Add Drop::pin_drop for pinned drops

[frank-king]

This PR is part of the pin_ergonomics experiment (the tracking issue is #130494). It allows implementing Drop with a pinned self receiver, which is required for safe pin-projection.

Implementations:

• At least and at most one of drop and pin_drop should be implemented.
• No direct call of drop or pin_drop. They should only be called by the drop glue.
• pin_drop must and must only be used with types that support pin-projection. It will be implemented after the pin-projection checks are ready (i.e. checking which types can be projected to Pin<&mut FieldType> from Pin<&mut Self>).
• Allows writing fn drop(&pin mut self) and desugars to fn pin_drop(&pin mut self).

Fixes #122630 (by accident).

Alternatives

Add a PinDrop trait that has the pin_drop method.

It is semantically clearer, but would duplicate all special checks of Drop.

It can be refactored if the trait hierarchy feature is ready.

[bors]

☔ The latest upstream changes (presumably #146071) made this pull request unmergeable. Please resolve the merge conflicts.

[frank-king]

The CI failed because Drop::drop becomes a provided method. I'm afraid it might not be a good way to hack into librustdoc. Is there any other good way to fix it?

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

##[endgroup]
[TIMING:end] tool::ToolBuild { build_compiler: Compiler { stage: 0, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target: aarch64-unknown-linux-gnu, tool: "linkchecker", path: "src/tools/linkchecker", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 0.156
[TIMING:end] tool::Linkchecker { compiler: Compiler { stage: 0, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target: aarch64-unknown-linux-gnu } -- 0.000
##[group]Testing stage1 Linkcheck (aarch64-unknown-linux-gnu)
book/print.html:3462: broken link fragment `#tymethod.drop` pointing to `std/ops/trait.Drop.html`
book/ch04-01-what-is-ownership.html:386: broken link fragment `#tymethod.drop` pointing to `std/ops/trait.Drop.html`
checked links in: 22.2s
number of HTML files scanned: 46122
number of HTML redirects found: 14532
number of links checked: 2531852
number of links ignored due to external: 121023
---
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/linkchecker /checkout/obj/build/aarch64-unknown-linux-gnu/doc` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1561:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:171:77

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:38:24
  local time: Tue Sep 23 01:38:46 UTC 2025
  network time: Tue, 23 Sep 2025 01:38:47 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

[bors]

☔ The latest upstream changes (presumably #146938) made this pull request unmergeable. Please resolve the merge conflicts.