clippy 0.0.41

A bunch of helpful lints to avoid common pitfalls in Rust failed to build clippy-0.0.41
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#rust-clippy Build Status Current Version License: MIT/Apache

A collection of lints to catch common mistakes and improve your Rust code.

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##Lints There are 120 lints included in this crate:

name default meaning
absurd_extreme_comparisons warn a comparison involving a maximum or minimum value involves a case that is always true or always false
approx_constant warn the approximate of a known float constant (in std::f64::consts or std::f32::consts) is found; suggests to use the constant
bad_bit_mask warn expressions of the form _ & mask == select that will only ever return true or false (because in the example select containing bits that mask doesn't have)
block_in_if_condition_expr warn braces can be eliminated in conditions that are expressions, e.g if { true } ...
block_in_if_condition_stmt warn avoid complex blocks in conditions, instead move the block higher and bind it with 'let'; e.g: if { let x = true; x } ...
bool_comparison warn comparing a variable to a boolean, e.g. if x == true
box_vec warn usage of Box<Vec<T>>, vector elements are already on the heap
boxed_local warn using Box where unnecessary
cast_possible_truncation allow casts that may cause truncation of the value, e.g x as u8 where x: u32, or x as i32 where x: f32
cast_possible_wrap allow casts that may cause wrapping around the value, e.g x as i32 where x: u32 and x > i32::MAX
cast_precision_loss allow casts that cause loss of precision, e.g x as f32 where x: u64
cast_sign_loss allow casts from signed types to unsigned types, e.g x as u32 where x: i32
char_lit_as_u8 warn Casting a character literal to u8
chars_next_cmp warn using .chars().next() to check if a string starts with a char
clone_double_ref warn using clone on &&T
clone_on_copy warn using clone on a Copy type
cmp_nan deny comparisons to NAN (which will always return false, which is probably not intended)
cmp_owned warn creating owned instances for comparing with others, e.g. x == "foo".to_string()
collapsible_if warn two nested if-expressions can be collapsed into one, e.g. if x { if y { foo() } } can be written as if x && y { foo() } and an else { if .. } expression can be collapsed to else if`
cyclomatic_complexity warn finds functions that should be split up into multiple functions
deprecated_semver warn Warn on #[deprecated(since = "x")] where x is not semver
derive_hash_not_eq warn deriving Hash but implementing PartialEq explicitly
drop_ref warn call to std::mem::drop with a reference instead of an owned value, which will not call the Drop::drop method on the underlying value
duplicate_underscore_argument warn Function arguments having names which only differ by an underscore
empty_loop warn empty loop {} detected
enum_glob_use allow finds use items that import all variants of an enum
enum_variant_names warn finds enums where all variants share a prefix/postfix
eq_op warn equal operands on both sides of a comparison or bitwise combination (e.g. x == x)
expl_impl_clone_on_copy warn implementing Clone explicitly on Copy types
explicit_counter_loop warn for-looping with an explicit counter when _.enumerate() would do
explicit_iter_loop warn for-looping over _.iter() or _.iter_mut() when &_ or &mut _ would do
extend_from_slice warn .extend_from_slice(_) is a faster way to extend a Vec by a slice
filter_next warn using filter(p).next(), which is more succinctly expressed as .find(p)
float_cmp warn using == or != on float values (as floating-point operations usually involve rounding errors, it is always better to check for approximate equality within small bounds)
for_kv_map warn looping on a map using iter when keys or values would do
for_loop_over_option warn for-looping over an Option, which is more clearly expressed as an if let
for_loop_over_result warn for-looping over a Result, which is more clearly expressed as an if let
identity_op warn using identity operations, e.g. x + 0 or y / 1
if_same_then_else warn if with the same then and else blocks
ifs_same_cond warn consecutive ifs with the same condition
ineffective_bit_mask warn expressions where a bit mask will be rendered useless by a comparison, e.g. `(x
inline_always warn #[inline(always)] is a bad idea in most cases
invalid_regex deny finds invalid regular expressions in Regex::new(_) invocations
items_after_statements warn finds blocks where an item comes after a statement
iter_next_loop warn for-looping over which is probably not intended
len_without_is_empty warn traits and impls that have .len() but not .is_empty()
len_zero warn checking .len() == 0 or .len() > 0 (or similar) when .is_empty() could be used instead
let_and_return warn creating a let-binding and then immediately returning it like let x = expr; x at the end of a block
let_unit_value warn creating a let binding to a value of unit type, which usually can't be used afterwards
linkedlist warn usage of LinkedList, usually a vector is faster, or a more specialized data structure like a VecDeque
map_clone warn using `.map(
map_entry warn use of contains_key followed by insert on a HashMap or BTreeMap
match_bool warn a match on boolean expression; recommends if..else block instead
match_overlapping_arm warn a match has overlapping arms
match_ref_pats warn a match or if let has all arms prefixed with &; the match expression can be dereferenced instead
match_same_arms warn match with identical arm bodies
min_max warn min(_, max(_, _)) (or vice versa) with bounds clamping the result to a constant
