README.md

   1 #rust-clippy
   2 [![Build Status](https://travis-ci.org/Manishearth/rust-clippy.svg?branch=master)](https://travis-ci.org/Manishearth/rust-clippy)
   3
   4 A collection of lints to catch common mistakes and improve your Rust code.
   5
   6 [Jump to usage instructions](#usage)
   7
   8 ##Lints
   9 There are 111 lints included in this crate:
  10
  11 name                                                                                                           | default | meaning
  12 ---------------------------------------------------------------------------------------------------------------|---------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  13 [absurd_unsigned_comparisons](https://github.com/Manishearth/rust-clippy/wiki#absurd_unsigned_comparisons)     | warn    | testing whether an unsigned integer is non-positive
  14 [approx_constant](https://github.com/Manishearth/rust-clippy/wiki#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
  15 [bad_bit_mask](https://github.com/Manishearth/rust-clippy/wiki#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)
  16 [block_in_if_condition_expr](https://github.com/Manishearth/rust-clippy/wiki#block_in_if_condition_expr)       | warn    | braces can be eliminated in conditions that are expressions, e.g `if { true } ...`
  17 [block_in_if_condition_stmt](https://github.com/Manishearth/rust-clippy/wiki#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 } ...`
  18 [box_vec](https://github.com/Manishearth/rust-clippy/wiki#box_vec)                                             | warn    | usage of `Box<Vec<T>>`, vector elements are already on the heap
  19 [boxed_local](https://github.com/Manishearth/rust-clippy/wiki#boxed_local)                                     | warn    | using Box<T> where unnecessary
  20 [cast_possible_truncation](https://github.com/Manishearth/rust-clippy/wiki#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`
  21 [cast_possible_wrap](https://github.com/Manishearth/rust-clippy/wiki#cast_possible_wrap)                       | allow   | casts that may cause wrapping around the value, e.g `x as i32` where `x: u32` and `x > i32::MAX`
  22 [cast_precision_loss](https://github.com/Manishearth/rust-clippy/wiki#cast_precision_loss)                     | allow   | casts that cause loss of precision, e.g `x as f32` where `x: u64`
  23 [cast_sign_loss](https://github.com/Manishearth/rust-clippy/wiki#cast_sign_loss)                               | allow   | casts from signed types to unsigned types, e.g `x as u32` where `x: i32`
  24 [char_lit_as_u8](https://github.com/Manishearth/rust-clippy/wiki#char_lit_as_u8)                               | warn    | Casting a character literal to u8
  25 [chars_next_cmp](https://github.com/Manishearth/rust-clippy/wiki#chars_next_cmp)                               | warn    | using `.chars().next()` to check if a string starts with a char
  26 [clone_double_ref](https://github.com/Manishearth/rust-clippy/wiki#clone_double_ref)                           | warn    | using `clone` on `&&T`
  27 [clone_on_copy](https://github.com/Manishearth/rust-clippy/wiki#clone_on_copy)                                 | warn    | using `clone` on a `Copy` type
  28 [cmp_nan](https://github.com/Manishearth/rust-clippy/wiki#cmp_nan)                                             | deny    | comparisons to NAN (which will always return false, which is probably not intended)
  29 [cmp_owned](https://github.com/Manishearth/rust-clippy/wiki#cmp_owned)                                         | warn    | creating owned instances for comparing with others, e.g. `x == "foo".to_string()`
  30 [collapsible_if](https://github.com/Manishearth/rust-clippy/wiki#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`
  31 [cyclomatic_complexity](https://github.com/Manishearth/rust-clippy/wiki#cyclomatic_complexity)                 | warn    | finds functions that should be split up into multiple functions
  32 [deprecated_semver](https://github.com/Manishearth/rust-clippy/wiki#deprecated_semver)                         | warn    | `Warn` on `#[deprecated(since = "x")]` where x is not semver
  33 [derive_hash_not_eq](https://github.com/Manishearth/rust-clippy/wiki#derive_hash_not_eq)                       | warn    | deriving `Hash` but implementing `PartialEq` explicitly
  34 [drop_ref](https://github.com/Manishearth/rust-clippy/wiki#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
  35 [duplicate_underscore_argument](https://github.com/Manishearth/rust-clippy/wiki#duplicate_underscore_argument) | warn    | Function arguments having names which only differ by an underscore
  36 [empty_loop](https://github.com/Manishearth/rust-clippy/wiki#empty_loop)                                       | warn    | empty `loop {}` detected
  37 [enum_variant_names](https://github.com/Manishearth/rust-clippy/wiki#enum_variant_names)                       | warn    | finds enums where all variants share a prefix/postfix
