Writing archive

I followed one u8 through the four stages of the Rust compiler: HIR, MIR, LLVM IR, and assembly. The type is a word, then a field, then a width, then nothing. The u8 and u32 versions of the same function compile to the same two instructions, and the one check a type does produce is the debug overflow guard.

A working record-and-replay harness for agent tests: a JSON cassette of decisions and tool results, deterministic replay with zero live calls, and a readable diff when the trajectory changes.

A simulation of a real 5 point improvement showing how often a fixed eval threshold gives a random red build, and why a paired comparison detects the gain at a much smaller sample size.

Measured with the same code as 1, 3 and 12 crates: splitting a Rust workspace narrows incremental rebuilds but increases total work, because generic instances are emitted once per crate.

Cargo can tell you exactly why it recompiled a crate. Here is the CARGO_LOG fingerprint command and six reproduced triggers, with the dirty reason each one prints.