Add an experimental feature gate for function delegation

[petrochenkov]

In accordance with the process.

Detailed description of the feature can be found in the RFC repo - rust-lang/rfcs#3530.

TODO: find a lang team liaison - https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/fn.20delegation.20liaison/near/402506959.

[rustbot]

r? @TaKO8Ki

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[TaKO8Ki]

After adding fn_delegation to sym, r=me

[joshtriplett]

In today's meeting, we approved this for experimentation. @traviscross will liaise with the lang team.

Thanks for working on this, @petrochenkov!

[petrochenkov]

@bors r=TaKO8Ki,joshtriplett

[bors]

📌 Commit edf6c9c has been approved by TaKO8Ki,joshtriplett

It is now in the queue for this repository.

[bors]

⌛ Testing commit edf6c9c with merge 237339f...

[bors]

☀️ Test successful - checks-actions
Approved by: TaKO8Ki,joshtriplett
Pushing 237339f to master...

[rust-timer]

Finished benchmarking commit (237339f): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 677.025s -> 676.283s (-0.11%)
Artifact size: 313.73 MiB -> 313.72 MiB (-0.00%)