Reland debug impl for fieldless enums with concatenated string without using discriminant_value

[panstromek]

Let's see whether this is still benefitial. If yes, then I'll clean this up.

relands #155452
reverts #162164

[panstromek]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 6ff0c29 (6ff0c293700f750741ca037a07d54262de940377)
Base parent: eca445e (eca445e5ae4a6679cc27d3a09106ce245e13a5a6)

[rust-timer]

Finished benchmarking commit (6ff0c29): comparison URL.

Overall result: ❌ regressions - 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

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.1%]	1
Regressions ❌ (secondary)	0.3%	[0.2%, 0.5%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.1%, 0.1%]	1

Max RSS (memory usage)

Results (primary 0.8%, secondary -0.0%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[0.5%, 5.2%]	4
Regressions ❌ (secondary)	3.8%	[3.8%, 3.8%]	1
Improvements ✅ (primary)	-2.2%	[-2.4%, -2.0%]	2
Improvements ✅ (secondary)	-1.9%	[-2.4%, -1.5%]	2
All ❌✅ (primary)	0.8%	[-2.4%, 5.2%]	6

Cycles

Results (primary 1.0%, secondary 5.9%)

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

	mean	range	count
Regressions ❌ (primary)	1.5%	[0.4%, 3.5%]	7
Regressions ❌ (secondary)	5.9%	[5.9%, 5.9%]	1
Improvements ✅ (primary)	-0.7%	[-0.7%, -0.6%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.0%	[-0.7%, 3.5%]	9

Binary size

Results (primary 0.1%, secondary 4.2%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 1.7%]	39
Regressions ❌ (secondary)	4.2%	[0.0%, 13.7%]	21
Improvements ✅ (primary)	-0.4%	[-0.7%, -0.0%]	16
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.7%, 1.7%]	55

Bootstrap: 481.923s -> 481.101s (-0.17%)
Artifact size: 403.28 MiB -> 403.16 MiB (-0.03%)

[panstromek]

Replacing discriminant_value with match => variant_index sadly doesn't help much. The generated MIR looks somewhat better than the default match => &str variant (there's less code in each arms basic block), but without the O(n) reduction in basic blocks, it doesn't seem to be worth it.

[rust-bors]

☔ The latest upstream changes (presumably #162631) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.