🔨 Make the Design System color tokens reachable from the rust code

The design system tokens are specified in the _base.scss file. To avoid to duplicate their value in a rust file, a new
step has been added to the building process to generate the `style_vars.rs` file which make the color tokens reachable
to the app.

Cargo.toml

@@ -31,3 +31,5 @@ futures = "0.3.29"
 [build]
 target = "x86_64-unknown-linux-gnu"
 
+[build-dependencies]
+regex = "1.10.3"

build.rs

@@ -1,5 +1,81 @@
+use std::env;
+use std::fs::File;
+use std::io::Write;
+use std::io::{self, BufRead};
+use std::path::Path;
+use std::path::PathBuf;
+
+use regex::Regex;
+
 fn main() {
     // Tell Cargo to rerun this build script if any SCSS file
     // in the 'src' directory or its subdirectories changes.
     println!("cargo:rerun-if-changed=src/**/*.scss");
+
+    let out_dir = env::var("OUT_DIR").unwrap();
+
+    let style_src_path = PathBuf::from("src/_base.scss");
+    let style_dst_path = Path::new(&out_dir).join("style_vars.rs");
+
+    export_color_variables(&style_src_path, &style_dst_path)
+}
+
+// From https://doc.rust-lang.org/rust-by-example/std_misc/file/read_lines.html
+// The output is wrapped in a Result to allow matching on errors.
+// Returns an Iterator to the Reader of the lines of the file.
+fn read_lines<P>(filename: P) -> io::Result<io::Lines<io::BufReader<File>>>
+where
+    P: AsRef<Path>,
+{
+    let file = File::open(filename)?;
+    Ok(io::BufReader::new(file).lines())
+}
+
+#[derive(Debug)]
+struct CssColorVariable<'a> {
+    name: &'a str,
+    value: &'a str,
+}
+
+impl<'a> CssColorVariable<'a> {
+    pub fn to_rust(&self) -> String {
+        format!(
+            "const {name}: &str = \"{value}\";",
+            name = self.name.replace("-", "_").to_uppercase(),
+            value = self.value
+        )
+    }
+}
+
+fn export_color_variables(src_path: &PathBuf, dst_path: &PathBuf) {
+    let mut dst_file = File::create(&dst_path).unwrap();
+    if let Err(err) = dst_file.write(b"#[allow(dead_code)]\nmod style {") {
+        println!("{}", err);
+        return;
+    };
+
+    let re = Regex::new(r"^\$([^:]+):[[:space:]]*#([^$]+);[[:space:]]*$").unwrap();
+
+    if let Ok(lines) = read_lines(src_path) {
+        for line in lines.flatten() {
+            let Some(groups) = re.captures(&line) else {
+                continue;
+            };
+
+            let var = CssColorVariable {
+                name: &groups[1],
+                value: &groups[2],
+            };
+
+            let rust_export = var.to_rust();
+            if let Err(err) = dst_file.write_fmt(format_args!("    pub {}\n", rust_export)) {
+                println!("{}", err);
+                break;
+            }
+        }
+    }
+
+    if let Err(err) = dst_file.write(b"}\n") {
+        println!("{}", err);
+    };
 }