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<!-- Please read the "Making a PR" section of [`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/main/CONTRIBUTING.md) before opening a Pull Request! * Keep your PR:s small and focused. * The PR title is what ends up in the changelog, so make it descriptive! * If applicable, add a screenshot or gif. * If it is a non-trivial addition, consider adding a demo for it to `egui_demo_lib`, or a new example. * Do NOT open PR:s from your `master` branch, as that makes it hard for maintainers to test and add commits to your PR. * Remember to run `cargo fmt` and `cargo clippy`. * Open the PR as a draft until you have self-reviewed it and run `./scripts/check.sh`. * When you have addressed a PR comment, mark it as resolved. Please be patient! I will review your PR, but my time is limited! --> * Closes <https://github.com/emilk/egui/issues/3964> * [X] I have followed the instructions in the PR template This PR adds support for syntax highlighting with custom `syntect::parsing::SyntaxSet` and `syntect::highlighting::ThemeSet`. It adds a new `egui_extras::highlight_with` function (enabled with `feature = "syntect"`), which takes a new public`struct SyntectSettings` containing the syntax and theme sets. ```rust let mut builder = SyntaxSetBuilder::new(); builder.add_from_folder("syntax", true).unwrap(); let ps = builder.build(); let ts = syntect::highlighting::ThemeSet::load_defaults(); let syntax = egui_extras::syntax_highlighting::SyntectSettings { ps, ts }; // ...elsewhere egui_extras::syntax_highlighting::highlight_with( ui.ctx(), ui.style(), &theme, buf, "rhai", &syntax, ); ``` There's a little bit of architectural complexity, but it all emerges naturally from the problem's constraints. Previously, the `Highlighter` both contained the `syntect` settings _and_ implemented `egui::cache::ComputerMut` to highlight a string; the settings would never change. After this change, the `syntect` settings have become part of the cache key, so we should redo highlighting if they change. The `Highlighter` becomes an empty `struct` which just serves to implement `ComputerMut`. `SyntaxSet` and `ThemeSet` are not hasheable themselves, so can't be used as cache keys direction. Instead, we can use the *address* of the `&SyntectSettings` as the key. This requires an object with a custom `Hash` implementation, so I added a new `HighlightSettings(&'a SyntectSettings)`, implementing `Hash` using `std::ptr::hash` on the reference. I think using the address is reasonable – it would be _weird_ for a user to be constantly moving around their `SyntectSettings`, and there's a warning in the docstring to this effect. To work _without_ custom settings, `SyntectSettings::default` preserves the same behavior as before, using `SyntaxSet::load_defaults_newlines` and `ThemeSet::load_defaults`. If the user doesn't provide custom settings, then we instantiate a singleton `SyntectSettings` in `data` and use it; this will only be constructed once. Finally, in cases where the `syntect` feature is disabled, `SyntectSettings` are replaced with a unit `()`. This adds a _tiny_ amount of overhead – one singleton `Arc<()>` allocation and a lookup in `data` per `highlight` – but I think that's better than dramatically different implementations. If this is an issue, I can refactor to make it zero-cost when the feature is disabled.