Synthetic Partial Drop Glue

[SoniEx2]

Proposal

Currently core::mem::drop_in_place requires types which are fully initialized. Instead of "calling drop glue", the compiler currently synthesizes drop glue every time when a value is dropped. This is obviously inefficient.

This MCP proposes allowing core::mem::drop_in_place to accept partially-moved-from types, and synthesizing the drop glue as part of monomorphization. (In other words, this requires typestate-based-ish monomorphization, tho we can still use drop flags to pick the correct typestate at runtime.)

This MCP also proposes using core::mem::drop_in_place in Box::drop, and taking the Drop for Box impl out of the compiler.

Note: This MCP is not Partially Initialized Types. It's much more lightweight and doesn't require any syntax changes. It also doesn't introduce anything like DerefMove.

The main benefits of this MCP are:

• Making Drop for Box just another impl.

This would likely make it less error-prone than having this impl in the compiler. (e.g. maybe reduce the likelihood of allocator_api-related ICEs? tho those seem to be mostly fixed by now, so maybe it isn't worth it...)

ref rust-lang/wg-allocators#103

Mentors or Reviewers

If you have a reviewer or mentor in mind for this work, mention then
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work.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors