hygiene: Ensure uniqueness of SyntaxContextDatas

[petrochenkov]

SyntaxContextDatas are basically interned with SyntaxContexts working as indices, so they are supposed to be unique.
However, currently duplicate SyntaxContextDatas can be created during decoding from metadata or incremental cache.
This PR fixes that.

cc #129827 (comment)

[rustbot]

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[petrochenkov]

I've added many asserts, I'll change them to debug asserts if they affect performance.
@bors try @rust-timer queue

[bors]

⌛ Trying commit e577b7a with merge b517457...

[bors]

☀️ Try build successful - checks-actions
Build commit: b517457 (b51745778d3c14275d7b8f9115c2aa8e3b760bfb)

[rust-timer]

Finished benchmarking commit (b517457): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

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

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.4%]	11
Regressions ❌ (secondary)	0.3%	[0.1%, 1.1%]	38
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	10
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.3%]	3
All ❌✅ (primary)	0.0%	[-0.4%, 0.4%]	21

Max RSS (memory usage)

Results (secondary 1.0%)

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)	3.7%	[3.7%, 3.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.6%	[-1.6%, -1.6%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -12.6%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-12.6%	[-15.5%, -2.5%]	7
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.5%, secondary -0.8%)

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-2.0%, -0.0%]	63
Improvements ✅ (secondary)	-0.8%	[-2.6%, -0.0%]	14
All ❌✅ (primary)	-0.5%	[-2.0%, -0.0%]	63

Bootstrap: 756.444s -> 757.208s (0.10%)
Artifact size: 341.13 MiB -> 341.19 MiB (0.02%)

[cjgillot]

I'm not super fond of the "hopefully" rhetoric...
SyntaxContexts form a tree structure, is there a way we could exploit it?
By refactoring all this into a DFS in metadata/cache to fetch the root, and then decode the children in the proper order?

[petrochenkov]

SyntaxContexts form a tree structure

Right now they are not a tree because the opaque(_and_semitransparent) are caches that often refer to the context itself.
With #129827 SyntaxContextKeys will probably be a tree - they should be a tree but I'm not sure that proc macro or built-in macro logic cannot mess up something, need to verify it with a bunch of asserts too.
So the whole idea will be easier to implement after #129827.

[petrochenkov]

FIXME: The holes left by decoder break the logic assigning $crate names in fn update_dollar_crate_names, it's not too important because it's just for pretty printing, but still better to fix it.

[petrochenkov]

and then decode the children in the proper order?

Ah, there's one more thing - not all contexts are coming from the decoder (during incremental compilation at least).

Many contexts come from the freshly redone compilation (which is typically done before incremental decoding starts) and then they need to "unify" with equivalent contexts coming from decoding - that's where the duplicates were coming from before this PR.

So even if all decoding is done in proper order, you can still decode and get a context that is equivalent to one of the freshly built ones, but you don't know it until you decode it and compare.

Maybe if #129827 eliminates recursion we'll be able to avoid reserving SyntaxContexts in advance, then there will be no holes.