Rust Programming

Question 1

What does let declare when mut is left off?

Accepted Answer

An immutable binding: assigning to it again is a compile error. Add mut to allow reassignment.

Question 2

What is shadowing?

Accepted Answer

Declaring a new variable with let using an existing name; the new binding hides the old one and may have a different type.

Question 3

Name Rust's integer types and the default for an unsuffixed literal.

Accepted Answer

Signed i8, i16, i32, i64, i128, unsigned u8 through u128, plus pointer-sized isize and usize. An unsuffixed integer literal defaults to i32.

Question 4

What is the unit type ()?

Accepted Answer

The empty tuple: the type of expressions that produce no value, such as a function with no return type or a block ending in a semicolon.

Question 5

String vs &str

Accepted Answer

String is an owned, growable, heap-allocated UTF-8 buffer. &str is a borrowed view (pointer plus length) into UTF-8 text, such as a literal or a slice of a String.

Question 6

State the three ownership rules.

Accepted Answer

Each value has exactly one owner; there is only one owner at a time; when the owner goes out of scope the value is dropped.

Question 7

What is a move?

Accepted Answer

Assigning or passing a non-Copy value transfers ownership; the original binding becomes invalid and using it is a compile error.

Question 8

Copy vs Clone

Accepted Answer

Clone is an explicit, possibly expensive duplicate made with .clone(). Copy is an implicit bitwise copy for simple types and is a marker subtrait of Clone.

Question 9

What does borrowing mean?

Accepted Answer

Taking a reference (&T or &mut T) to a value without taking ownership. The owner keeps the value and the reference must not outlive it.

Question 10

State the aliasing rule for references.

Accepted Answer

At any moment a value may have either any number of shared references &T or exactly one mutable reference &mut T, never both.

Question 11

What is a slice?

Accepted Answer

A reference to a contiguous range of a collection, written &[T] or &str, storing a pointer and a length.

Question 12

What is a lifetime?

Accepted Answer

A compile-time name for the scope during which a reference is valid. Annotations such as 'a relate the lifetimes of inputs and outputs so the borrow checker can verify them.

Question 13

When can lifetime annotations be omitted?

Accepted Answer

Under the elision rules: each reference parameter gets its own lifetime; if there is exactly one input lifetime it is assigned to all outputs; if there is a &self or &mut self, its lifetime goes to the outputs.

Question 14

What does 'static mean?

Accepted Answer

The lifetime of the whole program. String literals are &'static str, and a T: 'static bound means the type holds no non-static references (owned data qualifies).

Question 15

How do you define a struct and create an instance?

Accepted Answer

struct Name { field: Type }, then Name { field: value }. Field-init shorthand fills a field from a same-named variable.

Question 16

What does struct update syntax ..other do?

Accepted Answer

Fills every field you did not list from another instance of the same struct.

Question 17

What is a tuple struct?

Accepted Answer

A struct with unnamed positional fields, such as struct Meters(f64);, accessed with .0, .1 and so on.

Question 18

What is an enum in Rust?

Accepted Answer

A type with a fixed set of variants, each of which may carry data of a different shape (a tagged union).

Question 19

What is Option<T>?

Accepted Answer

An enum with variants Some(T) and None, Rust's replacement for null.

Question 20

What is Result<T, E>?

Accepted Answer

An enum with Ok(T) for success and Err(E) for failure, used for recoverable errors instead of exceptions.

Question 21

What happens if a match misses a variant?

Accepted Answer

Compile error. A match must be exhaustive: every possible value is covered, or a wildcard _ arm ends it.

Question 22

When would you use if let instead of match?

Accepted Answer

When you care about one pattern only. if let is sugar for a match with a single arm and an optional else.

Question 23

What does let else do?

Accepted Answer

Binds a refutable pattern, or runs the else block which must diverge (return, break, continue or panic). Stable since Rust 1.65.

Question 24

What does the ? operator do?

Accepted Answer

On a Result, returns early with Err(e.into()) when the value is an error, otherwise unwraps the Ok. On an Option it returns None early.

Question 25

unwrap vs expect

Accepted Answer

Both return the inner value or panic; expect(msg) prints your message in the panic, which makes the failure far easier to diagnose.

Question 26

How do you supply a fallback for an Option without panicking?

Accepted Answer

unwrap_or(v) for a ready value, unwrap_or_else(|| compute()) for one computed only when needed, unwrap_or_default() for the type's Default.

Question 27

map vs and_then on Option and Result

Accepted Answer

map transforms the inner value with a plain function. and_then chains a function that itself returns an Option or Result, flattening the nesting.

Question 28

When should code panic instead of returning a Result?

Accepted Answer

Panic for bugs and broken invariants in code you control; return Result for expected failures such as a missing file, bad input or a network error.

Question 29

What is Box<dyn Error> used for?

Accepted Answer

A heap-allocated trait object that can hold any error implementing std::error::Error, handy in applications that only need to propagate and print errors.

Question 30

How does ? convert one error type into another?

Accepted Answer

Through the From trait: ? calls From::from on the error, so implementing From<io::Error> for MyError lets ? convert automatically.