Clean up some FIXME notes on bootstrap

[onur-ozkan]

Fixing and removing some FIXME notes.

[rustbot]

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[rustbot]

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

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

[onur-ozkan]

This was asked on the PR for clarity, but no response has been provided.

[jieyouxu]

cc riscv32im-risc0-zkvm-elf target maintainers @flaub, @jbruestle and @SchmErik: could you elaborate why this is needed or if this is still needed?

Note that riscv32im-risc0-zkvm-elf is a Tier 3 compiler target, and while we would like to avoid unnecessarily breaking stuff we don't have to, we don't offer any guarantees that it will build. We'd like to trim down complexity in the bootstrap codebase where possible, because it's already very very complex.

[flaub]

@SchmErik Do you recall what this comment was about?

I don't think this is needed anymore.

[bors]

☔ The latest upstream changes (presumably #134830) made this pull request unmergeable. Please resolve the merge conflicts.

[onur-ozkan]

Rebased.

@rustbot ready

[Kobzol]

@bors r+

[bors]

📌 Commit eedafbc has been approved by Kobzol

It is now in the queue for this repository.

[bors]

⌛ Testing commit eedafbc with merge e7738af...

[bors]

☀️ Test successful - checks-actions
Approved by: Kobzol
Pushing e7738af to master...

[rust-timer]

Finished benchmarking commit (e7738af): 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 2.2%, secondary 0.8%)

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)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	2.1%	[1.9%, 2.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.7%	[-1.7%, -1.7%]	1
All ❌✅ (primary)	2.2%	[2.2%, 2.2%]	1

Cycles

Results (primary 2.4%)

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)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[2.4%, 2.4%]	1

Binary size

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

Bootstrap: 760.691s -> 760.612s (-0.01%)
Artifact size: 325.47 MiB -> 325.43 MiB (-0.01%)