Use infallible conversion to teach From trait

CHANGELOG.md

@@ -22,6 +22,7 @@
 - `structs3`: Rewrote the exercise to make users type method syntax themselves.
 - Rename the exercises for smart pointers and conversions so they're sorted alphabetically. [@foxfromworld](https://github.com/foxfromworld)
 - `vecs1`: Remove array literal. Some learners assumed their task is to convert it to a vector.
+- `conversions2`: Redesign the context such that infallible conversion makes sense.
 
 ## 6.5.0 (2025-08-21)
 

exercises/23_conversions/conversions2.rs

@@ -2,49 +2,36 @@
 // implemented, an implementation of `Into` is automatically provided.
 // You can read more about it in the documentation:
 // https://doc.rust-lang.org/std/convert/trait.From.html
+//
+// Frank the fairy would like to buy some truffles from Grace the gnome, a
+// world-renowned chocolatier. The truffles are priced in GnomeCoin though, and
+// Frank only has FairyCredit. Help Frank by providing a `From` implementation
+// to convert his FairyCredit to GnomeCoin. At the current exchange rate, one
+// FairyCredit is valued at 100 GnomeCoin.
 
 #[derive(Debug)]
-struct Person {
+struct FairyCredit(u32);
-    name: String,
+
-    age: u8,
+#[derive(Debug, PartialEq)]
+struct GnomeCoin(u64);
+
+impl From<FairyCredit> for GnomeCoin {
+    // TODO: implement From<FairyCredit> for GnomeCoin
 }
 
-// We implement the Default trait to use it as a fallback when the provided
+// Note that we shouldn't provide the opposite conversion: from GnomeCoin to
-// string is not convertible into a `Person` object.
+// FairyCredits. That's because less than 100 GnomeCoins cannot be represented
-impl Default for Person {
+// as FairyCredits, which would make the conversion lossy. The `From` trait is
-    fn default() -> Self {
+// only appropriate for infallible and lossless conversions.
-        Self {
-            name: String::from("John"),
-            age: 30,
-        }
-    }
-}
-
-// TODO: Complete this `From` implementation to be able to parse a `Person`
-// out of a string in the form of "Mark,20".
-// Note that you'll need to parse the age component into a `u8` with something
-// like `"4".parse::<u8>()`.
-//
-// Steps:
-// 1. Split the given string on the commas present in it.
-// 2. If the split operation returns less or more than 2 elements, return the
-//    default of `Person`.
-// 3. Use the first element from the split operation as the name.
-// 4. If the name is empty, return the default of `Person`.
-// 5. Parse the second element from the split operation into a `u8` as the age.
-// 6. If parsing the age fails, return the default of `Person`.
-impl From<&str> for Person {
-    fn from(s: &str) -> Self {}
-}
 
 fn main() {
     // Use the `from` function.
-    let p1 = Person::from("Mark,20");
+    let g1 = GnomeCoin::from(FairyCredit(12));
-    println!("{p1:?}");
+    println!("{g1:?}");
 
-    // Since `From` is implemented for Person, we are able to use `Into`.
+    // Since `From` is implemented for GnomeCoin, we are able to use `Into`.
-    let p2: Person = "Gerald,70".into();
+    let g2: GnomeCoin = FairyCredit(9).into();
-    println!("{p2:?}");
+    println!("{g2:?}");
 }
 
 #[cfg(test)]
@@ -52,79 +39,14 @@ mod tests {
     use super::*;
 
     #[test]
-    fn test_default() {
+    fn test_from() {
-        let dp = Person::default();
+        let g = GnomeCoin::from(FairyCredit(12));
-        assert_eq!(dp.name, "John");
+        assert_eq!(g, GnomeCoin(1200));
-        assert_eq!(dp.age, 30);
     }
 
     #[test]
-    fn test_bad_convert() {
+    fn test_into() {
-        let p = Person::from("");
+        let g: GnomeCoin = FairyCredit(9).into();
-        assert_eq!(p.name, "John");
+        assert_eq!(g, GnomeCoin(900));
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_good_convert() {
-        let p = Person::from("Mark,20");
-        assert_eq!(p.name, "Mark");
-        assert_eq!(p.age, 20);
-    }
-
-    #[test]
-    fn test_bad_age() {
-        let p = Person::from("Mark,twenty");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_comma_and_age() {
-        let p: Person = Person::from("Mark");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_age() {
-        let p: Person = Person::from("Mark,");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_name() {
-        let p: Person = Person::from(",1");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_name_and_age() {
-        let p: Person = Person::from(",");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_name_and_invalid_age() {
-        let p: Person = Person::from(",one");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_trailing_comma() {
-        let p: Person = Person::from("Mike,32,");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_trailing_comma_and_some_string() {
-        let p: Person = Person::from("Mike,32,dog");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
     }
 }

rustlings-macros/info.toml

@@ -1171,7 +1171,9 @@ Use the `as` operator to cast one of the operands in the last line of the
 name = "conversions2"
 dir = "23_conversions"
 hint = """
-Follow the steps provided right before the `From` implementation."""
+Implement From<FairyCredit> for GnomeCoin. Check the documentation of `From` to
+learn about its required items:
+https://doc.rust-lang.org/std/convert/trait.From.html"""
 
