Questions
Rust questions
Choose a question page, learn the concept, then test your understanding with a quiz.
What `let fields = fields;` Does in Rust
Learn what `let x = x;` means in Rust, how shadowing works, and why developers use `let fields = fields;` in real Rust code.
When Rust Closures Implement Fn, FnMut, and FnOnce
Learn when Rust closures implement Fn, FnMut, and FnOnce, how capture behavior affects each trait, and how to choose the right bound.
When to Use #[inline], #[inline(always)], and #[inline(never)] in Rust
Learn when to use Rust inline attributes, what they actually mean, and when #[inline], #[inline(always)], or #[inline(never)] help or hurt.
Why Capitalizing the First Letter of a String Is Different in Rust
Learn why Rust string capitalization is tricky, why &str cannot be indexed, and the simplest safe ways to uppercase the first letter.
Why Explicit Lifetimes Are Needed in Rust
Learn why Rust needs explicit lifetimes, when inference is enough, and when lifetime annotations are required for safe references.
Why Rust Avoids &String, &Vec, and &Box in Function Arguments
Learn why Rust functions usually take &str, slices, or plain references instead of &String, &Vec, and &Box, with examples and best practices.
Why Rust Cannot Store a Value and a Reference to It in the Same Struct
Learn why self-referential structs fail in Rust, what lifetime errors mean, and practical ways to model owned data safely.
Why Rust Executables Are So Large: Understanding Binary Size in Rust
Learn why Rust binaries can seem large, what gets included in a build, and how debug, release, stripping, and panic settings affect size.
Why Rust May Not Optimize Non-Aliasing Mutable References Yet
Learn why Rust mutable references imply no aliasing, why LLVM may still avoid some optimizations, and how this relates to UB and codegen.
Why You Can't Index a String in Rust and How to Access Characters Safely
Learn why Rust strings cannot be indexed by number and how to access characters safely using chars, bytes, and iterators.
Why `println!` Doesn’t Show in Rust Unit Tests
Learn why `println!` output is hidden in Rust unit tests, how test output capture works, and how to display debug prints when needed.
Why `println!` Uses an Exclamation Mark in Rust
Learn why `println!` ends with `!` in Rust, what macros are, and how they differ from functions with beginner-friendly examples.
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