Add StringBuilder.MoveChunks(StringBuilder) API

[Copilot]

Implements the approved API from #97570, enabling O(1) transfer of a StringBuilder's chunk chain to a fresh instance so that callers (e.g. Roslyn's StringBuilderText) can hold a no-copy, immutable view of the contents while releasing the original.

namespace System.Text;

public partial class StringBuilder
{
    public static StringBuilder MoveChunks(StringBuilder source);
}

Description

• StringBuilder.MoveChunks (src/libraries/System.Private.CoreLib/.../StringBuilder.cs)
  • Constructs the returned StringBuilder via the existing private StringBuilder(StringBuilder from) copy constructor, which transfers m_ChunkChars, m_ChunkPrevious, m_ChunkLength, m_ChunkOffset, and m_MaxCapacity from source and avoids the wasted 16-char default buffer that new StringBuilder() would allocate.
  • Drains source by zeroing m_ChunkChars (set to Array.Empty<char>()), m_ChunkPrevious, m_ChunkLength, and m_ChunkOffset, while preserving m_MaxCapacity. The drained source is left in an empty but fully usable state — distinct from Clear(), which retains the buffer. Subsequent append or insert operations on source will succeed, allocating new buffers as needed.
  • Throws ArgumentNullException when source is null.
• Ref assembly updated (System.Runtime.cs).
• Tests in StringBuilderTests cover null arg, empty source, single chunk, multi-chunk chain (using MemoryMarshal.TryGetArray to assert backing-array identity by reference, not just content), calling MoveChunks on an already-drained source, and that the drained source remains usable with its original MaxCapacity preserved.

Customer Impact

Unblocks the no-copy SourceText shape Roslyn has been waiting on (dotnet/roslyn#61326) and provides a general primitive for transferring ownership of a StringBuilder's contents without materializing a contiguous string or array of chunks.

Regression?

• Yes
• No

New API; no behavior change to existing members.

Risk

• Low
• Medium
• High

Pure addition; the implementation only manipulates internal fields already reachable via existing code paths. After the call, source retains its original MaxCapacity and is fully usable as an empty StringBuilder.

[Copilot]

Pull request overview

This PR adds a new public StringBuilder.MoveChunks(StringBuilder source) API to enable O(1) transfer of an existing StringBuilder’s chunk chain to a new StringBuilder, leaving the source drained.

Changes:

• Added StringBuilder.MoveChunks(StringBuilder) implementation in System.Private.CoreLib.
• Updated the System.Runtime reference assembly to expose the new API.
• Added new StringBuilderTests coverage for null handling, empty/single/multi-chunk transfers, and post-drain behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs	Adds the MoveChunks API and drains the source instance after transferring chunk state.
src/libraries/System.Runtime/ref/System.Runtime.cs	Adds the public ref signature for StringBuilder.MoveChunks.
src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Text/StringBuilderTests.cs	Adds tests for chunk transfer behavior and drained-source behavior (including reflection-based validation).

[tarekgh]

Left minor comments, LGTM, otherwise.

One last question, is there any thread safety worries? I mean the source chunks get mutated while moving?

[Copilot]

Left minor comments, LGTM, otherwise.

One last question, is there any thread safety worries? I mean the source chunks get mutated while moving?

StringBuilder has never been thread-safe, and MoveChunks is no different — it's a single-threaded ownership-transfer operation. The caller is responsible for ensuring no other thread is mutating source concurrently, just as with any other StringBuilder mutation. No additional synchronization is needed.

[tannergooding]

One last question, is there any thread safety worries? I mean the source chunks get mutated while moving?

@tarekgh, no more than any type has. Most types are not explicitly thread safe and so require some kind of using lock if they are expecting multithreaded access to be going on.

[tannergooding]

/ba-g net security tests failed with null log