Add an error on the BikeBuilder::build() function

src/main.rs

 #![doc = include_str!("../README.md")]
 
-
-#[derive(Debug)]
+#[derive(Debug, Clone, Copy)]
 pub enum Brake {
     Disk,
-    Pads
+    Pads,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone, Copy)]
 pub enum Wheel {
     Large,
     Small,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone, Copy)]
 pub enum Transmission {
     Standard,
     GearHub,
@@ -37,7 +36,7 @@ impl Bike {
     }
 }
 
-#[derive(Default,Debug)]
+#[derive(Default, Debug)]
 /// A bike builder that helps the configuration of a bike.
 pub struct BikeBuilder {
     brakes: Option<Brake>,
@@ -64,24 +63,26 @@ impl BikeBuilder {
     /// Builds the bike from the builder.
     ///
     /// Will panic if the builder configuration is not correct
-    pub fn build(self) -> Bike {
-        Bike {
-            wheels: self.wheels.unwrap(),
-            transmission: self.transmission.unwrap(),
-            brakes: self.brakes.unwrap(),
+    pub fn build(self) -> Result<Bike, String> {
+        match (self.wheels, self.transmission, self.brakes) {
+            (Some(w), Some(t), Some(b)) => Ok(Bike {
+                wheels: w,
+                transmission: t,
+                brakes: b,
+            }),
+            _ => Err(format!("The bike builder would create an invalid bike: {:?}", self))
         }
     }
 }
 
 fn main() {
     let builder = Bike::builder();
-    let bike = builder.with_brakes(Brake::Disk)
-        .with_transmission(Transmission::GearHub)
+    let bike = builder
+        .with_brakes(Brake::Disk)
+//        .with_transmission(Transmission::GearHub)
         .with_wheels(Wheel::Small)
-        .build();
-    
+        .build().unwrap();
+
     println!("{bike:#?}");
     bike.ride();
-
-    
 }