modulo_one warn taking a number modulo 1, which always returns 0
mut_mut allow usage of double-mut refs, e.g. &mut &mut ... (either copy'n'paste error, or shows a fundamental misunderstanding of references)
mutex_atomic warn using a Mutex where an atomic value could be used instead
mutex_integer allow using a Mutex for an integer type
needless_bool warn if-statements with plain booleans in the then- and else-clause, e.g. if p { true } else { false }
needless_lifetimes warn using explicit lifetimes for references in function arguments when elision rules would allow omitting them
needless_range_loop warn for-looping over a range of indices where an iterator over items would do
needless_return warn using a return statement like return expr; where an expression would suffice
needless_update warn using { ..base } when there are no missing fields
no_effect warn statements with no effect
non_ascii_literal allow using any literal non-ASCII chars in a string literal; suggests using the \u escape instead
nonsensical_open_options warn nonsensical combination of options for opening a file
ok_expect warn using ok().expect(), which gives worse error messages than calling expect directly on the Result
option_map_unwrap_or warn using, which is more succinctly expressed as map_or(a, f)
option_map_unwrap_or_else warn using, which is more succinctly expressed as map_or_else(g, f)
option_unwrap_used allow using Option.unwrap(), which should at least get a better message using expect()
or_fun_call warn using any *or method when the *or_else would do
out_of_bounds_indexing deny out of bound constant indexing
panic_params warn missing parameters in panic!
precedence warn catches operations where precedence may be unclear. See the wiki for a list of cases caught
print_stdout allow printing on stdout
ptr_arg warn fn arguments of the type &Vec<...> or &String, suggesting to use &[...] or &str instead, respectively
range_step_by_zero warn using Range::step_by(0), which produces an infinite iterator
range_zip_with_len warn zipping iterator with a range when enumerate() would do
redundant_closure warn using redundant closures, i.e. `
redundant_pattern warn using name @ _ in a pattern
regex_macro warn finds use of regex!(_), suggests Regex::new(_) instead
result_unwrap_used allow using Result.unwrap(), which might be better handled
reverse_range_loop warn Iterating over an empty range, such as 10..0 or 5..5
search_is_some warn using an iterator search followed by is_some(), which is more succinctly expressed as a call to any()
shadow_reuse allow rebinding a name to an expression that re-uses the original value, e.g. let x = x + 1
shadow_same allow rebinding a name to itself, e.g. let mut x = &mut x
shadow_unrelated allow The name is re-bound without even using the original value
should_implement_trait warn defining a method that should be implementing a std trait
single_match warn a match statement with a single nontrivial arm (i.e, where the other arm is _ => {}) is used; recommends if let instead
single_match_else allow a match statement with a two arms where the second arm's pattern is a wildcard; recommends if let instead
str_to_string warn using to_string() on a str, which should be to_owned()
string_add allow using x + .. where x is a String; suggests using push_str() instead
string_add_assign allow using x = x + .. where x is a String; suggests using push_str() instead
string_lit_as_bytes warn calling as_bytes on a string literal; suggests using a byte string literal instead
string_to_string warn calling String::to_string which is inefficient
temporary_assignment warn assignments to temporaries
toplevel_ref_arg warn An entire binding was declared as ref, in a function argument (fn foo(ref x: Bar)), or a let statement (let ref x = foo()). In such cases, it is preferred to take references with &.
trivial_regex warn finds trivial regular expressions in Regex::new(_) invocations
type_complexity warn usage of very complex types; recommends factoring out parts into type definitions
unicode_not_nfc allow using a unicode literal not in NFC normal form (see for further information)
unit_cmp warn comparing unit values (which is always true or false, respectively)
unnecessary_mut_passed warn an argument is passed as a mutable reference although the function/method only demands an immutable reference
unneeded_field_pattern warn Struct fields are bound to a wildcard instead of using ..
unstable_as_mut_slice warn as_mut_slice is not stable and can be replaced by &mut v[..]see
unstable_as_slice warn as_slice is not stable and can be replaced by & v[..]see
unused_collect warn collect()ing an iterator without using the result; this is usually better written as a for loop
unused_lifetimes warn unused lifetimes in function definitions
use_debug allow use Debug-based formatting
used_underscore_binding warn using a binding which is prefixed with an underscore
useless_transmute warn transmutes that have the same to and from types
useless_vec warn useless vec!
while_let_loop warn loop { if let { ... } else break } can be written as a while let loop
while_let_on_iterator warn using a while-let loop instead of a for loop on an iterator
wrong_pub_self_convention allow defining a public method named with an established prefix (like "into_") that takes self with the wrong convention
wrong_self_convention warn defining a method named with an established prefix (like "into_") that takes self with the wrong convention
zero_divided_by_zero warn usage of 0.0 / 0.0 to obtain NaN instead of std::f32::NaN or std::f64::NaN
zero_width_space deny using a zero-width space in a string literal, which is confusing