 [[exercises]]
 name = "conversions3"

solutions/23_conversions/conversions2.rs

@@ -2,55 +2,38 @@
 // implemented, an implementation of `Into` is automatically provided.
 // You can read more about it in the documentation:
 // https://doc.rust-lang.org/std/convert/trait.From.html
+//
+// Frank the fairy would like to buy some truffles from Grace the gnome, a
+// world-renowned chocolatier. The truffles are priced in GnomeCoin though, and
+// Frank only has FairyCredit. Help Frank by providing a `From` implementation
+// to convert his FairyCredit to GnomeCoin. At the current exchange rate, one
+// FairyCredit is valued at 100 GnomeCoin.
 
 #[derive(Debug)]
-struct Person {
+struct FairyCredit(u32);
-    name: String,
-    age: u8,
-}
 
-// We implement the Default trait to use it as a fallback when the provided
+#[derive(Debug, PartialEq)]
-// string is not convertible into a `Person` object.
+struct GnomeCoin(u64);
-impl Default for Person {
+
-    fn default() -> Self {
+impl From<FairyCredit> for GnomeCoin {
-        Self {
+    fn from(value: FairyCredit) -> Self {
-            name: String::from("John"),
+        Self(value.0 as u64 * 100)
-            age: 30,
-        }
     }
 }
 
-impl From<&str> for Person {
+// Note that we shouldn't provide the opposite conversion: from GnomeCoin to
-    fn from(s: &str) -> Self {
+// FairyCredits. That's because less than 100 GnomeCoins cannot be represented
-        let mut split = s.split(',');
+// as FairyCredits, which would make the conversion lossy. The `From` trait is
-        let (Some(name), Some(age), None) = (split.next(), split.next(), split.next()) else {
+// only appropriate for infallible and lossless conversions.
-            //                      ^^^^ there should be no third element
-            return Self::default();
-        };
-
-        if name.is_empty() {
-            return Self::default();
-        }
-
-        let Ok(age) = age.parse() else {
-            return Self::default();
-        };
-
-        Self {
-            name: name.into(),
-            age,
-        }
-    }
-}
 
 fn main() {
     // Use the `from` function.
-    let p1 = Person::from("Mark,20");
+    let g1 = GnomeCoin::from(FairyCredit(12));
-    println!("{p1:?}");
+    println!("{g1:?}");
 
-    // Since `From` is implemented for Person, we are able to use `Into`.
+    // Since `From` is implemented for GnomeCoin, we are able to use `Into`.
-    let p2: Person = "Gerald,70".into();
+    let g2: GnomeCoin = FairyCredit(9).into();
-    println!("{p2:?}");
+    println!("{g2:?}");
 }
 
 #[cfg(test)]
@@ -58,79 +41,14 @@ mod tests {
     use super::*;
 
     #[test]
-    fn test_default() {
+    fn test_from() {
-        let dp = Person::default();
+        let g = GnomeCoin::from(FairyCredit(12));
-        assert_eq!(dp.name, "John");
+        assert_eq!(g, GnomeCoin(1200));
-        assert_eq!(dp.age, 30);
     }
 
     #[test]
-    fn test_bad_convert() {
+    fn test_into() {
-        let p = Person::from("");
+        let g: GnomeCoin = FairyCredit(9).into();
-        assert_eq!(p.name, "John");
+        assert_eq!(g, GnomeCoin(900));
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_good_convert() {
-        let p = Person::from("Mark,20");
-        assert_eq!(p.name, "Mark");
-        assert_eq!(p.age, 20);
-    }
-
-    #[test]
-    fn test_bad_age() {
-        let p = Person::from("Mark,twenty");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_comma_and_age() {
-        let p: Person = Person::from("Mark");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_age() {
-        let p: Person = Person::from("Mark,");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_name() {
-        let p: Person = Person::from(",1");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_name_and_age() {
-        let p: Person = Person::from(",");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_missing_name_and_invalid_age() {
-        let p: Person = Person::from(",one");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_trailing_comma() {
-        let p: Person = Person::from("Mike,32,");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
-    }
-
-    #[test]
-    fn test_trailing_comma_and_some_string() {
-        let p: Person = Person::from("Mike,32,dog");
-        assert_eq!(p.name, "John");
-        assert_eq!(p.age, 30);
     }
 }