[PERF] Rework unsizing coercions in new solver

[compiler-errors]

r? @ghost

[compiler-errors]

@bors2 try @rust-timer queue

[rust-bors]

⌛ Trying commit f4a5840 with merge 8eb675b…

[rust-bors]

☀️ Try build successful (CI)
Build commit: 8eb675b (8eb675b570eea2b53a1dbb5187eb3f82d2303b41)

[rust-timer]

Finished benchmarking commit (8eb675b): comparison URL.

Overall result: ✅ improvements - no 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.

@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
Regressions ❌ (secondary)	1.3%	[1.3%, 1.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.6%	[-1.5%, -0.2%]	33
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 1.2%, 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)	1.2%	[1.2%, 1.2%]	1
Regressions ❌ (secondary)	0.7%	[0.5%, 0.8%]	8
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.0%	[-2.1%, -0.4%]	5
All ❌✅ (primary)	1.2%	[1.2%, 1.2%]	1

Cycles

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)	0.8%	[0.5%, 1.8%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.4%	[-2.2%, -0.5%]	4
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 743.192s -> 743.81s (0.08%)
Artifact size: 372.27 MiB -> 371.77 MiB (-0.13%)

[compiler-errors]

Minor perf improvement.... I'll try to clean this up because I also think it simplifies the implementation quite a lot.