Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::double_must_use: this function has a `#[must_use]` attribute with no message, but returns a type already considered as `#[must_use]`
What it meansChecks for a #[must_use] attribute without further information on functions and methods that return a type already considered as #[must_use]. It is in the style group and warns by default.
What you see
warning: this function has a `#[must_use]` attribute with no message, but returns a type already considered as `#[must_use]` --> main.rs:4:1 | 3 | #[must_use] | ----------- help: remove the attribute 4 | fn double_must_use() -> Result<(), ()> { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ | = note: alternatively, you may add an explicit reason to the `must_use` attribute = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#double_must_use = note: `#[warn(clippy::double_must_use)]` on by default
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?
The attribute isn’t needed. Not using the result will already be reported. Alternatively, one can add some text to the attribute to improve the lint message.
Code that works but is not idiomatic Rust. 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
#[must_use]
fn double_must_use() -> Result<(), ()> {
unimplemented!();
}
Turn it off, or on
Allow it where it does not fit your code, as narrowly as you can:
On one item
#[allow(clippy::double_must_use)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
double_must_use = "allow"One run
cargo clippy -- -A clippy::double-must-useThe [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.
Clippy's own documentation
What it does
Checks for a #[must_use] attribute without
further information on functions and methods that return a type already
considered as #[must_use].
Why is this bad?
The attribute isn’t needed. Not using the result will already be reported. Alternatively, one can add some text to the attribute to improve the lint message.
Examples
#[must_use]
fn double_must_use() -> Result<(), ()> {
unimplemented!();
}
Note
The compiler may consider a type as being indirectly #[must_use]. For
example, although Box<_> itself is not #[must_use], Box<T> will be
considered #[must_use] if T is.
From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.
Related lints
- needless_returnunneeded `return` statement
- new_without_defaultChecks for public types with a pub fn new() -> Self method and no implementation of Default
- len_zeroChecks for getting the length of something via .len() just to compare to zero, and suggests using .is_empty() where applicable
- disallowed_typesDenies the configured types in clippy.toml. Note: Even though this lint is warn-by-default, it will only trigger if types are defined in the clippy.toml file
- doc_lazy_continuationdoc quote line without `>` marker
- doc_overindented_list_itemsDetects overindented list items in doc comments where the continuation lines are indented more than necessary
- duplicate_underscore_argument`a` already exists, having another argument having almost the same name makes code comprehension and documentation more difficult
- enum_variant_namesvariant name ends with the enum's name
- err_expectcalled `.err().expect()` on a `Result` value
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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