Oleks
dcf3a897d1
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.
67 lines
2.0 KiB
Markdown
67 lines
2.0 KiB
Markdown
# Cranelift Codegen Backend Limitations
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## Background
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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.
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## Known Unsupported Features
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### `global_asm!` with `sym` operands
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**Status**: Not supported as of March 2026
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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:
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```
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error: asm! and global_asm! sym operands are not yet supported
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```
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**Affected crates** (non-exhaustive):
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- `wasmtime-fiber` (all versions, including v42+) — uses `global_asm!` with `sym` for fiber context switching
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- Any crate using `global_asm!` with function symbol references
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**Source**: [rust-lang/rust global_asm.rs](https://github.com/rust-lang/rust/blob/main/compiler/rustc_codegen_cranelift/src/global_asm.rs)
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### Diagnostic confusion
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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:
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1. `cargo check` and `cargo clippy` don't invoke codegen → no error
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2. `cargo build` / `cargo test` invoke codegen → Cranelift fails
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3. The error message doesn't mention Cranelift, so it looks like a rustc bug
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## Workarounds
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### Per-invocation override
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```bash
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CARGO_PROFILE_DEV_CODEGEN_BACKEND=llvm cargo test
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CARGO_PROFILE_DEV_CODEGEN_BACKEND=llvm cargo build
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```
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### Per-project override
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Create `.cargo/config.toml` in the project:
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```toml
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[profile.dev]
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codegen-backend = "llvm"
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```
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### Selective override (keep Cranelift for most builds)
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Use LLVM only for test profile:
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```toml
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[profile.test]
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codegen-backend = "llvm"
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```
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## Detection
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If `cargo check` passes but `cargo test` fails with `global_asm!` errors, check:
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```bash
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grep -r "codegen-backend" ~/.cargo/config.toml .cargo/config.toml 2>/dev/null
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```
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