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Stylance

Stylance is a library and cli tool for working with scoped CSS in rust.

Features:

• Import hashed class names from css files into your rust code as string constants.
  • Trying to use a class name that doesn't exist in the css file becomes an error.
  • Unused class names become warnings.
• Bundle your css module files into a single output css file with all the class names transformed to include a hash (by using stylance cli).
• Class name hashes are deterministic and based on the relative path between the css file and your crate's manifest dir (where the Cargo.toml resides)
• CSS Bundle generation is independent of the rust build process, allowing for blazingly fast iteration when modifying the contents of a css style rule.

Usage

Stylance is divided in two parts:

1. Rust proc macros for importing scoped class names from css files as string constants into your rust code.
2. A cli tool that finds all css modules in your crate and generates an output css file with hashed class names.

Proc macro

Add stylance as a dependency:

cargo add stylance

Then use the import_crate_style proc macro to read a css/scss file and bring the classes from within that file as constants.

src/component/card/card.module.scss file's content:

.header {
    background-color: red;
}

src/component/card/card.rs file's contents:

// Import a css file's classes:
stylance::import_crate_style!(my_style, "src/component/card/card.module.scss");

fn use_style() {
	// Use the classnames:
	println!("{}", my_style::header) // prints header-f45126d
}

All class names found inside the file src/component/card/card.module.scss will be included as constants inside a module named as the identifier passed as first argument to import_style.

The proc macro has no side effects, to generate the transformed css file we then use the stylance cli.

Accessing global classnames

Sometimes you might want to target classnames that are defined globally and outside of your css module. To do this you can wrap them with :global()

.my_scoped_class :global(.paragraph) {
    color: red;
}

this will transform to:

.my_scoped_class-f45126d .paragraph {
    color: red;
}

.my_scoped_class got the module hash attached but .paragraph was left alone while the :global() was removed.

Unused classname warnings

The import style macros will crate constants which, if left unused, will produce warnings.

This is helpful if you don't want to have css classes left unused but you are able to silence this warning by adding #[allow(dead_code)] before the module identifier in the macro.

Example:

import_crate_style!(#[allow(dead_code)] my_style, "src/component/card/card.module.scss");

Any attribute is allowed, if you want to deny instead you can do it too:

import_crate_style!(#[deny(dead_code)] my_style, "src/component/card/card.module.scss");

Nightly feature

If you are using rust nightly you can enable the nightly feature to get access to the import_style! macro which lets you specify the css module file as relative to the current file.

Enable the nightly feature:

stylance = { version = "<version here>", features = ["nightly"] }

Then import style as relative:

src/component/card/card.rs:

stylance::import_style!(my_style, "card.module.scss");

Stylance cli

Install stylance cli:

cargo install stylance-cli

Run stylance cli:

stylance ./path/to/crate/dir/ --output-file ./bundled.scss

The first argument is the path to the directory containing the Cargo.toml of your package/crate.

This will find all the files ending with .module.scss and .module.cssand bundle them into ./bundled.scss, all classes will be modified to include a hash that matches the one the import_crate_style! macro produces.

Resulting ./bundled.scss:

.header-f45126d {
    background-color: red;
}

By default stylance cli will only look for css modules inside the crate's ./src/ folder. This can be configured.

<a name="SASS"></a> Use output-dir for better SASS compatibility

If you plan to use the output of stylance in a SASS project (by importing it from a .scss file), then I recommend using the output-dir option instead of output-file.

stylance ./path/to/crate/dir/ --output-dir ./styles/

This will create the folder ./styles/stylance/.

When using --output-dir (or output_dir in package.metadata.stylance) stylance will not bundle the transformed module files, instead it will create a "stylance" folder in the specified output-dir path which will contain all the transformed css modules inside as individual files.

This "stylance" folder also includes an _index.scss file that imports all the transformed scss modules.

You can then use @use "path/to/the/folder/stylance" to import the css modules into your sass project.

Watching for changes

During development it is convenient to use sylance cli in watch mode:

stylance --watch --output-file ./bundled.scss ./path/to/crate/dir/

The stylance process will then watch any .module.css and .module.scss files for changes and automatically rebuild the output file.

<a name="configuration"></a> Configuration

Stylance configuration lives inside the Cargo.toml file of your crate.

All configuration settings are optional.

[package.metadata.stylance]

# output_file
# When set, stylance-cli will bundle all css module files
# into by concatenating them and put the result in this file.
output_file = "./styles/bundle.scss"

# output_dir
# When set, stylance-cli will create a folder named "stylance" inside
# the output_dir directory.
# The stylance folder will be populated with one file per detected css module
# and one _all.scss file that contains one `@use "file.module-hash.scss";` statement
# per module file.
# You can use that file to import all your modules into your main scss project.
output_dir = "./styles/"

# folders
# folders in which stylance cli will look for css module files.
# defaults to ["./src/"]
folders = ["./src/", "./styles/"]

# extensions
# files ending with these extensions will be considered to be
# css modules by stylance cli and will be included in the output
# bundle
# defaults to [".module.scss", ".module.css"]
extensions = [".module.scss", ".module.css"]

# scss_prelude
# When generating an scss file stylance-cli will prepend this string
# Useful to include a @use statement to all scss modules.
scss_prelude = '@use "../path/to/prelude" as *;'

# hash_len
# Controls how long the hash name used in scoped classes should be.
# It is safe to lower this as much as you want, stylance cli will produce an
# error if two files end up with colliding hashes.
# defaults to 7
hash_len = 7

# class_name_pattern
# Controls the shape of the transformed scoped class names.
# [name] will be replaced with the original class name
# [hash] will be replaced with the hash of css module file path.
# defaults to "[name]-[hash]"
class_name_pattern = "my-project-[name]-[hash]"

Rust analyzer completion issues

Nightly import_style!

Rust analyzer will not produce any completion for import_style!, this is because it doesn't support the nightly features used to obtain the current rust file path.

Stable import_crate_style!

Rust analyzer will expand the import_crate_style!(style, "src/mystyle.module.css") macro properly the first time, which means you'll be able to get completion when typing style::|.

Unfortunately RA will cache the result and will not realize that it needs to reevaluate the proc macro when the contents of src/mystyle.module.css change.

This only affects completion, errors from cargo check will properly update.

The only way to force RA to reevaluate the macros is to restart the server or to rebuild all proc macros. Sadly this takes a really long time.

It is my opinion that no completion would be better than outdated completion.

Supposedly one should be able to disable the expansion of the macro by adding this to .vscode/settings.json

"rust-analyzer.procMacro.ignored": {
   "stylance": ["import_style_classes"]
},

Unfortunately this doesn't seem to work at the moment, this rust analyzer feature might fix the issue: https://github.com/rust-lang/rust-analyzer/pull/15923

In the meantime the nightly import_style is my recommended way to work with this crate.