Data structures

Initialize an empty growable collection for subsequent element insertion

Building a list incrementally in a loop, returning an empty collection from a function, starting an accumulator for fold/reduce operations

Rust standard library, systems programming, CLI tools, web services, embedded Rust

Vec::new()

Vec::new()

none (fully fixed)

• vec.push()
• vec.extend()
• vec.iter()
• for loops
• collect()

• vec![] macro (more idiomatic for inline)
• Vec::with_capacity(n) when approximate size known

Forgetting type annotation when inference fails (e.g., let v = Vec::new(); without later use), using Vec::new() when capacity is predictable (wastes reallocations)

vec![] macro — identical runtime behavior, preferred for inline initialization; Vec::with_capacity(n) — pre-allocates memory, use when size estimate exists

Coming from C++: std::vector() may allocate; Vec::new() is guaranteed zero-allocation until first push. Coming from Python: [] creates list immediately; Vec::new() defers allocation entirely.

• vec![]
• Vec::with_capacity
• Vec::from
• Vec::into_iter
• Vec::push

Zero-cost until first element inserted. Type must be inferrable from subsequent usage or explicitly annotated (e.g., Vec::<i32>::new()). Prefer vec![] for readability when initializing inline.

Zero-cost abstraction for empty vector creation; enables type-safe incremental building

Empty vec, zero alloc, push to grow

Vec::with_capacity(n) when approximate final size is known