Merge pull request rust-lang#2386 from senekor/senekor/lxuqllrrmroy

Use infallible conversion to teach From trait

Author: senekor

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

@@ -2,129 +2,53 @@
 // 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
+//
+// Representing units of measurements with separate types is a common practice.
+// It avoids accidentally mixing up values of different units of measurement.
 
-#[derive(Debug)]
-struct Person {
-    name: String,
-    age: u8,
-}
+struct Celsius(f64);
 
-// We implement the Default trait to use it as a fallback when the provided
-// string is not convertible into a `Person` object.
-impl Default for Person {
-    fn default() -> Self {
-        Self {
-            name: String::from("John"),
-            age: 30,
-        }
-    }
+struct Fahrenheit(f64);
+
+impl From<Celsius> for Fahrenheit {
+    // TODO: Convert Celsius to Fahrenheit. Don't worry about floating-point
+    // precision. The formula is: F = C * 1.8 + 32
 }
 
-// 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 {}
+impl From<Fahrenheit> for Celsius {
+    // TODO: Convert Fahrenheit to Celsius.
 }
 
 fn main() {
-    // Use the `from` function.
-    let p1 = Person::from("Mark,20");
-    println!("{p1:?}");
-
-    // Since `From` is implemented for Person, we are able to use `Into`.
-    let p2: Person = "Gerald,70".into();
-    println!("{p2:?}");
+    // You can optionally experiment here.
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    #[test]
-    fn test_default() {
-        let dp = Person::default();
-        assert_eq!(dp.name, "John");
-        assert_eq!(dp.age, 30);
-    }
-
-    #[test]
-    fn test_bad_convert() {
-        let p = Person::from("");
-        assert_eq!(p.name, "John");
-        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);
-    }
+    const CASES: [(f64, f64); 6] = [
+        (-50.0, -58.0),
+        (0.0, 32.0),
+        (20.0, 68.0),
+        (100.0, 212.0),
+        (400.0, 752.0),
+        (1000.0, 1832.0),
+    ];
 
     #[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);
+    fn celsius_to_fahrenheit() {
+        for (celsius, fahrenheit) in CASES {
+            let Fahrenheit(actual) = Celsius(celsius).into();
+            assert_eq!(actual.round(), fahrenheit);
+        }
     }
 
     #[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);
+    fn fahrenheit_to_celsius() {
+        for (celsius, fahrenheit) in CASES {
+            let Celsius(actual) = Fahrenheit(fahrenheit).into();
+            assert_eq!(actual.round(), celsius);
+        }
     }
 }

rustlings-macros/info.toml

@@ -1171,7 +1171,13 @@ 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."""
+The formula for converting from Fahrenheit to Celsius is: C = (F - 32) / 1.8
+This can be derived from the first formula:
+
+             F = C * 1.8 + 32   // now subtract 32 on both sides
+        F - 32 = C * 1.8        // then divide by 1.8
+(F - 32) / 1.8 = C
+"""
 
 [[exercises]]
 name = "conversions3"

solutions/23_conversions/conversions2.rs

@@ -2,135 +2,56 @@
 // 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
+//
+// Representing units of measurements with separate types is a common practice.
+// It avoids accidentally mixing up values of different units of measurement.
 
-#[derive(Debug)]
-struct Person {
-    name: String,
-    age: u8,
-}
+struct Celsius(f64);
 
-// We implement the Default trait to use it as a fallback when the provided
-// string is not convertible into a `Person` object.
-impl Default for Person {
-    fn default() -> Self {
-        Self {
-            name: String::from("John"),
-            age: 30,
-        }
+struct Fahrenheit(f64);
+
+impl From<Celsius> for Fahrenheit {
+    fn from(Celsius(celsius): Celsius) -> Self {
+        Fahrenheit(celsius * 1.8 + 32.0)
     }
 }
 
-impl From<&str> for Person {
-    fn from(s: &str) -> Self {
-        let mut split = s.split(',');
-        let (Some(name), Some(age), None) = (split.next(), split.next(), split.next()) else {
-            //                      ^^^^ 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,
-        }
+impl From<Fahrenheit> for Celsius {
+    fn from(Fahrenheit(fahrenheit): Fahrenheit) -> Self {
+        Celsius((fahrenheit - 32.0) / 1.8)
     }
 }
 
 fn main() {
-    // Use the `from` function.
-    let p1 = Person::from("Mark,20");
-    println!("{p1:?}");
-
-    // Since `From` is implemented for Person, we are able to use `Into`.
-    let p2: Person = "Gerald,70".into();
-    println!("{p2:?}");
+    // You can optionally experiment here.
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    #[test]
-    fn test_default() {
-        let dp = Person::default();
-        assert_eq!(dp.name, "John");
-        assert_eq!(dp.age, 30);
-    }
+    const CASES: [(f64, f64); 6] = [
+        (-50.0, -58.0),
+        (0.0, 32.0),
+        (20.0, 68.0),
+        (100.0, 212.0),
+        (400.0, 752.0),
+        (1000.0, 1832.0),
+    ];
 
     #[test]
-    fn test_bad_convert() {
-        let p = Person::from("");
-        assert_eq!(p.name, "John");
-        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);
+    fn celsius_to_fahrenheit() {
+        for (celsius, fahrenheit) in CASES {
+            let Fahrenheit(actual) = Celsius(celsius).into();
+            assert_eq!(actual.round(), fahrenheit);
+        }
     }
 
     #[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);
+    fn fahrenheit_to_celsius() {
+        for (celsius, fahrenheit) in CASES {
+            let Celsius(actual) = Fahrenheit(fahrenheit).into();
+            assert_eq!(actual.round(), celsius);
+        }
     }
 }