Use ThinVec more in the AST

[nnethercote]

For consistency.

[nnethercote]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 973fa09 (973fa097ee83e915d5a9ba83a9cbb259b4573170)
Base parent: c397dae (c397dae808f70caebab1fc4e11b3edf7e59f58c7)

[rust-timer]

Finished benchmarking commit (973fa09): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (primary 0.3%, secondary -5.6%)

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

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.8%, 1.0%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.4%	[-1.4%, -1.4%]	1
Improvements ✅ (secondary)	-5.6%	[-5.7%, -5.5%]	2
All ❌✅ (primary)	0.3%	[-1.4%, 1.0%]	4

Cycles

Results (primary 2.2%, secondary -3.5%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.5%	[-5.5%, -2.4%]	3
All ❌✅ (primary)	2.2%	[2.2%, 2.2%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 485.944s -> 484.62s (-0.27%)
Artifact size: 393.37 MiB -> 393.83 MiB (0.12%)