Flip order of s[a..b].as_bytes()

[A-Walrus]

What it does

Suggests replacing slicing a str followed by as_bytes with the opposite order

s[a..b].as_bytes()

with

&s.as_bytes()[a..b]

Advantage

• Removes unnecessary utf8 indexing check
• won't panic if a and b are not on utf8 boundaries

Drawbacks

Technically possible someone relied on it panicking, but find that unlikely.

Example

let s = "Hello World";
let bytes = s[1..3].as_bytes();

Could be written as:

let s = "Hello World";
let bytes = &s.as_bytes()[1..3];

[A-Walrus]

I'd like to work on this but I'm new to the codebase and might need some guidance.
Want to make sure I'm on the right track... this lint should implement the LateLintPass check_expr method right?

[y21]

This seems like it would need a LateLintPass, since you need to know that the type of the expression being indexed is str (type information are only available in late lint passes), and check_expr sounds right to me. I would implement the check_expr method, check if the expression being checked is a method call to str::as_bytes and the receiver of the call is an index expression, with the indexed type being str. The clippy documentation (specifically "common tools") is probably helpful here. Good luck!

[wowinter13]

@rustbot claim