Profiling
If you have a file that's particularly slow, you can use cargo-flamegraph to see which functions are slow.
$ CARGO_PROFILE_RELEASE_DEBUG=true cargo flamegraph --bin difft sample_files/slow_before.rs sample_files/slow_after.rs
It's also worth looking at memory usage, as graph traversal bugs can lead to huge memory consumption.
$ /usr/bin/time -v ./target/release/difft sample_files/slow_before.rs sample_files/slow_after.rs
If timing measurement are noisy, Linux's perf tool will report
instructions executed, which is more stable.
$ perf stat ./target/release/difft sample_files/slow_before.rs sample_files/slow_after.rs
$ perf stat ./target/release/difft sample_files/typing_old.ml sample_files/typing_new.ml
Many more profiling techniques are discussed in The Rust Performance Book.