rustdoc: remove ItemInner.

[nnethercote]

The Item struct is 48 bytes and contains a Box<ItemInner>; ItemInner is 104 bytes. This is an odd arrangement. Normally you'd have one of the following.

• A single large struct, which avoids the allocation for the Box, but can result in lots of wasted space in unused parts of a container like Vec<Item>, HashSet<Item>, etc.

• Or, something like struct Item(Box<ItemInner>), which requires the Box allocation but gives a very small Item size, which is good for containers like Vec<Item>. (Vec<Box<Item>> would also work.)

Item/ItemInner currently gets the worst of both worlds: it always requires a Box, but Item is also pretty big and so wastes space in containers. It would make sense to push it in one direction or the other. This commit does the first option, a single large struct.

r? @ghost

[nnethercote]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 0c21f21 with merge fd7707c...

[bors]

☀️ Try build successful - checks-actions
Build commit: fd7707c (fd7707c71f9f502c2a3ccfd21d74318f3bdb2e3e)

[rust-timer]

Finished benchmarking commit (fd7707c): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.2%, 2.2%]	18
Regressions ❌ (secondary)	0.4%	[0.3%, 0.9%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.2%, 2.2%]	18

Max RSS (memory usage)

Results (primary 2.0%, secondary 1.8%)

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)	2.0%	[1.2%, 3.6%]	5
Regressions ❌ (secondary)	1.8%	[1.6%, 1.9%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[1.2%, 3.6%]	5

Cycles

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

Binary size

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

Bootstrap: 777.153s -> 777.694s (0.07%)
Artifact size: 365.84 MiB -> 365.84 MiB (-0.00%)

[nnethercote]

This was a clear regression for icounts, cycles, wall-time, and max-rss. #138927 takes the other approach and improves perf.