Callgraph analysis

Ferrous Systems explores multiple approaches to statically verifying that certain functions are never called in a Rust program, motivated by Ferrocene's need to certify a subset of `core` to IEC 61508 (SIL-2). Approaches covered include Clippy lints, effect type systems, linker-based tricks (no-panic), cfg hacks, and ultimately a custom rustc compiler driver. The chosen solution implements a callgraph analysis with two passes: a pre-monomorphization THIR pass for fast `cargo check` feedback, and a post-monomorphization MIR pass for reliable certification-grade analysis. The tool catches panics, unvalidated function calls, function pointer coercions, and dynamic trait object casts, even across crate boundaries and through generics.