clippy::char_indices_as_byte_indices: Checks for usage of a character position yielded by .chars().enumerate() in a context where a byte index is expected, such as an argument to a specific str method or indexing into a str or String

Does it matter?

A character (more specifically, a Unicode scalar value) that is yielded by str::chars can take up multiple bytes, so a character position does not necessarily have the same byte index at which the character is stored. Thus, using the character position where a byte index is expected can unexpectedly return wrong values or panic when the string consists of multibyte characters. For example, the character a in äa is stored at byte index 2 but has the character position 1. Using the character position 1 to index into the string will lead to a panic as it is in the middle of the first character. Instead of .chars().enumerate(), the correct iterator to use is .char_indices(), which yields byte indices. This pattern is technically fine if the strings are known to only use the ASCII subset, though in those cases it would be better to use bytes() directly to make the intent clearer, but there is also no downside to just using .char_indices() directly and supporting non-ASCII strings. You may also want to read the chapter on strings in the Rust Book which goes into this in more detail.

Code that is almost certainly wrong. Denied by default: the build fails until you fix it or allow it.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

for (idx, c) in s.chars().enumerate() {
    let _ = s[idx..]; // ⚠️ Panics for strings consisting of multibyte characters
}

Write this instead

for (idx, c) in s.char_indices() {
    let _ = s[idx..];
}

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

#[allow(clippy::char_indices_as_byte_indices)]
fn my_function() { /* ... */ }

[lints.clippy]
char_indices_as_byte_indices = "allow"

cargo clippy -- -A clippy::char-indices-as-byte-indices

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.