Generics with traits in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of generic function with trait bound

What is the output of this Rust program?

Rust

use std::fmt::Display;

fn print_value<T: Display>(value: T) {
    println!("Value: {}", value);
}

fn main() {
    print_value(42);
    print_value("hello");
}

A Value: 42\nValue: hello

B Value: 42\nValue: \"hello\"

C Compilation error: trait bound not satisfied

D Value: 42\nValue: hello\nValue: 42

💡 Hint

Look at the trait bound Display and how it formats values.

The function print_value accepts any type T that implements the Display trait. Both i32 and &str implement Display, so the function prints their values directly without quotes.

🧠 Conceptual

intermediate

Trait bounds in generic structs

Which statement about the following Rust code is true?

struct Container {
    value: T,
}

impl Container {
    fn duplicate(&self) -> (T, T) {
        (self.value.clone(), self.value.clone())
    }
}

A The duplicate() method will fail to compile for all types.

B The struct can hold any type, but duplicate() requires Clone.

C The struct requires Copy trait, not Clone.

D The struct can only hold types that implement the Clone trait.

💡 Hint

Look at the trait bound on the struct definition.

The trait bound T: Clone on the struct means Container can only be created with types that implement Clone. This ensures duplicate() can call clone() safely.

🔧 Debug

advanced

Fix the trait bound error

What error does this Rust code produce?

fn compare(a: T, b: T) -> bool {
    a == b
}

fn main() {
    println!("{}", compare(5, 5));
}

A error: function main must return ()

B error: missing semicolon after println!

C error: binary operation `==` cannot be applied to type `T`

D No error, prints true

💡 Hint

Think about what traits are needed to use == operator.

The generic type T must implement the PartialEq trait to use the == operator. Without this bound, the compiler errors.

❓ Predict Output

advanced

Output of generic trait implementation

What is the output of this Rust program?

Rust

trait Summary {
    fn summarize(&self) -> String;
}

struct NewsArticle {
    headline: String,
    location: String,
}

impl Summary for NewsArticle {
    fn summarize(&self) -> String {
        format!("{} - {}", self.headline, self.location)
    }
}

fn notify<T: Summary>(item: T) {
    println!("Breaking news: {}", item.summarize());
}

fn main() {
    let article = NewsArticle {
        headline: String::from("Rust 1.60 Released"),
        location: String::from("Internet"),
    };
    notify(article);
}

A Breaking news: Rust 1.60 Released, Internet

B Breaking news: Rust 1.60 Released - Internet

C Compilation error: missing trait implementation

D Breaking news: NewsArticle

💡 Hint

Look at the summarize method implementation and the notify function.

The Summary trait is implemented for NewsArticle. The notify function calls summarize, which returns the formatted headline and location.

🧠 Conceptual

expert

Trait object vs generic parameter

Which statement correctly explains the difference between using a trait object &dyn Trait and a generic parameter T: Trait in Rust?

A <p><code>&dyn Trait</code> uses dynamic dispatch and allows different types at runtime; <code>T: Trait</code> uses static dispatch and requires a single type at compile time.</p>

B <p><code>&dyn Trait</code> requires the type to implement <code>Copy</code>; <code>T: Trait</code> does not.</p>

C <p><code>T: Trait</code> uses dynamic dispatch; <code>&dyn Trait</code> uses static dispatch.</p>

D <p>Both are identical in performance and usage.</p>

💡 Hint

Think about when Rust decides which method to call: compile time or runtime.

Trait objects like &dyn Trait use dynamic dispatch, meaning the exact method to call is decided at runtime. Generic parameters with trait bounds use static dispatch, where the compiler generates code for each concrete type.

Practice

1. What is the main purpose of using generics with traits in Rust?

easy

A. To restrict functions to only work with primitive types

B. To allow functions or structs to work with different types that implement specific behavior

C. To enable dynamic typing in Rust

D. To automatically convert types without explicit code

Solution

1. Step 1: Understand generics

   Generics allow writing code that works with multiple types without repeating code.

2. Step 2: Understand traits with generics

   Traits define behavior. Using traits with generics means the generic type must implement certain behavior.

3. Final Answer:

   To allow functions or structs to work with different types that implement specific behavior -> Option B

4. Quick Check:

   Generics + traits = flexible code with behavior constraints [OK]

Hint: Generics + traits = flexible code with behavior constraints [OK]

Common Mistakes:

• Thinking generics only work with primitive types
• Confusing traits with dynamic typing
• Assuming automatic type conversion

2. Which of the following is the correct syntax to declare a generic function in Rust that requires the generic type T to implement the Display trait?

easy

A. fn print_value(value: T) where Display { println!("{}", value); }

B. fn print_value<T>(value: T) where T: Display { println!("{}", value); }

C. fn print_value<T>(value: T: Display) { println!("{}", value); }

D. fn print_value(value: T) { println!("{}", value); }

Solution

1. Step 1: Recall generic syntax with trait bounds

   The correct syntax places the trait bound after the generic type with a colon: T: Display.

