Hack, the PHP dialect, uses a persistent hh_server so linting, finding definitions etc is fast.
Whilst this is a more common model today (language server protocol, YouCompleteMe), it's an interesting design considering PHP is based on short-lived processes.
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The niche of short, environment-specific scripts, and the consequences for programming language design choices: https://brrt-to-the-future.blogspot.com/2021/03/why-bother-with-scripting.html
I've just squeezed another 5% of performance out of difftastic by finding a few HashSet values that weren't FxHashSet.
I do wonder whether hash DoS resistance is a good default. Sure, Rust programs are often pretty fast anyway, but it feels like a different threat model to the rest of Rust.
Whilst LLMs don't always give an accurate answer, the UI is really compelling. I keep finding users whose favourite way of doing research is an LLM.