Remove unneeded clone in macro deriving

[Kobzol]

Found this while looking at the parser and macro expansion. Note that newitem is already an owned Box<Item>, so there is no need to clone it.

I found a few other places where we clone unnecessarily before dropping a value, but they were all in diagnostics code and the clone was very cheap, so it isn't IMO worth the churn. None of them were detected by the redundant_clone Clippy lint, which is a shame - it should be more powerful!

r? nnethercote

[Kobzol]

@bors try @rust-timer queue

[Kobzol]

This is a perf. run where I removed all the clones I found. I think/hope that this PR should cover most of those small perf. wins.

[nnethercote]

@bors r+

[rust-bors]

📌 Commit f32a095 has been approved by nnethercote

It is now in the queue for this repository.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 70a39cf (70a39cfb7b741e95fcb6af8b15f5f694a3c2b271)
Base parent: 3cabe36 (3cabe36ceb022e2f56d4d330b1e2886f31117f18)

[rust-timer]

Finished benchmarking commit (70a39cf): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.5%, -0.1%]	37
Improvements ✅ (secondary)	-0.7%	[-2.1%, -0.1%]	37
All ❌✅ (primary)	-0.3%	[-0.5%, -0.1%]	37

Max RSS (memory usage)

Results (primary 0.9%, secondary -1.5%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[3.0%, 3.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.2%	[-1.2%, -1.2%]	1
Improvements ✅ (secondary)	-1.5%	[-1.8%, -1.1%]	2
All ❌✅ (primary)	0.9%	[-1.2%, 3.0%]	2

Cycles

Results (primary 3.1%, secondary -0.1%)

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

	mean	range	count
Regressions ❌ (primary)	3.1%	[3.1%, 3.1%]	1
Regressions ❌ (secondary)	2.1%	[2.1%, 2.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.3%, -2.3%]	1
All ❌✅ (primary)	3.1%	[3.1%, 3.1%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 475.389s -> 473.445s (-0.41%)
Artifact size: 402.85 MiB -> 402.88 MiB (0.01%)