  38 [eq_op](https://github.com/Manishearth/rust-clippy/wiki#eq_op)                                                 | warn    | equal operands on both sides of a comparison or bitwise combination (e.g. `x == x`)
  39 [expl_impl_clone_on_copy](https://github.com/Manishearth/rust-clippy/wiki#expl_impl_clone_on_copy)             | warn    | implementing `Clone` explicitly on `Copy` types
  40 [explicit_counter_loop](https://github.com/Manishearth/rust-clippy/wiki#explicit_counter_loop)                 | warn    | for-looping with an explicit counter when `_.enumerate()` would do
  41 [explicit_iter_loop](https://github.com/Manishearth/rust-clippy/wiki#explicit_iter_loop)                       | warn    | for-looping over `_.iter()` or `_.iter_mut()` when `&_` or `&mut _` would do
  42 [extend_from_slice](https://github.com/Manishearth/rust-clippy/wiki#extend_from_slice)                         | warn    | `.extend_from_slice(_)` is a faster way to extend a Vec by a slice
  43 [filter_next](https://github.com/Manishearth/rust-clippy/wiki#filter_next)                                     | warn    | using `filter(p).next()`, which is more succinctly expressed as `.find(p)`
  44 [float_cmp](https://github.com/Manishearth/rust-clippy/wiki#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)
  45 [for_loop_over_option](https://github.com/Manishearth/rust-clippy/wiki#for_loop_over_option)                   | warn    | for-looping over an `Option`, which is more clearly expressed as an `if let`
  46 [for_loop_over_result](https://github.com/Manishearth/rust-clippy/wiki#for_loop_over_result)                   | warn    | for-looping over a `Result`, which is more clearly expressed as an `if let`
  47 [identity_op](https://github.com/Manishearth/rust-clippy/wiki#identity_op)                                     | warn    | using identity operations, e.g. `x + 0` or `y / 1`
  48 [ineffective_bit_mask](https://github.com/Manishearth/rust-clippy/wiki#ineffective_bit_mask)                   | warn    | expressions where a bit mask will be rendered useless by a comparison, e.g. `(x | 1) > 2`
  49 [inline_always](https://github.com/Manishearth/rust-clippy/wiki#inline_always)                                 | warn    | `#[inline(always)]` is a bad idea in most cases
  50 [invalid_regex](https://github.com/Manishearth/rust-clippy/wiki#invalid_regex)                                 | deny    | finds invalid regular expressions in `Regex::new(_)` invocations
  51 [items_after_statements](https://github.com/Manishearth/rust-clippy/wiki#items_after_statements)               | warn    | finds blocks where an item comes after a statement
  52 [iter_next_loop](https://github.com/Manishearth/rust-clippy/wiki#iter_next_loop)                               | warn    | for-looping over `_.next()` which is probably not intended
  53 [len_without_is_empty](https://github.com/Manishearth/rust-clippy/wiki#len_without_is_empty)                   | warn    | traits and impls that have `.len()` but not `.is_empty()`
  54 [len_zero](https://github.com/Manishearth/rust-clippy/wiki#len_zero)                                           | warn    | checking `.len() == 0` or `.len() > 0` (or similar) when `.is_empty()` could be used instead
  55 [let_and_return](https://github.com/Manishearth/rust-clippy/wiki#let_and_return)                               | warn    | creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block
  56 [let_unit_value](https://github.com/Manishearth/rust-clippy/wiki#let_unit_value)                               | warn    | creating a let binding to a value of unit type, which usually can't be used afterwards
  57 [linkedlist](https://github.com/Manishearth/rust-clippy/wiki#linkedlist)                                       | warn    | usage of LinkedList, usually a vector is faster, or a more specialized data structure like a VecDeque
  58 [map_clone](https://github.com/Manishearth/rust-clippy/wiki#map_clone)                                         | warn    | using `.map(|x| x.clone())` to clone an iterator or option's contents (recommends `.cloned()` instead)
  59 [map_entry](https://github.com/Manishearth/rust-clippy/wiki#map_entry)                                         | warn    | use of `contains_key` followed by `insert` on a `HashMap` or `BTreeMap`
  60 [match_bool](https://github.com/Manishearth/rust-clippy/wiki#match_bool)                                       | warn    | a match on boolean expression; recommends `if..else` block instead
  61 [match_overlapping_arm](https://github.com/Manishearth/rust-clippy/wiki#match_overlapping_arm)                 | warn    | a match has overlapping arms
  62 [match_ref_pats](https://github.com/Manishearth/rust-clippy/wiki#match_ref_pats)                               | warn    | a match or `if let` has all arms prefixed with `&`; the match expression can be dereferenced instead
  63 [min_max](https://github.com/Manishearth/rust-clippy/wiki#min_max)                                             | warn    | `min(_, max(_, _))` (or vice versa) with bounds clamping the result to a constant
