Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::manual_strip: stripping a prefix manually

What it meansSuggests using strip_{prefix,suffix} over str::{starts,ends}_with and slicing using the pattern’s length. It is in the complexity group and warns by default.

What you see

warning: stripping a prefix manually
 --> main.rs:5:16
  |
5 |     assert_eq!(s["hello, ".len()..].to_uppercase(), "WORLD!");
  |                ^^^^^^^^^^^^^^^^^^^^
  |
note: the prefix was tested here
 --> main.rs:4:1
  |
4 | if s.starts_with("hello, ") {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#manual_strip
  = note: `#[warn(clippy::manual_strip)]` on by default
help: try using the `strip_prefix` method
  |
4 ~ if let Some(<stripped>) = s.strip_prefix("hello, ") {
5 ~     assert_eq!(<stripped>.to_uppercase(), "WORLD!");
  |

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

Using str:strip_{prefix,suffix} is safer and may have better performance as there is no slicing which may panic and the compiler does not need to insert this panic code. It is also sometimes more readable as it removes the need for duplicating or storing the pattern used by str::{starts,ends}_with and in the slicing.

Code that does something simple in a complicated way. Warns by default.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

let s = "hello, world!";
if s.starts_with("hello, ") {
    assert_eq!(s["hello, ".len()..].to_uppercase(), "WORLD!");
}

Write this instead

let s = "hello, world!";
if let Some(end) = s.strip_prefix("hello, ") {
    assert_eq!(end.to_uppercase(), "WORLD!");
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
manual_strip = "allow"

One run

cargo clippy -- -A clippy::manual-strip

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.

Settings in clippy.toml

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Complexitygroup

warndefault level

yesfixed by cargo clippy --fix

1.48.0added in Rust

Clippy's own documentation

What it does

Suggests using strip_{prefix,suffix} over str::{starts,ends}_with and slicing using the pattern’s length.

Why is this bad?

Using str:strip_{prefix,suffix} is safer and may have better performance as there is no slicing which may panic and the compiler does not need to insert this panic code. It is also sometimes more readable as it removes the need for duplicating or storing the pattern used by str::{starts,ends}_with and in the slicing.

Example

let s = "hello, world!";
if s.starts_with("hello, ") {
    assert_eq!(s["hello, ".len()..].to_uppercase(), "WORLD!");
}

Use instead:

let s = "hello, world!";
if let Some(end) = s.strip_prefix("hello, ") {
    assert_eq!(end.to_uppercase(), "WORLD!");
}

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

(default: current version)

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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