Virtual Slicer

The virtual slicer is a RUSTC_WRAPPER that stubs unreachable functions at the MIR level before LLVM sees them. It does not modify your source files or your Cargo.toml.

What "unreachable" means

Starting from the binary's main function, cargo-slicer traces the call graph across all workspace crates. Any function that cannot be reached from main is replaced with an abort() body. LLVM skips compilation, optimisation, and code emission for those functions entirely.

The analysis is conservative: trait impls, generics, async functions, closures, and any function called through a function pointer are always kept.

How it plugs into cargo

RUSTC_WRAPPER=cargo_slicer_dispatch  ← stable binary, < 1 ms startup
    │
    └─ local workspace crate?
           CARGO_SLICER_DRIVER=cargo-slicer-rustc  ← nightly driver, ~300 ms startup
               └─ BFS reachability analysis
               └─ MIR stub replacement
               └─ CGU filtering (skip codegen units with only stubs)

The dispatch binary keeps the nightly driver out of the fast path. Registry crates (which change rarely and are cached) never pay the 300 ms driver load.

Cross-crate pre-analysis

For accurate reachability across crate boundaries, run pre-analysis before the build:

cargo-slicer pre-analyze

This uses syn-based parsing to build a call graph across all workspace crates in seconds, writing results to .slicer-cache/. The driver reads these files at build time instead of re-analysing from scratch for every crate.

Without pre-analysis the slicer falls back to conservative per-crate analysis, which still works but produces fewer stubs.

Tuning

Environment variable	Effect
`CARGO_SLICER_VIRTUAL=1`	Enable virtual slicing
`CARGO_SLICER_CODEGEN_FILTER=1`	Skip CGUs that contain only stubs
`CARGO_SLICER_DEBUG=1`	Write a debug log to `.cargo-slicer-debug.log`
`CARGO_SLICER_SKIP_THRESHOLD=auto`	Skip driver for crates with no predicted stubs (default)
`CARGO_SLICER_SKIP_THRESHOLD=0`	Always load the driver for every local crate

What cannot be stubbed

The slicer never stubs:

Trait impl associated functions (vtable entries)
Generic functions (monomorphised at the call site)
async fn and closures
unsafe fn (unless CARGO_SLICER_RELAX_UNSAFE=1)
Any function reachable through a function pointer

These constraints are intentional. Stubbing them would either cause linker errors or produce incorrect binaries.

Upstream proposal

The virtual slicing logic has been extracted into a proposed rustc patch behind a -Z dead-fn-elimination flag. If accepted upstream, the install story becomes:

RUSTFLAGS="-Z dead-fn-elimination" cargo +nightly build --release

No extra binary, no nightly ABI compatibility shims.