IntoIterator & Owned vs Borrowed

IntoIterator converts into iterator. Three modes: iter() shared borrow, iter_mut() mutable borrow, into_iter() owned consume.

Recipe

fn main() {
    let v = vec![1, 2, 3];
    for x in &v { print!("{x} "); }
    for x in v.into_iter() { print!("{x} "); }
}

When to reach for this: Choosing whether for moves, borrows, or mutates elements.

Working Example

fn total_borrowed(v: &Vec<i32>) -> i32 {
    v.iter().sum()
}
fn total_owned(v: Vec<i32>) -> i32 {
    v.into_iter().sum()
}
 
fn main() {
    let data = vec![1, 2, 3];
    println!("{} {}", total_borrowed(&data), data.len());
    println!("{}", total_owned(data));
}

What this demonstrates:

iter keeps data usable after sum
into_iter consumes data
API signature documents ownership

Deep Dive

for x in collection calls into_iter on collection. &collection uses IntoIterator for &Vec yielding &T.

Arrays [T; N] implement IntoIterator copying elements if Copy.

Gotchas

for x in vec moves vec - Cannot use after. Fix: for x in &vec.
into_iter on &mut - Yields &mut T.
Collect after into_iter - Source consumed.
Iterator Item vs IntoIterator - Different associated types per impl.
Generic fn + for - Use impl IntoIterator or generic I: IntoIterator.

Alternatives

Alternative	Use When	Don't Use When
Index loop	Need index + neighbors	Idiomatic for each
`drain`	Remove subset	Full consume
`as_slice` iteration	Explicit slice	`iter()` same

FAQs

three forms table?

iter/&T, iter_mut/&mut T, into_iter/T owned.

for moves?

for x in v moves if v not ref.

IntoIterator bound?

fn f<I: IntoIterator<Item=T>>(i: I).

array into_iter?

Moves elements if not Copy.

str into_iter?

chars(), bytes(), lines().

Option into_iter?

0 or 1 item.

Result into_iter?

1 item Ok or Err as single element patterns rare.

extend from iter?

vec.extend(iter).

cloned()?

Iterator over clones of &T.

copied()?

For Copy types from &T.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Recipe

fn main() {
    let v = vec![1, 2, 3];
    for x in &v { print!("{x} "); }
    for x in v.into_iter() { print!("{x} "); }
}

When to reach for this: Choosing whether for moves, borrows, or mutates elements.

Working Example

fn total_borrowed(v: &Vec<i32>) -> i32 {
    v.iter().sum()
}
fn total_owned(v: Vec<i32>) -> i32 {
    v.into_iter().sum()
}
 
fn main() {
    let data = vec![1, 2, 3];
    println!("{} {}", total_borrowed(&data), data.len());
    println!("{}", total_owned(data));
}

What this demonstrates:

iter keeps data usable after sum
into_iter consumes data
API signature documents ownership

Deep Dive

for x in collection calls into_iter on collection. &collection uses IntoIterator for &Vec yielding &T.

Arrays [T; N] implement IntoIterator copying elements if Copy.

Gotchas

for x in vec moves vec - Cannot use after. Fix: for x in &vec.
into_iter on &mut - Yields &mut T.
Collect after into_iter - Source consumed.
Iterator Item vs IntoIterator - Different associated types per impl.
Generic fn + for - Use impl IntoIterator or generic I: IntoIterator.

Alternatives

Alternative	Use When	Don't Use When
Index loop	Need index + neighbors	Idiomatic for each
`drain`	Remove subset	Full consume
`as_slice` iteration	Explicit slice	`iter()` same

FAQs

three forms table?

iter/&T, iter_mut/&mut T, into_iter/T owned.

for moves?

for x in v moves if v not ref.

IntoIterator bound?

fn f<I: IntoIterator<Item=T>>(i: I).

array into_iter?

Moves elements if not Copy.

str into_iter?

chars(), bytes(), lines().

Option into_iter?

0 or 1 item.

Result into_iter?

1 item Ok or Err as single element patterns rare.

extend from iter?

vec.extend(iter).

cloned()?

Iterator over clones of &T.

copied()?

For Copy types from &T.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

IntoIterator & Owned vs Borrowed

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IntoIterator & Owned vs Borrowed