_ARRAY 0x100

FORCEINLINE BOOL MARKS_VB_BYTE_ARRAY(WCHAR x)
{
    return static_cast<BOOL>(x & SPECIAL_STRING_VB_BYTE_ARRAY);
}

FORCEINLINE WCHAR MAKE_VB_TRAIL_BYTE(BYTE x)
{
    return static_cast<WCHAR>(x) | SPECIAL_STRING_VB_BYTE_ARRAY;
}

FORCEINLINE BYTE GET_VB_TRAIL_BYTE(WCHAR x)
{
    return static_cast<BYTE>(x & 0xFF);
}

Do we have a test for this already?

Thanks