compute temporary scopes when building MIR, not THIR

[dianne]

Based on #148658; only the last commit is new.

This accomplishes two things:

• Makes the THIR slightly smaller by not attaching a full TempLifetime to every expression.
• Reduces the number of traversals of the ScopeTree by only calling ScopeTree::temporary_scope when building MIR for something that needs to be dropped in a temporary scope.

[dianne]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 9de26a5 (9de26a5918b5655a37eefd1b02cdb1ff0cdcdc74, parent: 87f9dcd5e28a301fce81515bd4bea792444dcf29)

[rust-timer]

Finished benchmarking commit (9de26a5): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@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)	1.2%	[1.2%, 1.2%]	1
Regressions ❌ (secondary)	0.7%	[0.2%, 1.2%]	2
Improvements ✅ (primary)	-0.4%	[-2.0%, -0.1%]	56
Improvements ✅ (secondary)	-0.9%	[-2.5%, -0.0%]	81
All ❌✅ (primary)	-0.4%	[-2.0%, 1.2%]	57

Max RSS (memory usage)

Results (primary -0.5%, secondary -2.8%)

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

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.4%, 1.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.5%	[-1.9%, -1.2%]	2
Improvements ✅ (secondary)	-2.8%	[-4.8%, -1.3%]	28
All ❌✅ (primary)	-0.5%	[-1.9%, 1.4%]	3

Cycles

Results (primary -2.3%, secondary -4.0%)

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)	4.9%	[4.2%, 5.5%]	2
Improvements ✅ (primary)	-2.3%	[-2.4%, -2.2%]	3
Improvements ✅ (secondary)	-4.6%	[-8.7%, -2.3%]	29
All ❌✅ (primary)	-2.3%	[-2.4%, -2.2%]	3

Binary size

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

Bootstrap: 475.809s -> 473.629s (-0.46%)
Artifact size: 390.93 MiB -> 390.83 MiB (-0.03%)

[dianne]

This could be removed entirely from thir::Expr, and we could use thir::ExprKind::Scopes to set up temporary scopes, but it's a little subtle/inconvenient for certain THIR lowerings (looking at &pin mut here) so for now I've opted to keep it simple.