UTF-8 & Indexing

Rust str is UTF-8. Indexing s[i] is byte-based and panics on invalid char boundaries.

Recipe

fn safe_prefix(s: &str, max_bytes: usize) -> &str {
    if s.len() <= max_bytes { return s; }
    let mut end = max_bytes;
    while !s.is_char_boundary(end) { end -= 1; }
    &s[..end]
}

When to reach for this: Truncating UI strings, parsing protocols, any substring by length.

Working Example

fn main() {
    let s = "Hello 🦀";
    // let bad = &s[0..7]; // may panic
    for (i, ch) in s.char_indices() {
        println!("{i}: {ch}");
    }
}

What this demonstrates:

char_indices gives byte index at char boundary
Emoji uses 4 bytes in UTF-8
Safe slicing only at is_char_boundary

Deep Dive

len() returns bytes. Use chars(), bytes(), or graphemes (unicode-segmentation crate) for user-perceived characters.

Gotchas

s[n] char access - Not available. Fix: s.chars().nth(n).
Assume 1 byte per char - Wrong for UTF-8. Fix: Iterate chars.
Truncation mid-codepoint - Invalid str if forced. Fix: is_char_boundary.
OsString not UTF-8 - Platform bytes. Fix: Separate type.
Byte index from user - Dangerous. Fix: Char/grapheme index API.

Alternatives

Alternative	Use When	Don't Use When
`chars().nth`	Char by index	Need O(1) random char
`unicode-segmentation`	User graphemes	ASCII-only internal
`Vec<char>`	Repeated char index	Memory overhead ok

FAQs

why no s[i]?

O(1) char index impossible UTF-8.

is_char_boundary?

Check before slice at byte index.

chars nth O(n)?

Yes linear scan.

encode utf16?

encode_utf16 iterator.

from_utf8?

Validate bytes to &str Result.

lossy?

String::from_utf8_lossy replacement char.

str ref lifetime?

Borrowed from String or static.

ascii fast path?

is_ascii check optimize.

serde str?

UTF-8 JSON strings.

clippy?

indexing slices warns on str sometimes.

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+.

Revisado por Chris St. John·Última actualización: 16 jul 2026

Was this article helpful?

Rust Utf 8 Strings

UTF-8 & Indexing

Rust str is UTF-8. Indexing s[i] is byte-based and panics on invalid char boundaries.

Recipe

fn safe_prefix(s: &str, max_bytes: usize) -> &str {
    if s.len() <= max_bytes { return s; }
    let mut end = max_bytes;
    while !s.is_char_boundary(end) { end -= 1; }
    &s[..end]
}

When to reach for this: Truncating UI strings, parsing protocols, any substring by length.

Working Example

fn main() {
    let s = "Hello 🦀";
    // let bad = &s[0..7]; // may panic
    for (i, ch) in s.char_indices() {
        println!("{i}: {ch}");
    }
}

What this demonstrates:

char_indices gives byte index at char boundary
Emoji uses 4 bytes in UTF-8
Safe slicing only at is_char_boundary

Deep Dive

len() returns bytes. Use chars(), bytes(), or graphemes (unicode-segmentation crate) for user-perceived characters.

Gotchas

s[n] char access - Not available. Fix: s.chars().nth(n).
Assume 1 byte per char - Wrong for UTF-8. Fix: Iterate chars.
Truncation mid-codepoint - Invalid str if forced. Fix: is_char_boundary.
OsString not UTF-8 - Platform bytes. Fix: Separate type.
Byte index from user - Dangerous. Fix: Char/grapheme index API.

Alternatives

Alternative	Use When	Don't Use When
`chars().nth`	Char by index	Need O(1) random char
`unicode-segmentation`	User graphemes	ASCII-only internal
`Vec<char>`	Repeated char index	Memory overhead ok

FAQs

why no s[i]?

O(1) char index impossible UTF-8.

is_char_boundary?

Check before slice at byte index.

chars nth O(n)?

Yes linear scan.

encode utf16?

encode_utf16 iterator.

from_utf8?

Validate bytes to &str Result.

lossy?

String::from_utf8_lossy replacement char.

str ref lifetime?

Borrowed from String or static.

ascii fast path?

is_ascii check optimize.

serde str?

UTF-8 JSON strings.

clippy?

indexing slices warns on str sometimes.

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+.

Revisado por Chris St. John·Última actualización: 16 jul 2026

Was this article helpful?

UTF-8 & Indexing

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UTF-8 & Indexing