If you can answer these without flipping back, you've internalised more Rust than you think.
These are twenty multiple-choice questions about the topics from the previous chapters: ownership and borrowing, Option and Result, lifetimes, traits, enums, modules.
Pick an answer to lock it in and reveal the explanation for every choice, not just the right one.
The distractor explanations show why the plausible alternatives do not fit.
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.const is for compile-time constants and is never mutable.var keyword.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.Result<T, E>. Option only models absence, not failure.as, From/Into, or try_from.How do you borrow a value immutably?
* dereferences a reference; it doesn't create one.& operator produces a shared, immutable reference..clone() makes an independent owned copy, not a reference to the original.Which statement about Rust's ownership is correct?
Copy, so they're duplicated on assignment, no manual management.Copy types, assignment moves ownership; the original binding can't be used afterwards.What's the difference between String and &str?
"...") are baked into the binary as &'static str, but String values are very much runtime.String can be made mutable with let mut, but the headline difference is ownership.String owns its bytes; &str is a view into bytes owned by somebody else (a String, a literal, etc.).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).match res { Ok(v) => v, Err(e) => return Err(e.into()) }..ok(), .err()), but ? is not one of them.? returns early, it doesn't unwind.Which of these creates an empty Vec?
Vec, not Vector.vec! macro with no elements gives you an empty vector.new keyword. Constructors are just associated functions: Vec::new().What happens if you call HashMap::get for a key that doesn't exist?
Rust prefers types over panics for expected absence.
Option<&V>, so missing keys are part of the API.get never panics. Indexing (map["key"]) does, but get returns an Option.entry API, not the default behaviour of get.entry().or_default() is opt-in.)How do you create an owned String from a string literal?
String::from("hello").new keyword in Rust.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.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?
match on the variants.String/str methods, not in control flow.regex crate. Not built into the language.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 says nothing about where the memory lives, only how long.'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.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?
pub opens up cross-module visibility; you can also scope it with pub(crate) or pub(super).global keyword.statics still default to private; they need pub static to escape the module.extern is for FFI bindings, not module visibility.Which of these does not create a reference?
* is the dereference operator.
AsRef::as_ref returns a reference.*x dereferences x and (for Copy types) gives you the value. r is a value, not a reference.