Add sandbox crate + workflow to test ephemeral podman runners

sandbox/src/lib.rs

@@ -0,0 +1,49 @@
+//! Tiny crate used to exercise CI caching: enough deps to make a warm
+//! build obviously faster than a cold one.
+use anyhow::{bail, Result};
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct Reading {
+    pub sensor: String,
+    pub celsius: f64,
+}
+
+pub fn to_fahrenheit(celsius: f64) -> f64 {
+    celsius * 9.0 / 5.0 + 32.0
+}
+
+pub fn parse(json: &str) -> Result<Reading> {
+    let r: Reading = serde_json::from_str(json)?;
+    if r.celsius < -273.15 {
+        bail!("{} C is below absolute zero", r.celsius);
+    }
+    Ok(r)
+}
+
+pub fn jitter(amount: f64) -> f64 {
+    use rand::Rng;
+    rand::thread_rng().gen_range(-amount..=amount)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn converts_units() {
+        assert!((to_fahrenheit(0.0) - 32.0).abs() < 1e-9);
+        assert!((to_fahrenheit(100.0) - 212.0).abs() < 1e-9);
+    }
+
+    #[test]
+    fn parses_reading() {
+        let r = parse(r#"{"sensor":"attic","celsius":21.5}"#).unwrap();
+        assert_eq!(r.sensor, "attic");
+    }
+
+    #[test]
+    fn rejects_impossible_temperature() {
+        assert!(parse(r#"{"sensor":"x","celsius":-300.0}"#).is_err());
+    }
+}