Tolerate changes in the state file

src/info_file.rs

@@ -0,0 +1,81 @@
+use anyhow::{bail, Context, Error, Result};
+use serde::Deserialize;
+use std::{fs, path::PathBuf};
+
+// The mode of the exercise.
+#[derive(Deserialize, Copy, Clone)]
+#[serde(rename_all = "lowercase")]
+pub enum Mode {
+    // The exercise should be compiled as a binary
+    Run,
+    // The exercise should be compiled as a test harness
+    Test,
+    // The exercise should be linted with clippy
+    Clippy,
+}
+
+// Deserialized from the `info.toml` file.
+#[derive(Deserialize)]
+pub struct ExerciseInfo {
+    // Name of the exercise
+    pub name: String,
+    // The exercise's directory inside the `exercises` directory
+    pub dir: Option<String>,
+    // The mode of the exercise
+    pub mode: Mode,
+    // The hint text associated with the exercise
+    pub hint: String,
+}
+
+impl ExerciseInfo {
+    pub fn path(&self) -> PathBuf {
+        let path = if let Some(dir) = &self.dir {
+            format!("exercises/{dir}/{}.rs", self.name)
+        } else {
+            format!("exercises/{}.rs", self.name)
+        };
+
+        PathBuf::from(path)
+    }
+}
+
+#[derive(Deserialize)]
+pub struct InfoFile {
+    pub welcome_message: Option<String>,
+    pub final_message: Option<String>,
+    pub exercises: Vec<ExerciseInfo>,
+}
+
+impl InfoFile {
+    pub fn parse() -> Result<Self> {
+        // Read a local `info.toml` if it exists.
+        let slf: Self = match fs::read_to_string("info.toml") {
+            Ok(file_content) => toml_edit::de::from_str(&file_content)
+                .context("Failed to parse the `info.toml` file")?,
+            Err(e) => match e.kind() {
+                std::io::ErrorKind::NotFound => {
+                    toml_edit::de::from_str(include_str!("../info.toml"))
+                        .context("Failed to parse the embedded `info.toml` file")?
+                }
+                _ => return Err(Error::from(e).context("Failed to read the `info.toml` file")),
+            },
+        };
+
+        if slf.exercises.is_empty() {
+            bail!("{NO_EXERCISES_ERR}");
+        }
+
+        let mut names_set = hashbrown::HashSet::with_capacity(slf.exercises.len());
+        for exercise in &slf.exercises {
+            if !names_set.insert(exercise.name.as_str()) {
+                bail!("Exercise names must all be unique!")
+            }
+        }
+        drop(names_set);
+
+        Ok(slf)
+    }
+}
+
+const NO_EXERCISES_ERR: &str = "There are no exercises yet!
+If you are developing third-party exercises, add at least one exercise before testing.";