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

[EFanZh]

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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 main 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.

[tgross35]

#141348 (comment)
@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: d1342e2 (d1342e224f0872b9979c7609992a6bd882531f6c)
Base parent: 1a82b5d (1a82b5d4f4c79ead5ce15b6e586af24d2a20300c)

[rust-timer]

Finished benchmarking commit (d1342e2): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.2%, 2.1%]	136
Regressions ❌ (secondary)	0.5%	[0.2%, 3.2%]	62
Improvements ✅ (primary)	-0.7%	[-1.3%, -0.4%]	5
Improvements ✅ (secondary)	-0.7%	[-1.5%, -0.3%]	4
All ❌✅ (primary)	0.4%	[-1.3%, 2.1%]	141

Max RSS (memory usage)

Results (primary -2.7%, secondary 4.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.0%	[0.9%, 3.0%]	2
Regressions ❌ (secondary)	4.3%	[2.3%, 6.6%]	4
Improvements ✅ (primary)	-5.1%	[-5.8%, -4.0%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.7%	[-5.8%, 3.0%]	6

Cycles

Results (primary 2.7%, secondary 3.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.7%	[2.7%, 2.7%]	1
Regressions ❌ (secondary)	3.6%	[1.8%, 6.8%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.7%	[2.7%, 2.7%]	1

Binary size

Results (primary 1.0%, secondary 0.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	1.2%	[0.0%, 4.0%]	47
Regressions ❌ (secondary)	1.2%	[0.4%, 6.4%]	50
Improvements ✅ (primary)	-0.5%	[-1.1%, -0.3%]	5
Improvements ✅ (secondary)	-4.8%	[-6.3%, -0.0%]	4
All ❌✅ (primary)	1.0%	[-1.1%, 4.0%]	52

Bootstrap: 490.182s -> 486.498s (-0.75%)
Artifact size: 406.51 MiB -> 406.71 MiB (0.05%)