Remove confusing use_polonius flag and do less cloning

[amandasystems]

The use_polonius flag is both redundant and confusing since every function it's propagated to also checks if all_facts is Some, the true test of whether to generate Polonius facts for Polonius or for external consumers. This PR makes that path clearer by simply doing away with the argument and handling the logic in precisely two places: where facts are populated (check for Some), and where all_facts are initialised. It also delays some statements until after that check to avoid the miniscule performance penalty of executing them when Polonius is disabled.

This also addresses @lqd's concern in #125652 by reducing the size of what is cloned out of Polonius facts to just the facts being added, as opposed to the entire vector of potential inputs, and added descriptive comments.

Reviewer note: the comments in add_extra_drop_facts should be inspected by a reviewer, in particular the one on L#259 in this PR, which should be trivial for someone with the right background knowledge to address.

I also included some lints I found on the way there that I couldn't help myself from addressing.

[rustbot]

r? @jieyouxu

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[jieyouxu]

I'm going to reroll because I'm not familiar with this part of the compiler. r? compiler

[amandasystems]

Ping @nikomatsakis ready for review if you have the time (this is not super important, but you asked me to ping you without a scope so I am)

[compiler-errors]

Sorry, was gonna review this but I got hella distracted. If Niko can review it, great, otherwise you can bounce it back to me and bother me until I review it.

r? nikomatsakis

[nikomatsakis]

@bors r+

[bors]

📌 Commit 7b5b7a7 has been approved by nikomatsakis

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7b5b7a7 with merge d49994b...

[bors]

☀️ Test successful - checks-actions
Approved by: nikomatsakis
Pushing d49994b to master...

[rust-timer]

Finished benchmarking commit (d49994b): comparison URL.

Overall result: ❌ regressions - 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)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.3%, 0.3%]	1

Max RSS (memory usage)

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

Cycles

Results (secondary 8.0%)

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)	8.0%	[7.1%, 9.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (secondary -0.0%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-	-	0

Bootstrap: 692.369s -> 693.26s (0.13%)
Artifact size: 326.71 MiB -> 326.75 MiB (0.01%)