Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::same_length_and_capacity: Checks for usages of Vec::from_raw_parts and String::from_raw_parts where the same expression is used for the length and the capacity

What it meansChecks for usages of Vec::from_raw_parts and String::from_raw_parts where the same expression is used for the length and the capacity. It is in the pedantic group and off by default.

Does it matter?

If the same expression is being passed for the length and capacity, it is most likely a semantic error. In the case of a Vec, for example, the only way to end up with one that has the same length and capacity is by going through a boxed slice, e.g. Box::from(some_vec), which shrinks the capacity to match the length.

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

#![feature(vec_into_raw_parts)]
let mut original: Vec::<i32> = Vec::with_capacity(20);
original.extend([1, 2, 3, 4, 5]);

let (ptr, mut len, cap) = original.into_raw_parts();

// I will add three more integers:
unsafe {
   let ptr = ptr as *mut i32;

   for i in 6..9 {
       *ptr.add(i - 1) = i as i32;
       len += 1;
   }
}

// But I forgot the capacity was separate from the length:
let reconstructed = unsafe { Vec::from_raw_parts(ptr, len, len) };

Write this instead

#![feature(vec_into_raw_parts)]
let mut original: Vec::<i32> = Vec::with_capacity(20);
original.extend([1, 2, 3, 4, 5]);

let (ptr, mut len, cap) = original.into_raw_parts();

// I will add three more integers:
unsafe {
   let ptr = ptr as *mut i32;

   for i in 6..9 {
       *ptr.add(i - 1) = i as i32;
       len += 1;
   }
}

// This time, leverage the previously saved capacity:
let reconstructed = unsafe { Vec::from_raw_parts(ptr, len, cap) };

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
same_length_and_capacity = "warn"

In code

#![warn(clippy::same_length_and_capacity)]

One run

cargo clippy -- -W clippy::same-length-and-capacity

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.

Pedanticgroup

allowdefault level

nofixed by cargo clippy --fix

1.94.0added in Rust

Clippy's own documentation

What it does

Checks for usages of Vec::from_raw_parts and String::from_raw_parts where the same expression is used for the length and the capacity.

Why is this bad?

If the same expression is being passed for the length and capacity, it is most likely a semantic error. In the case of a Vec, for example, the only way to end up with one that has the same length and capacity is by going through a boxed slice, e.g. Box::from(some_vec), which shrinks the capacity to match the length.

Example

#![feature(vec_into_raw_parts)]
let mut original: Vec::<i32> = Vec::with_capacity(20);
original.extend([1, 2, 3, 4, 5]);

let (ptr, mut len, cap) = original.into_raw_parts();

// I will add three more integers:
unsafe {
   let ptr = ptr as *mut i32;

   for i in 6..9 {
       *ptr.add(i - 1) = i as i32;
       len += 1;
   }
}

// But I forgot the capacity was separate from the length:
let reconstructed = unsafe { Vec::from_raw_parts(ptr, len, len) };

Use instead:

#![feature(vec_into_raw_parts)]
let mut original: Vec::<i32> = Vec::with_capacity(20);
original.extend([1, 2, 3, 4, 5]);

let (ptr, mut len, cap) = original.into_raw_parts();

// I will add three more integers:
unsafe {
   let ptr = ptr as *mut i32;

   for i in 6..9 {
       *ptr.add(i - 1) = i as i32;
       len += 1;
   }
}

// This time, leverage the previously saved capacity:
let reconstructed = unsafe { Vec::from_raw_parts(ptr, len, cap) };

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

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