Delete the cfg(not(parallel)) serial compiler

[Noratrieb]

Since it's inception a long time ago, the parallel compiler and its cfgs have been a maintenance burden. This was a necessary evil the allow iteration while not degrading performance because of synchronization overhead.

But this time is over. Thanks to the amazing work by the parallel working group (and the dyn sync crimes), the parallel compiler has now been fast enough to be shipped by default in nightly for quite a while now.
Stable and beta have still been on the serial compiler, because they can't use -Zthreads anyways.
But this is quite suboptimal:

• the maintenance burden still sucks
• we're not testing the serial compiler in nightly

Because of these reasons, it's time to end it. The serial compiler has served us well in the years since it was split from the parallel one, but it's over now.

Let the knight slay one head of the two-headed dragon!

#113349

Note that the default is still 1 thread, as more than 1 thread is still fairly broken.

cc @onur-ozkan to see if i did the bootstrap field removal correctly, @SparrowLii on the sync parts

[rustbot]

r? @cjgillot

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[rustbot]

rustc_error_messages was changed

cc @davidtwco, @compiler-errors, @TaKO8Ki

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[rustbot]

rustc_error_messages was changed

cc @davidtwco, @compiler-errors, @TaKO8Ki

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[compiler-errors]

cc rust-lang/compiler-team#681

[rustbot]

This PR modifies config.example.toml.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[bors]

☔ The latest upstream changes (presumably #132513) made this pull request unmergeable. Please resolve the merge conflicts.

[Noratrieb]

@bors p=1 many changes

[cjgillot]

I trust you cleaned-up the cfgs correctly.
Some cases of missed formatting for macros there and there, if you don't mind fixing them.
Does this need some kind of heads-up to the team?
r=me if doesn't.

[SparrowLii]

From the discussion I think we can land this. @Noratrieb Could you help doing the format work as @cjgillot suggested?

[Noratrieb]

Yes, I will do it, but it may take me a few days. Feel free to push the fix to my branch and approve it if you want to.

[clubby789]

@bors r=cjgillot rollup=never

[bors]

📌 Commit 505b8e1 has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit 505b8e1 with merge 6503543...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 6503543 to master...

[rust-timer]

Finished benchmarking commit (6503543): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary -1.0%)

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)	1.0%	[1.0%, 1.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.0%	[-3.0%, -3.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.0%	[-3.0%, 1.0%]	2

Cycles

Results (secondary 2.0%)

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)	2.0%	[1.8%, 2.2%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 787.726s -> 784.558s (-0.40%)
Artifact size: 335.35 MiB -> 335.34 MiB (-0.00%)