clippy::undocumented_unsafe_blocks: Checks for unsafe blocks and impls without a // SAFETY: comment explaining why the unsafe operations performed inside the block are safe. Note the comment must appear on the line(s) preceding the unsafe block with nothing appearing in between

foo(
    // SAFETY:
    // This is a valid safety comment
    unsafe { *x }
)

// SAFETY:
// This is not a valid safety comment
foo(
    /* SAFETY: Neither is this */ unsafe { *x },
);

[lints.clippy]
undocumented_unsafe_blocks = "warn"

#![warn(clippy::undocumented_unsafe_blocks)]

cargo clippy -- -W clippy::undocumented-unsafe-blocks

Clippy's own documentation

What it does

Checks for unsafe blocks and impls without a // SAFETY: comment explaining why the unsafe operations performed inside the block are safe.

Note the comment must appear on the line(s) preceding the unsafe block with nothing appearing in between. The following is ok:

foo(
    // SAFETY:
    // This is a valid safety comment
    unsafe { *x }
)

But neither of these are:

// SAFETY:
// This is not a valid safety comment
foo(
    /* SAFETY: Neither is this */ unsafe { *x },
);

Why restrict this?

Undocumented unsafe blocks and impls can make it difficult to read and maintain code. Writing out the safety justification may help in discovering unsoundness or bugs.

Example

use std::ptr::NonNull;
let a = &mut 42;

let ptr = unsafe { NonNull::new_unchecked(a) };

Use instead:

use std::ptr::NonNull;
let a = &mut 42;

// SAFETY: references are guaranteed to be non-null.
let ptr = unsafe { NonNull::new_unchecked(a) };

Configuration

• accept-comment-above-attributes: Whether to accept a safety comment to be placed above the attributes for the unsafe block

  (default: true)

• accept-comment-above-statement: Whether to accept a safety comment to be placed above the statement containing the unsafe block

  (default: true)

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

Related lints

• unwrap_usedChecks for .unwrap() or .unwrap_err() calls on Results and .unwrap() call on Options
• module_name_repetitionsDetects public item names that are prefixed or suffixed by the containing public module’s name
• expect_usedChecks for .expect() or .expect_err() calls on Results and .expect() call on Options
• tests_outside_test_moduleTriggers when a testing function (marked with the #[test] attribute) isn’t inside a testing module (marked with #[cfg(test)])
• todo`todo` should not be present in production code
• try_errreturning an `Err(_)` with the `?` operator
• unimplemented`unimplemented` should not be present in production code
• unnecessary_rest_patternunnecessary rest pattern (`..`)
• unnecessary_safety_commentstatement has unnecessary safety comment

All 847 Clippy lints · Rust compiler errors · Cargo.toml checker