2. Step 2: Check function signature correctness

   fn print_value(value: T) { println!("{}", value); } correctly uses fn print_value(value: T). fn print_value<T>(value: T) where T: Display { println!("{}", value); } uses where clause but incorrectly placed. fn print_value<T>(value: T: Display) { println!("{}", value); } and D have syntax errors.

3. Final Answer:

   fn print_value<T: Display>(value: T) { println!("{}", value); } -> Option D

4. Quick Check:

   Generic with trait bound syntax = fn print_value(value: T) { println!("{}", value); } [OK]

Hint: Trait bounds go after generic type with colon: T: Trait [OK]

Common Mistakes:

• Placing trait bound inside function parameters
• Misusing where clause syntax
• Omitting trait bound for required behavior

3. What will be the output of the following Rust code?

use std::fmt::Display;

fn print_info(item: T) {
    println!("Item: {}", item);
}

fn main() {
    print_info(42);
    print_info("hello");
}

medium

A. Item: 42
Item: hello

B. Item: 42
Item: "hello"

C. Compilation error due to missing trait implementation

D. Runtime error

Solution

1. Step 1: Check trait implementation for types

   Both i32 (42) and &str ("hello") implement the Display trait, so the function can print both.

2. Step 2: Understand println! macro output

   The macro prints the value without quotes for Display. So output is two lines: "Item: 42" and "Item: hello".

3. Final Answer:

   Item: 42 Item: hello -> Option A

4. Quick Check:

   Display trait prints values plainly = Item: 42 Item: hello [OK]

Hint: Display prints values without quotes, works for i32 and &str [OK]

Common Mistakes:

• Expecting quotes around string output
• Thinking trait is not implemented for primitive types
• Confusing compile and runtime errors

4. Identify the error in the following Rust code and select the correct fix:

use std::fmt::Display;

fn print_twice<T>(item: T) {
    println!("{} {}", item, item);
}

fn main() {
    print_twice(10);
}

medium

A. Add trait bound: fn print_twice(item: T) {...}

B. Change println! to println!("{:?} {:?}", item, item);

C. Remove generic and use i32 directly

D. No error, code runs fine

Solution

1. Step 1: Understand trait requirements for println!

   The format string "{}" requires the type to implement Display trait.

2. Step 2: Check generic function signature

   The function lacks a trait bound on T, so compiler cannot guarantee T implements Display.

3. Step 3: Fix by adding trait bound

   Add T: Display to function signature to fix the error.

4. Final Answer:

   Add trait bound: fn print_twice<T: Display>(item: T) {...} -> Option A

5. Quick Check:

   Format string "{}" needs Display trait bound [OK]

Hint: Add trait bound when using {} in println! with generics [OK]

Common Mistakes:

• Ignoring missing trait bound error
• Using {:?} without Debug trait bound
• Assuming generic works without constraints

5. Given the following trait and struct, how can you implement a generic function compare_and_print that accepts any two items of the same type implementing both PartialOrd and Display, and prints which one is greater or if they are equal?

use std::fmt::Display;

struct Item<T> {
    value: T,
}

// Implement compare_and_print here

hard

A. fn compare_and_print(a: T, b: T) {
println!("Comparing {} and {}", a, b);
}

B. fn compare_and_print<T>(a: T, b: T) where T: PartialOrd + Display {
if a >= b {
println!("{} is greater", a);
} else {
println!("{} is greater", b);
}
}

C. fn compare_and_print(a: T, b: T) {
if a > b {
println!("{} is greater", a);
} else if a < b {
println!("{} is greater", b);
} else {
println!("Both are equal: {}", a);
}
}

D. fn compare_and_print(a: T, b: T) {
if a > b {
println!("a is greater");
} else {
println!("b is greater");
}
}

Solution

1. Step 1: Define trait bounds for generic function

   The function must require T to implement both PartialOrd (for comparison) and Display (for printing).

2. Step 2: Implement comparison logic with all cases

   Check if a > b, print a is greater; else if a < b, print b is greater; else print both are equal.

3. Step 3: Verify printing uses Display trait

   Use {} in println! to print values, which requires Display.

4. Final Answer:

   fn compare_and_print<T: PartialOrd + Display>(a: T, b: T) { if a > b { println!("{} is greater", a); } else if a < b { println!("{} is greater", b); } else { println!("Both are equal: {}", a); } } -> Option C

5. Quick Check:

   Use PartialOrd + Display for comparison and printing [OK]

Hint: Use T: PartialOrd + Display to compare and print generics [OK]

Common Mistakes:

• Omitting one of the required trait bounds
• Not handling equality case
• Using incorrect comparison operators