Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::assert_is_empty: Checks assertions that only test whether a supported value is empty. The lint handles assert! and debug_assert! calls on strings, slices, arrays, and Vec

What it meansChecks assertions that only test whether a supported value is empty. The lint handles assert! and debug_assert! calls on strings, slices, arrays, and Vec. It is in the pedantic group and off by default.

Does it matter?

A boolean assertion only reports that the emptiness check failed. It does not show what the asserted value contained. In CI or another remote test service, the failure output tells you that the value was unexpectedly empty or non-empty, but not which values were present. The next step is often to reproduce the failure locally, add temporary logging, or change the test so it exposes the value. That extra investigation can be much slower than fixing the problem from the original CI failure. The emptiness check also commonly appears before a deeper contents assertion: assert!(!items.is_empty()); assert_eq!(items[0], "bar"); If the first assertion fails, the second assertion never runs, so the failure can hide the check that would have shown more useful context. Instead, compare the value with an empty value using assert_eq!, assert_ne!, debug_assert_eq!, or debug_assert_ne!. These macros print the asserted value on failure.

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

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

assert!(!items.is_empty());
assert_eq!(items[0], "bar");

Write this instead

assert!(items.is_empty());
assert!(!items.is_empty());

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]
assert_is_empty = "warn"

In code

#![warn(clippy::assert_is_empty)]

One run

cargo clippy -- -W clippy::assert-is-empty

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

yesfixed by cargo clippy --fix

1.98.0added in Rust

Clippy's own documentation

What it does

Checks assertions that only test whether a supported value is empty.

The lint handles assert! and debug_assert! calls on strings, slices, arrays, and Vec.

Why is this bad?

A boolean assertion only reports that the emptiness check failed. It does not show what the asserted value contained.

In CI or another remote test service, the failure output tells you that the value was unexpectedly empty or non-empty, but not which values were present. The next step is often to reproduce the failure locally, add temporary logging, or change the test so it exposes the value. That extra investigation can be much slower than fixing the problem from the original CI failure.

The emptiness check also commonly appears before a deeper contents assertion:

assert!(!items.is_empty());
assert_eq!(items[0], "bar");

If the first assertion fails, the second assertion never runs, so the failure can hide the check that would have shown more useful context.

Instead, compare the value with an empty value using assert_eq!, assert_ne!, debug_assert_eq!, or debug_assert_ne!. These macros print the asserted value on failure.

Known problems

Printing the asserted value can be undesirable outside tests, especially when the value may be very large or contain sensitive information. If the assertion failure should not reveal the value, keep the boolean assertion and allow this lint at that assertion.

Example

assert!(items.is_empty());
assert!(!items.is_empty());

Use instead:

assert_eq!(items, [] as [i32; 0]);
assert_ne!(items, [] as [i32; 0]);

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

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