01 / Foundations
Strings, slices, and collections
Work with UTF-8 text and choose when collections should own or borrow data.
By Robert DeVore · Download Markdown
Text is not an array of letters
A String owns text. A str is a dynamically sized sequence of UTF-8 bytes, usually accessed through &str. Our parser will accept &str because it only reads a record. A caller can pass a string literal, a slice of an owned string, or other borrowed text without first allocating another string.
Run the example
cargo run --locked --example 05_text
fn main() {
let text = "café";
assert_eq!(text.len(), 5);
assert_eq!(text.chars().count(), 4);
assert_eq!(text.get(..3), Some("caf"));
assert_eq!(text.get(..4), None);
let mut rows = vec![String::from("INFO ok")];
rows.push("WARN retry".into());
let sizes: Vec<_> = rows.iter().map(|row| row.len()).collect();
assert_eq!(sizes, [7, 10]);
}
The final character in café occupies two UTF-8 bytes. len() reports bytes, while chars() iterates Unicode scalar values. Neither necessarily reports user-perceived grapheme clusters: combining marks and multi-scalar emoji make that distinction visible. A byte range must end at UTF-8 boundaries. get returns None for an invalid range; direct slicing can panic. There is no general constant-time “character at index” operation on UTF-8 text.
Choose a collection
Vec<T> owns a growable contiguous sequence. [T; N] has a fixed length as part of its type. &[T] borrows a sequence without requiring a particular owning container. An interface taking a slice is often more useful than one taking &Vec<T>.
A HashMap provides key-based lookup without a stable iteration order. A BTreeMap maintains keys in sorted order. If output must always have the same order, sort it or use an ordered collection. Our three severity counters do not need any map at all: three named fields make their meaning clear.
iter() borrows elements, iter_mut() allows mutation of elements under an exclusive collection borrow, and into_iter() consumes the value on which it is called. The receiver matters: consuming an owned vector yields its elements, while iterating a borrowed collection yields references. Look at the iterator's Item type when unsure.
Limit how much input you store
read_to_string is convenient when inputs are intentionally small. It is not a bounded-memory solution for arbitrary files. Likewise, “read one line at a time” can still allocate an enormous buffer if an input contains no newline. The capstone checks a byte limit as it reads each record, before the buffer can grow too large.
A vector's capacity is reserved storage, not initialized elements. Its length describes initialized values available to safe callers. Never treat spare capacity as initialized data. Changing the length with unsafe code requires proving that the new elements are valid and initialized. Our application does not need this technique.
Exercise
Collect only records beginning with WARN , using borrowed slices rather than new String values. Test with WARNING ignored to prove the separator matters. Then explain what must stay alive while your collected slices are used.
Solution and acceptance check
Use an iterator over the input records and filter with starts_with("WARN "). Collect &str items. Only WARN retry should match among INFO ok, WARN retry, and WARNING ignored. The backing input strings must remain valid and cannot be mutated in ways conflicting with those borrows. If the result must outlive the input, owning strings may be the correct design.
Sources: str, Vec, and collections.