Clippy lints / Style
Clippy lint · Style · checked with Clippy 0.1.99
clippy::partialeq_to_none: binary comparison to literal `Option::None`
What it meansChecks for binary comparisons to a literal Option::None. It is in the style group and warns by default.
What you see
warning: binary comparison to literal `Option::None` --> main.rs:4:8 | 4 | if f != None { "yay" } else { "nay" } | ^^^^^^^^^ help: use `Option::is_some()` instead: `f.is_some()` | = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#partialeq_to_none = note: `#[warn(clippy::partialeq_to_none)]` 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?
A programmer checking if some foo is None via a comparison foo == None is usually inspired from other programming languages (e.g. foo is None in Python). Checking if a value of type Option<T> is (not) equal to None in that way relies on T: PartialEq to do the comparison, which is unneeded.
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
fn foo(f: Option<u32>) -> &'static str {
if f != None { "yay" } else { "nay" }
}
Write this instead
fn foo(f: Option<u32>) -> &'static str {
if f.is_some() { "yay" } else { "nay" }
}
Turn it off, or on
Allow it where it does not fit your code, as narrowly as you can:
On one item
#[allow(clippy::partialeq_to_none)]
fn my_function() { /* ... */ }Whole crate (Cargo.toml)
[lints.clippy]
partialeq_to_none = "allow"One run
cargo clippy -- -A clippy::partialeq-to-noneThe [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 binary comparisons to a literal Option::None.
Why is this bad?
A programmer checking if some foo is None via a comparison foo == None
is usually inspired from other programming languages (e.g. foo is None
in Python).
Checking if a value of type Option<T> is (not) equal to None in that
way relies on T: PartialEq to do the comparison, which is unneeded.
Example
fn foo(f: Option<u32>) -> &'static str {
if f != None { "yay" } else { "nay" }
}
Use instead:
fn foo(f: Option<u32>) -> &'static str {
if f.is_some() { "yay" } else { "nay" }
}
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
- op_refChecks for arguments to == which have their address taken to satisfy a bound and suggests to dereference the other argument instead
- option_map_or_noneChecks for usage of _.map_or(None, _)
- owned_cowneedlessly owned Cow type
- print_literalliteral with an empty format string
- print_with_newlineThis lint warns when you use print!() with a format string that ends in a newline
- println_empty_stringempty string literal in `println!`
All 847 Clippy lints · Rust compiler errors · Cargo.toml checker
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