Replace some LLVMRust wrappers with calls to the LLVM C API

[Zalathar]

This PR removes the LLVMRust wrapper functions for getting/setting linkage and visibility, and replaces them with direct calls to the corresponding functions in LLVM's C API.

To make this convenient and sound, two pieces of supporting code have also been added:

• A simple proc-macro that derives TryFrom<u32> for fieldless enums
• A wrapper type for C enum values returned by LLVM functions, to ensure soundness if LLVM returns an enum value we don't know about

In a few places, the use of safe wrapper functions means that an unsafe block is no longer needed, so the affected code has changed its indentation level.

[rustbot]

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[Zalathar]

For historical context, these wrappers were added by #36200 and #38117.

There appears to have been a fear of the enums going out of sync with LLVM, but the actual desyncs appear to have been self-inflicted, due to people modifying the Rust-side enum in ways that reflect the unstable C++ enum rather than the stable C enum.

[jieyouxu]

Thanks, this looks like a very nice cleanup. I have some minor suggestions, otherwise this LGTM.

[jieyouxu]

Thanks. You can r=me after PR CI is green.

[Zalathar]

🟩
@bors r=jieyouxu

[bors]

📌 Commit 3e51f6f has been approved by jieyouxu

It is now in the queue for this repository.

[Zalathar]

Rebased after trivial conflict with #132168.

@bors r=jieyouxu

[bors]

📌 Commit 2f06853 has been approved by jieyouxu

It is now in the queue for this repository.

[jieyouxu]

error: `gen` is a keyword in the 2024 edition
  --> compiler/rustc_macros/src/try_from.rs:43:9
   |
43 |         gen impl TryFrom<u32> for @Self {
   |         ^^^ help: you can use a raw identifier to stay compatible: `r#gen`
   = warning: this is accepted in the current edition (Rust 2021) but is a hard error in Rust 2024!
   = note: for more information, see issue #49716 <https://github.com/rust-lang/rust/issues/49716>
   = note: for more information, see issue #49716 <https://github.com/rust-lang/rust/issues/49716>
   = note: `-D keyword-idents-2024` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(keyword_idents_2024)]`

@bors r- (failed in #132177)

[Zalathar]

Fixing this is easy enough, but I'm a bit baffled as to why it doesn't fail/warn locally or in PR CI.

[Zalathar]

Addressed keyword_idents_2024 warning about gen in the macro (diff).

Since this wasn't found by PR CI, I'll run a side job at #114917 and then reapprove this when that passes.

[Zalathar]

Looks like I am not the first person to have this problem with gen and synstructure: #127729.

Given that there's precedent in rustc_macros, I'll just add another #[allow(keyword_idents_2024)] here. Whatever work is needed to fix the existing cases will also fix this one.

[Zalathar]

Workaround for the synstructure/gen issue, which already has precedent elsewhere in rustc_macros: (diff).

[jieyouxu]

This is fine, I'll revisit how bootstrap sets that lint level... it seems somewhat suspicious.

@bors r+ rollup=iffy

[bors]

📌 Commit d976ca8 has been approved by jieyouxu

It is now in the queue for this repository.

[bors]

⌛ Testing commit d976ca8 with merge be33e4f...

[bors]

☀️ Test successful - checks-actions
Approved by: jieyouxu
Pushing be33e4f to master...

[rust-timer]

Finished benchmarking commit (be33e4f): 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 2.7%, 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)	2.7%	[2.7%, 2.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -1.9%]	2
All ❌✅ (primary)	2.7%	[2.7%, 2.7%]	1

Cycles

Results (secondary 1.5%)

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)	2.8%	[2.6%, 3.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 782.934s -> 782.771s (-0.02%)
Artifact size: 333.73 MiB -> 333.71 MiB (-0.00%)