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 (or &str). Bare String means a move.String by value transfers ownership; the caller can't use it again unless the function returns it.Which of these creates a mutable variable in Rust?
mut keyword opts a binding into mutability.var keyword.const is for compile-time constants and is never mutable.What is Option<T> for?
Think about what other languages reach for null to express.
as, From/Into, or try_from.Some(value) when the value is present, None when it isn't. No nulls needed.Result<T, E>. Option only models absence, not failure.How do you borrow a value immutably?
& operator produces a shared, immutable reference.* dereferences a reference; it doesn't create one..clone() makes an independent owned copy, not a reference to the original.Which statement about Rust's ownership is correct?
Copy types, assignment moves ownership; the original binding can't be used afterwards.Copy, so they're duplicated on assignment, no manual management.What's the difference between String and &str?
"...") are baked into the binary as &'static str, but String values are very much runtime.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.What does #[derive(Debug)] do?
#[warn(...)], #[deny(...)]).Debug is implemented you can format the value with {:?} or {:#?}.What does the ? operator do on a Result<T, E>?
It collapses a very common match pattern.
? only operates on Result (and Option).? returns early, it doesn't unwind.match res { Ok(v) => v, Err(e) => return Err(e.into()) }..ok(), .err()), but ? is not one of them.Which of these creates an empty Vec?
vec! macro with no elements gives you an empty vector.new keyword. Constructors are just associated functions: Vec::new().Vec, not 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.Option<&V>, so missing keys are part of the API.entry().or_default() is opt-in.)entry API, not the default behaviour of get.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.to_string(), from the ToString trait.new keyword in Rust.String::from("hello").Which of these is true of Rust enums?
Rust enums are closer to algebraic data types than to C-style enums.
Option, Result, and many parser ASTs are enums.match instead, and == only works if the enum derives PartialEq.Enum trait.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 adds.'a as the shorter of the two and unifies via subtyping.'a is the intersection of the two input lifetimes, so the returned reference is tied to whichever input is dropped first and can't be used past it.What does .clone() typically do?
clone() produces a second value alongside the original.Clone impls are deep; they duplicate owned heap data so the new value is fully independent.&value. .clone() creates a new value.String, clone() allocates fresh memory and copies the contents in.What is match primarily for?
regex crate. Not built into the language.String/str methods, not in control flow.match on the variants.When is a value's drop method called?
Same answer that explains why Rust doesn't need a garbage collector.
drop call at the end of the value's scope.Drop::drop directly. Use std::mem::drop if you need to drop early.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 references are either to data baked into the binary (string literals, const data) or to leaked heap allocations.'static says nothing about where the memory lives, only how long.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?
extern is for FFI bindings, not module visibility.statics still default to private; they need pub static to escape the module.pub opens up cross-module visibility; you can also scope it with pub(crate) or pub(super).global keyword.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.