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Showing content from https://doc.rust-lang.org/nightly/std/macro.vec.html below:

vec in std - Rust

macro_rules! vec {
    () => { ... };
    ($elem:expr; $n:expr) => { ... };
    ($($x:expr),+ $(,)?) => { ... };
}

Creates a Vec containing the arguments.

vec! allows Vecs to be defined with the same syntax as array expressions. There are two forms of this macro:

• Create a Vec containing a given list of elements:

let v = vec![1, 2, 3];
assert_eq!(v[0], 1);
assert_eq!(v[1], 2);
assert_eq!(v[2], 3);

• Create a Vec from a given element and size:

let v = vec![1; 3];
assert_eq!(v, [1, 1, 1]);

Note that unlike array expressions this syntax supports all elements which implement Clone and the number of elements doesn’t have to be a constant.

This will use clone to duplicate an expression, so one should be careful using this with types having a nonstandard Clone implementation. For example, vec![Rc::new(1); 5] will create a vector of five references to the same boxed integer value, not five references pointing to independently boxed integers.

Also, note that vec![expr; 0] is allowed, and produces an empty vector. This will still evaluate expr, however, and immediately drop the resulting value, so be mindful of side effects.

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