More to come, please file an issue if you have ideas!


Compiler plugins are highly unstable and will only work with a nightly Rust for now. Since stable Rust is backwards compatible, you should be able to compile your stable programs with nightly Rust with clippy plugged in to circumvent this.

Add in your Cargo.toml:

clippy = "*"

You then need to add #![feature(plugin)] and #![plugin(clippy)] to the top of your crate entry point ( or




fn main(){
    let x = Some(1u8);
    match x {
        Some(y) => println!("{:?}", y),
        _ => ()

Produces this warning:

src/ 11:6 warning: you seem to be trying to use match for destructuring a single type. Consider using `if let`, #[warn(single_match)] on by default
src/     match x {
src/         Some(y) => println!("{:?}", y),
src/         _ => ()
src/     }
src/ 11:6 help: Try
if let Some(y) = x { println!("{:?}", y) }

An alternate way to use clippy is by compiling and using cargo clippy, a custom cargo subcommand that runs clippy on a given project.

You can add options to allow/warn/deny:

  • the whole set of Warn lints using the clippy lint group (#![deny(clippy)])
  • all lints using both the clippy and clippy_pedantic lint groups (#![deny(clippy)], #![deny(clippy_pedantic)]). Note that clippy_pedantic contains some very aggressive lints prone to false positives.
  • only some lints (#![deny(single_match, box_vec)], etc)
  • allow/warn/deny can be limited to a single function or module using #[allow(...)], etc

Note: deny produces errors instead of warnings

To have cargo compile your crate with clippy without needing #![plugin(clippy)] in your code, you can use:

cargo rustc -- -L /path/to/clippy_so -Z extra-plugins=clippy

Note: Be sure that clippy was compiled with the same version of rustc that cargo invokes here!

If you want to make clippy an optional dependency, you can do the following:

In your Cargo.toml:

clippy = {version = "*", optional = true}


And, in your or

#![cfg_attr(feature="clippy", feature(plugin))]

#![cfg_attr(feature="clippy", plugin(clippy))]

##License Licensed under MPL. If you're having issues with the license, let me know and I'll try to change it to something more permissive.