rust-clippy

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

Table of contents:

• Lint list
• Usage instructions
• Configuration
• License

Usage

Since this is a tool for helping the developer of a library or application write better code, it is recommended not to include clippy as a hard dependency. Options include using it as an optional dependency, as a cargo subcommand, or as an included feature during build. All of these options are detailed below.

As a general rule clippy will only work with the latest Rust nightly for now.

Optional dependency

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

In your Cargo.toml:

[dependencies]
clippy = {version = "*", optional = true}

[features]
default = []

And, in your main.rs or lib.rs:

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

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

Then build by enabling the feature: cargo build --features "clippy"

Instead of adding the cfg_attr attributes you can also run clippy on demand: cargo rustc --features clippy -- -Z no-trans -Z extra-plugins=clippy (the -Z no trans, while not neccessary, will stop the compilation process after typechecking (and lints) have completed, which can significantly reduce the runtime).

As a cargo subcommand (cargo clippy)

An alternate way to use clippy is by installing clippy through cargo as a cargo subcommand.

cargo install clippy

Now you can run clippy by invoking cargo clippy, or rustup run nightly cargo clippy directly from a directory that is usually compiled with stable.

In case you are not using rustup, you need to set the environment flag SYSROOT during installation so clippy knows where to find librustc and similar crates.

SYSROOT=/path/to/rustc/sysroot cargo install clippy

Running clippy from the command line without installing

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!

As a Compiler Plugin

Note: This is not a recommended installation method.

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:

[dependencies]
clippy = "*"

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

Sample main.rs:

#![feature(plugin)]

#![plugin(clippy)]


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

Produces this warning:

src/main.rs:8:5: 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/main.rs:8     match x {
src/main.rs:9         Some(y) => println!("{:?}", y),
src/main.rs:10         _ => ()
src/main.rs:11     }
src/main.rs:8:5: 11:6 help: Try
if let Some(y) = x { println!("{:?}", y) }

Configuration

Some lints can be configured in a clippy.toml file. It contains basic variable = value mapping eg.

blacklisted-names = ["toto", "tata", "titi"]
cyclomatic-complexity-threshold = 30

See the wiki for more information about which lints can be configured and the meaning of the variables.

You can also specify the path to the configuration file with:

#![plugin(clippy(conf_file="path/to/clippy's/configuration"))]

To deactivate the “for further information visit wiki-link” message you can define the CLIPPY_DISABLE_WIKI_LINKS environment variable.

Allowing/denying lints

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.

For convenience, cargo clippy automatically defines a cargo-clippy features. This lets you set lints level and compile with or without clippy transparently:

#[cfg_attr(feature = "cargo-clippy", allow(needless_lifetimes))]

Lints

There are 186 lints included in this crate:

