Make Rc<T>::deref and Arc<T>::deref zero-cost

[EFanZh]

Currently, Rc<T> and Arc<T> store pointers to RcInner<T> and ArcInner<T>. This PR changes the pointers so that they point to T directly instead.

This is based on the assumption that we access the T value more frequently than accessing reference counts. With this change, accessing the data can be done without offsetting pointers from RcInner<T> and ArcInner<T> to their contained data. This change might also enables some possibly useful future optimizations, such as:

• Convert &[Rc<T>] into &[&T] within O(1) time.
• Convert &[Rc<T>] into Vec<&T> utilizing memcpy.
• Convert &Option<Rc<T>> into Option<&T> without branching.
• Make Rc<T> and Arc<T> FFI compatible types where T: Sized.

[rustbot]

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[marmeladema]

Would it potentially enable those types to have an ffi compatible ABI? So that they could be returned and passed directly from /to ffi function, like Box?

[EFanZh]

Would it potentially enable those types to have an ffi compatible ABI? So that they could be returned and passed directly from /to ffi function, like Box?

I think in theory it is possible, at least for sized types, but I am not familiar with how to formally make it so.

[jhpratt]

r? libs

[joboet]

@EFanZh Is this ready for review? If so, please un-draft the PR.

[EFanZh]

@joboet: The source code part is mostly done, but I haven’t finished updating LLDB and CDB pretty printers. The CI doesn’t seem to run those tests.

[joboet]

No worries! I just didn't want to keep you waiting in case you had forgotten to change the state.
@rustbot author

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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

finished building tool typos
error: `aligment` should be `alignment`
##[error]  --> library/alloc/src/raw_rc/mod.rs:131:24
    |
131 |         // SAFETY: The aligment is guaranteed by both trait implementor and caller. Trait
    |                        ^^^^^^^^
    |
some tidy checks failed
tidy error: checks with external tool 'typos' failed
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1550:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1281:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:02:34
  local time: Tue Sep  9 16:50:33 UTC 2025
  network time: Tue, 09 Sep 2025 16:50:33 GMT
##[error]Process completed with exit code 1.