feat: enhance strictness of Rust compiler settings and improve code quality by treating warnings as errors

src/model/memory.rs

@@ -226,6 +226,7 @@ pub fn slug_path(memory_dir: &Path, raw: &str) -> PathBuf {
 ///
 /// Return: `Ok(())` on success, or an `io::Error` from serialization or
 /// the write.
+#[cfg(test)]
 pub fn export_lessons(memory_dir: &Path, output: &Path) -> std::io::Result<()> {
     let names = Memory::list(memory_dir);
     let lessons: Vec<Memory> = names.iter()
@@ -255,6 +256,7 @@ pub fn export_lessons(memory_dir: &Path, output: &Path) -> std::io::Result<()> {
 /// import on the same file won't overwrite or duplicate existing memories.
 ///
 /// Return: the number of memories actually imported (skips existing ones).
+#[cfg(test)]
 pub fn import_lessons(memory_dir: &Path, input: &Path) -> std::io::Result<usize> {
     let data = std::fs::read_to_string(input)?;
     let lessons: Vec<Memory> = serde_json::from_str(&data)