If you can answer these without flipping back, you've internalised more Rust than you think.
Twenty multiple-choice questions covering the ground we've walked
together: ownership and borrowing, Option and Result, lifetimes,
traits, enums, modules. Pick an answer to lock it in and reveal the
explanations — not just for the right answer, but for every
distractor too, because the wrong answers are usually where the
learning lives.
No grade, no time limit, no record kept. Skim the explanations even for the ones you got right — sometimes the why is more interesting than the what.
What happens when you pass a String to a function in Rust?
Think about ownership rules.
String by value transfers ownership; the caller can't use it again unless the function returns it.&String (or &str). Bare String means a move.Which of these creates a mutable variable in Rust?
var keyword.const is for compile-time constants and is never mutable.mut keyword opts a binding into mutability.What is Option<T> for?
Think about what other languages reach for null to express.
Some(value) when the value is present, None when it isn't. No nulls needed.as, From/Into, or try_from.Result<T, E>. Option only models absence, not failure.How do you borrow a value immutably?
* dereferences a reference; it doesn't create one.& operator produces a shared, immutable reference..borrow() is a RefCell method, not the general way to take a reference.Which statement about Rust's ownership is correct?
Copy, so they're duplicated on assignment, no manual management.SharedPtr is a C++ thing. Rust uses Rc<T> (single-threaded) or Arc<T> (across threads).Rc/Arc only for shared handles).What's the difference between String and &str?
String owns its bytes; &str is a view into bytes owned by somebody else (a String, a literal, etc.).String can be made mutable with let mut, but the headline difference is ownership."...") are baked into the binary as &'static str, but String values are very much runtime.What does #[derive(Debug)] do?
Debug is implemented you can format the value with {:?} or {:#?}.#[warn(...)], #[deny(...)]).What does the ? operator do on a Result<T, E>?
It collapses a very common match pattern.
.ok(), .err()), but ? is not one of them.match res { Ok(v) => v, Err(e) => return Err(e.into()) }.? only operates on Result (and Option).? returns early, it doesn't unwind.Which of these creates an empty Vec?
new keyword. Constructors are just associated functions: Vec::new().Vec, not Vector.vec! macro with no elements gives you an empty vector.What happens if you call HashMap::get for a key that doesn't exist?
Rust prefers types over panics for expected absence.
get never panics. Indexing (map["key"]) does, but get returns an Option.entry API, not the default behaviour of get.Option<&V>, so missing keys are part of the API.entry().or_default() is opt-in.)How do you create an owned String from a string literal?
to_owned() clones the &str into a heap-allocated String. String::from("hello") and "hello".to_string() also work.new keyword in Rust.to_string(), from the ToString trait.String::from("hello").Which of these is true of Rust enums?
Rust enums are closer to algebraic data types than to C-style enums.
Enum trait.Option, Result, and many parser ASTs are enums.match instead, and == only works if the enum derives PartialEq.What does the lifetime in this signature say?
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str
'a is the shorter of x's and y's lifetimes.
'a says "at least as long as", not "strictly less than".'a as the shorter of the two and unifies via subtyping.'a is the intersection of the two input lifetimes, and the return reference is valid for that whole window.'a adds.What does .clone() typically do?
String, clone() allocates fresh memory and copies the contents in.Clone impls are deep; they duplicate owned heap data so the new value is fully independent.&value. .clone() creates a new value.clone() produces a second value alongside the original.What is match primarily for?
match on the variants.regex crate. Not built into the language.String/str methods, not in control flow.When is a value's drop method called?
Same answer that explains why Rust doesn't need a garbage collector.
Drop::drop directly. Use std::mem::drop if you need to drop early.drop call at the end of the value's scope.drop() is one way to force early dropping, but normal end-of-scope drops happen automatically.What does the 'static lifetime mean?
'static is about lifetime, not mutability. A static mut is 'static and mutable (but unsafe to touch).Sized trait, which is unrelated.'static says nothing about where the memory lives, only how long.'static references are either to data baked into the binary (string literals, const data) or to leaked heap allocations.What is a trait in Rust?
Send, Sync, Copy, …) are a kind of trait, but traits in general aren't about memory management.How do you make an item visible from outside its module?
global keyword.statics still default to private; they need pub static to escape the module.extern is for FFI bindings, not module visibility.pub opens up cross-module visibility; you can also scope it with pub(crate) or pub(super).Which of these does not create a reference?
* is the dereference operator.
*x dereferences x and (for Copy types) gives you the value. r is a value, not a reference.AsRef::as_ref returns a reference.