I’m skeptical this is just codegen units, since the non-incremental result was with -C codgen-units=4 on a 4 cores (8 threads) machine. With -C codegen-units=200 non-incremental, I get 161 seconds, slower than 147 seconds for 4 units.
Back to incremental builds, -Zincremental-info says everything is reused: incremental: re-using 1252 out of 1252 modules. Here is the output with RUSTFLAGS="-Zincremental-info -Ztime-passes" for the largest crate (script) https://gist.github.com/anonymous/996ddd7dae5da732c02e517f368a08bf Extract:
time: 3.367; rss: 830MB expansion
time: 2.508; rss: 1086MB compute_incremental_hashes_map
time: 16.472; rss: 1607MB load_dep_graph
time: 7.488; rss: 2957MB item-bodies checking
time: 7.160; rss: 4005MB translation
time: 17.666; rss: 3945MB serialize dep graph
time: 9.442; rss: 2305MB linking
Finished dev [unoptimized + debuginfo] target(s) in 79.13 secs