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Oleks dcf3a897d1 Initial commit: rust-craft plugin

Rust-on-NixOS knowledge plugin. Relocates plugin.json into .claude-plugin/
so it loads as a proper plugin, and adds README/LICENSE/.gitignore for
distribution. Skill rust-nix-toolchain (Cranelift vs LLVM codegen, fenix
nightly pins, cargo/clippy gates) with cranelift-limitations reference.

2026-06-01 11:28:48 +03:00

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Cranelift Codegen Backend Limitations

Background

The Cranelift codegen backend (rustc-codegen-cranelift-preview) is an alternative to LLVM for Rust compilation. It's significantly faster for debug builds but has feature gaps.

Known Unsupported Features

`global_asm!` with `sym` operands

Status: Not supported as of March 2026

The error originates in compiler/rustc_codegen_cranelift/src/global_asm.rs in the rust-lang/rust repo. When Cranelift encounters a GlobalAsmOperandRef::SymFn operand, it emits:

error: asm! and global_asm! sym operands are not yet supported

Affected crates (non-exhaustive):

wasmtime-fiber (all versions, including v42+) — uses global_asm! with sym for fiber context switching
Any crate using global_asm! with function symbol references

Source: rust-lang/rust global_asm.rs

Diagnostic confusion

The error message says "not yet supported" which looks like a Rust nightly regression. It's NOT — it's a Cranelift limitation that has existed since the backend was introduced. The confusion arises because:

cargo check and cargo clippy don't invoke codegen → no error
cargo build / cargo test invoke codegen → Cranelift fails
The error message doesn't mention Cranelift, so it looks like a rustc bug

Workarounds

Per-invocation override

CARGO_PROFILE_DEV_CODEGEN_BACKEND=llvm cargo test
CARGO_PROFILE_DEV_CODEGEN_BACKEND=llvm cargo build

Per-project override

Create .cargo/config.toml in the project:

[profile.dev]
codegen-backend = "llvm"

Selective override (keep Cranelift for most builds)

Use LLVM only for test profile:

[profile.test]
codegen-backend = "llvm"

Detection

If cargo check passes but cargo test fails with global_asm! errors, check:

grep -r "codegen-backend" ~/.cargo/config.toml .cargo/config.toml 2>/dev/null

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