name	default	triggers on
absurd_extreme_comparisons	warn	a comparison with a maximum or minimum value that is always true or false
almost_swapped	warn	foo = bar; bar = foo sequence
approx_constant	warn	the approximate of a known float constant (in std::fXX::consts)
assign_op_pattern	warn	assigning the result of an operation on a variable to that same variable
assign_ops	allow	any compound assignment operation
bad_bit_mask	warn	expressions of the form _ & mask == select that will only ever return true or false
blacklisted_name	warn	usage of a blacklisted/placeholder name
block_in_if_condition_expr	warn	braces that can be eliminated in conditions, e.g if { true } ...
block_in_if_condition_stmt	warn	complex blocks in conditions, 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<T> where unnecessary
builtin_type_shadow	warn	shadowing a builtin type
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, probably not intended
cmp_null	warn	comparing a pointer to a null pointer, suggesting to use .is_null() instead.
cmp_owned	warn	creating owned instances for comparing with others, e.g. x == "foo".to_string()
collapsible_if	warn	ifs that can be collapsed (e.g. if x { if y { ... } } and else { if x { ... } })
crosspointer_transmute	warn	transmutes that have to or from types that are a pointer to the other
cyclomatic_complexity	warn	functions that should be split up into multiple functions
deprecated_semver	warn	use of #[deprecated(since = "x")] where x is not semver
deref_addrof	warn	use of *& or *&mut in an expression
derive_hash_xor_eq	warn	deriving Hash but implementing PartialEq explicitly
diverging_sub_expression	warn	whether an expression contains a diverging sub expression
doc_markdown	warn	presence of _, :: or camel-case outside backticks in documentation
double_neg	warn	--x, which is a double negation of x and not a pre-decrement as in C/C++
double_parens	warn	Warn on unnecessary double parentheses
drop_ref	warn	calls to std::mem::drop with a reference instead of an owned value
duplicate_underscore_argument	warn	function arguments having names which only differ by an underscore
empty_enum	allow	enum with no variants
empty_loop	warn	empty loop {}, which should block or sleep
enum_clike_unportable_variant	warn	C-like enums that are repr(isize/usize) and have values that don't fit into an i32
enum_glob_use	allow	use items that import all variants of an enum
enum_variant_names	warn	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)
eval_order_dependence	warn	whether a variable read occurs before a write depends on sub-expression evaluation order
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_into_iter_loop	warn	for-looping over _.into_iter() when _ would do
explicit_iter_loop	warn	for-looping over _.iter() or _.iter_mut() when &_ or &mut _ would do
filter_map	allow	using combinations of filter, map, filter_map and flat_map which can usually be written as a single method call
filter_next	warn	using filter(p).next(), which is more succinctly expressed as .find(p)
float_arithmetic	allow	any floating-point arithmetic statement
float_cmp	warn	using == or != on float values instead of comparing difference with an epsilon
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
forget_ref	warn	calls to std::mem::forget with a reference instead of an owned value
get_unwrap	warn	using .get().unwrap() or .get_mut().unwrap() when using [] would work instead
identity_op	warn	using identity operations, e.g. x + 0 or y / 1
if_let_redundant_pattern_matching	warn	use the proper utility function avoiding an if let
if_let_some_result	warn	usage of ok() in if let Some(pat) statements is unnecessary, match on Ok(pat) instead
if_not_else	allow	if branches that could be swapped so no negation operation is necessary on the condition
if_same_then_else	warn	if with the same then and else blocks
ifs_same_cond	warn	consecutive ifs with the same condition
indexing_slicing	allow	indexing/slicing usage
ineffective_bit_mask	warn	expressions where a bit mask will be rendered useless by a comparison, e.g. `(x
inline_always	warn	use of #[inline(always)]
integer_arithmetic	allow	any integer arithmetic statement
invalid_regex	deny	invalid regular expressions
invalid_upcast_comparisons	allow	a comparison involving an upcast which is always true or false
items_after_statements	allow	blocks where an item comes after a statement
iter_next_loop	warn	for-looping over _.next() which is probably not intended
iter_nth	warn	using .iter().nth() on a standard library type with O(1) element access
iter_skip_next	warn	using .skip(x).next() on an iterator
large_enum_variant	warn	large variants on an enum
len_without_is_empty	warn	traits or impls with a public len method but no corresponding is_empty method
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
logic_bug	warn	boolean expressions that contain terminals which can be eliminated
manual_swap	warn	manual swap of two variables
many_single_char_names	warn	too many single character bindings
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 a boolean expression instead of an if..else block
match_overlapping_arm	warn	a match with overlapping arms
match_ref_pats	warn	a match or if let with all arms prefixed with & instead of deref-ing the match expression
match_same_arms	warn	match with identical arm bodies
mem_forget	allow	mem::forget usage on Drop types, likely to cause memory leaks
min_max	warn	min(_, max(_, _)) (or vice versa) with bounds clamping the result to a constant
misrefactored_assign_op	warn	having a variable on both sides of an assign op
missing_docs_in_private_items	allow	detects missing documentation for public and private members
mixed_case_hex_literals	warn	hex literals whose letter digits are not consistently upper- or lowercased
module_inception	warn	modules that have the same name as their parent module
modulo_one	warn	taking a number modulo 1, which always returns 0
mut_mut	allow	usage of double-mut refs, e.g. &mut &mut ...
