Type fallback refactorings

[WaffleLapkin]

r? types

• Don't merge different kinds of infer variables into Ty before matching them back
• Do fallback only on root vars (this is semantically equivalent to what we currently do; unifying any of the infer vars from the same equivalence class necesserily affects all of them) (this makes Implement "low priority impls" #159003 a bit less hacky though)
• Do some simplification that has been allowed by the previous changes

cc @lcnr

[WaffleLapkin]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 6a25a06 (6a25a0621d4f43872d80fc0aba25c7aedfc92c0f)
Base parent: 3664b37 (3664b37017c529cad0f0ed259769f15743c21a6a)

[rust-timer]

Finished benchmarking commit (6a25a06): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

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.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@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)	0.2%	[0.2%, 0.3%]	4
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.3%, -2.0%]	12
All ❌✅ (primary)	0.2%	[0.2%, 0.3%]	4

Max RSS (memory usage)

Results (primary 3.4%)

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

	mean	range	count
Regressions ❌ (primary)	3.4%	[2.6%, 4.1%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.4%	[2.6%, 4.1%]	2

Cycles

Results (secondary -5.3%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.3%	[-14.7%, -2.8%]	7
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, 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)	0.1%	[0.0%, 0.1%]	59
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	22
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.1%]	59

Bootstrap: 487.967s -> 488.861s (0.18%)
Artifact size: 388.37 MiB -> 388.42 MiB (0.01%)

[WaffleLapkin]

I think the perf regression is noise.
@rustbot label: +perf-regression-triaged