replace rustlings-macros with build script

This turns rustlings into a single crate, which is less complicated to
publish releases for. A follow-up step might be to remove the workspace
configuration.

The motivation was this discussion on Zulip:
https://rust-lang.zulipchat.com/#narrow/channel/334454-t-lang-docs.2Frustlings/topic/Trusted.20publishing/near/638396712

build.rs

@@ -1,5 +1,98 @@
+use quote::quote;
+use serde::Deserialize;
+
 fn main() {
     // Fix building from source on Windows because it can't handle file links.
     #[cfg(windows)]
     let _ = std::fs::copy("dev/Cargo.toml", "dev-Cargo.toml");
+
+    process_info_toml();
+}
+
+#[derive(Deserialize)]
+struct ExerciseInfo<'a> {
+    name: &'a str,
+    dir: &'a str,
+    #[serde(default)]
+    input_files: Vec<&'a str>,
+}
+
+#[derive(Deserialize)]
+struct InfoFile<'a> {
+    #[serde(borrow)]
+    exercises: Vec<ExerciseInfo<'a>>,
+}
+
+/// Translates the data in info.toml to Rust data structures to be embedded in
+/// the final binary.
+pub fn process_info_toml() {
+    let info_file = include_str!("info.toml");
+    let info = toml::de::from_str::<InfoFile>(info_file).expect("Failed to parse `info.toml`");
+    let exercises = info.exercises;
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+
+    let exercise_files = exercises.iter().map(|exercise| {
+        format!(
+            "{manifest_dir}/exercises/{}/{}.rs",
+            exercise.dir, exercise.name
+        )
+    });
+    let solution_files = exercises.iter().map(|exercise| {
+        format!(
+            "{manifest_dir}/solutions/{}/{}.rs",
+            exercise.dir, exercise.name
+        )
+    });
+
+    let mut dirs = Vec::with_capacity(32);
+    let mut dir_inds = vec![0; exercises.len()];
+
+    for (exercise, dir_ind) in exercises.iter().zip(&mut dir_inds) {
+        // The directory is often the last one inserted.
+        if let Some(ind) = dirs.iter().rev().position(|dir| *dir == exercise.dir) {
+            *dir_ind = dirs.len() - 1 - ind;
+            continue;
+        }
+
+        dirs.push(exercise.dir);
+        *dir_ind = dirs.len() - 1;
+    }
+
+    let input_files = exercises.iter().map(|exercise| {
+        let names = exercise.input_files.iter();
+        let paths = exercise
+            .input_files
+            .iter()
+            .map(|f| format!("{manifest_dir}/exercises/{}/{}", exercise.dir, f));
+        quote! {
+            &[#(InputFile {
+                name: #names,
+                content: include_str!(#paths),
+            }),*]
+        }
+    });
+
+    let readmes = dirs
+        .iter()
+        .map(|dir| format!("{manifest_dir}/exercises/{dir}/README.md"));
+
+    let embedded_files = quote! {
+        EmbeddedFiles {
+            info_file: #info_file,
+            exercise_files: &[#(ExerciseFiles {
+                exercise: include_bytes!(#exercise_files),
+                solution: include_bytes!(#solution_files),
+                dir_ind: #dir_inds,
+                input_files: #input_files,
+            }),*],
+            exercise_dirs: &[#(ExerciseDir { name: #dirs, readme: include_bytes!(#readmes) }),*],
+        }
+    };
+
+    let out_dir = std::env::var("OUT_DIR").unwrap();
+    std::fs::write(
+        format!("{out_dir}/embedded_files.rs"),
+        embedded_files.to_string(),
+    )
+    .unwrap();
 }