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_borrow	warn	taking a reference that is going to be automatically dereferenced
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 Foo { ..base } when there are no missing fields
neg_multiply	warn	multiplying integers with -1
new_ret_no_self	warn	not returning Self in a new method
new_without_default	warn	fn new() -> Self method without Default implementation
new_without_default_derive	warn	fn new() -> Self without #[derive]able Default implementation
no_effect	warn	statements with no effect
non_ascii_literal	allow	using any literal non-ASCII chars in a string literal instead of using the \\u escape
nonminimal_bool	allow	boolean expressions that can be written more concisely
nonsensical_open_options	warn	nonsensical combination of options for opening a file
not_unsafe_ptr_arg_deref	warn	public functions dereferencing raw pointer arguments but not marked unsafe
ok_expect	warn	using ok().expect(), which gives worse error messages than calling expect directly on the Result
option_map_unwrap_or	allow	using Option.map(f).unwrap_or(a), which is more succinctly expressed as map_or(a, f)
option_map_unwrap_or_else	allow	using Option.map(f).unwrap_or_else(g), 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 with a function call, which suggests *or_else
out_of_bounds_indexing	deny	out of bounds constant indexing
overflow_check_conditional	warn	overflow checks inspired by C which are likely to panic
panic_params	warn	missing parameters in panic! calls
partialeq_ne_impl	warn	re-implementing PartialEq::ne
possible_missing_comma	warn	possible missing comma in array
precedence	warn	operations where precedence may be unclear
print_stdout	allow	printing on stdout
print_with_newline	warn	using print!() with a format string that ends in a newline
ptr_arg	warn	fn arguments of the type &Vec<...> or &String, suggesting to use &[...] or &str instead, respectively
pub_enum_variant_names	allow	enums where all variants share a prefix/postfix
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	redundant closures, i.e. `
redundant_closure_call	warn	throwaway closures called in the expression they are defined
redundant_pattern	warn	using name @ _ in a pattern
regex_macro	warn	use of regex!(_) instead of Regex::new(_)
result_unwrap_used	allow	using Result.unwrap(), which might be better handled
reverse_range_loop	warn	iteration 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()
serde_api_misuse	warn	various things that will negatively affect your serde experience
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	rebinding a name without even using the original value
short_circuit_statement	warn	using a short circuit boolean condition as a statement
should_implement_trait	warn	defining a method that should be implementing a std trait
similar_names	allow	similarly named items and bindings
single_char_pattern	warn	using a single-character str where a char could be used, e.g. _.split("x")
single_match	warn	a match statement with a single nontrivial arm (i.e, where the other arm is _ => {}) instead of if let
single_match_else	allow	a match statement with a two arms where the second arm's pattern is a wildcard instead of if let
string_add	allow	using x + .. where x is a String instead of push_str()
string_add_assign	allow	using x = x + .. where x is a String instead of push_str()
string_extend_chars	warn	using x.extend(s.chars()) where s is a &str or String
string_lit_as_bytes	warn	calling as_bytes on a string literal instead of using a byte string literal
stutter	allow	type names prefixed/postfixed with their containing module's name
suspicious_assignment_formatting	warn	suspicious formatting of *=, -= or !=
suspicious_else_formatting	warn	suspicious formatting of else if
temporary_assignment	warn	assignments to temporaries
temporary_cstring_as_ptr	warn	getting the inner pointer of a temporary CString
too_many_arguments	warn	functions with too many arguments
toplevel_ref_arg	warn	an entire binding declared as ref, in a function argument or a let statement
transmute_ptr_to_ref	warn	transmutes from a pointer to a reference type
trivial_regex	warn	trivial regular expressions
type_complexity	warn	usage of very complex types that might be better factored into type definitions
unicode_not_nfc	allow	using a unicode literal not in NFC normal form (see unicode tr15 for further information)
unit_cmp	warn	comparing unit values
unnecessary_mut_passed	warn	an argument passed as a mutable reference although the callee only demands an immutable reference
unnecessary_operation	warn	outer expressions with no effect
unneeded_field_pattern	warn	struct fields bound to a wildcard instead of using ..
unsafe_removed_from_name	warn	unsafe removed from API names on import
unseparated_literal_suffix	allow	literals whose suffix is not separated by an underscore
unused_collect	warn	collect()ing an iterator without using the result; this is usually better written as a for loop
unused_io_amount	deny	unused written/read amount
unused_label	warn	unused labels
unused_lifetimes	warn	unused lifetimes in function definitions
use_debug	allow	use of Debug-based formatting
used_underscore_binding	allow	using a binding which is prefixed with an underscore
useless_attribute	warn	use of lint attributes on extern crate items
useless_format	warn	useless use of format!
useless_let_if_seq	warn	unidiomatic let mut declaration followed by initialization in if
useless_transmute	warn	transmutes that have the same to and from types or could be a cast/coercion
useless_vec	warn	useless vec!
while_let_loop	warn	loop { if let { ... } else break }, which 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
wrong_transmute	warn	transmutes that are confusing at best, undefined behaviour at worst and always useless
zero_divided_by_zero	warn	usage of 0.0 / 0.0 to obtain NaN instead of std::f32::NaN or std::f64::NaN
zero_prefixed_literal	warn	integer literals starting with 0
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!

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.