  64 [modulo_one](https://github.com/Manishearth/rust-clippy/wiki#modulo_one)                                       | warn    | taking a number modulo 1, which always returns 0
  65 [mut_mut](https://github.com/Manishearth/rust-clippy/wiki#mut_mut)                                             | allow   | usage of double-mut refs, e.g. `&mut &mut ...` (either copy'n'paste error, or shows a fundamental misunderstanding of references)
  66 [mutex_atomic](https://github.com/Manishearth/rust-clippy/wiki#mutex_atomic)                                   | warn    | using a Mutex where an atomic value could be used instead
  67 [mutex_integer](https://github.com/Manishearth/rust-clippy/wiki#mutex_integer)                                 | allow   | using a Mutex for an integer type
  68 [needless_bool](https://github.com/Manishearth/rust-clippy/wiki#needless_bool)                                 | warn    | if-statements with plain booleans in the then- and else-clause, e.g. `if p { true } else { false }`
  69 [needless_lifetimes](https://github.com/Manishearth/rust-clippy/wiki#needless_lifetimes)                       | warn    | using explicit lifetimes for references in function arguments when elision rules would allow omitting them
  70 [needless_range_loop](https://github.com/Manishearth/rust-clippy/wiki#needless_range_loop)                     | warn    | for-looping over a range of indices where an iterator over items would do
  71 [needless_return](https://github.com/Manishearth/rust-clippy/wiki#needless_return)                             | warn    | using a return statement like `return expr;` where an expression would suffice
  72 [needless_update](https://github.com/Manishearth/rust-clippy/wiki#needless_update)                             | warn    | using `{ ..base }` when there are no missing fields
  73 [no_effect](https://github.com/Manishearth/rust-clippy/wiki#no_effect)                                         | warn    | statements with no effect
  74 [non_ascii_literal](https://github.com/Manishearth/rust-clippy/wiki#non_ascii_literal)                         | allow   | using any literal non-ASCII chars in a string literal; suggests using the \\u escape instead
  75 [nonsensical_open_options](https://github.com/Manishearth/rust-clippy/wiki#nonsensical_open_options)           | warn    | nonsensical combination of options for opening a file
  76 [ok_expect](https://github.com/Manishearth/rust-clippy/wiki#ok_expect)                                         | warn    | using `ok().expect()`, which gives worse error messages than calling `expect` directly on the Result
  77 [option_map_unwrap_or](https://github.com/Manishearth/rust-clippy/wiki#option_map_unwrap_or)                   | warn    | using `Option.map(f).unwrap_or(a)`, which is more succinctly expressed as `map_or(a, f)`
  78 [option_map_unwrap_or_else](https://github.com/Manishearth/rust-clippy/wiki#option_map_unwrap_or_else)         | warn    | using `Option.map(f).unwrap_or_else(g)`, which is more succinctly expressed as `map_or_else(g, f)`
  79 [option_unwrap_used](https://github.com/Manishearth/rust-clippy/wiki#option_unwrap_used)                       | allow   | using `Option.unwrap()`, which should at least get a better message using `expect()`
  80 [or_fun_call](https://github.com/Manishearth/rust-clippy/wiki#or_fun_call)                                     | warn    | using any `*or` method when the `*or_else` would do
  81 [out_of_bounds_indexing](https://github.com/Manishearth/rust-clippy/wiki#out_of_bounds_indexing)               | deny    | out of bound constant indexing
  82 [panic_params](https://github.com/Manishearth/rust-clippy/wiki#panic_params)                                   | warn    | missing parameters in `panic!`
  83 [precedence](https://github.com/Manishearth/rust-clippy/wiki#precedence)                                       | warn    | catches operations where precedence may be unclear. See the wiki for a list of cases caught
  84 [print_stdout](https://github.com/Manishearth/rust-clippy/wiki#print_stdout)                                   | allow   | printing on stdout
  85 [ptr_arg](https://github.com/Manishearth/rust-clippy/wiki#ptr_arg)                                             | warn    | fn arguments of the type `&Vec<...>` or `&String`, suggesting to use `&[...]` or `&str` instead, respectively
  86 [range_step_by_zero](https://github.com/Manishearth/rust-clippy/wiki#range_step_by_zero)                       | warn    | using Range::step_by(0), which produces an infinite iterator
  87 [range_zip_with_len](https://github.com/Manishearth/rust-clippy/wiki#range_zip_with_len)                       | warn    | zipping iterator with a range when enumerate() would do
  88 [redundant_closure](https://github.com/Manishearth/rust-clippy/wiki#redundant_closure)                         | warn    | using redundant closures, i.e. `|a| foo(a)` (which can be written as just `foo`)
  89 [redundant_pattern](https://github.com/Manishearth/rust-clippy/wiki#redundant_pattern)                         | warn    | using `name @ _` in a pattern
  90 [result_unwrap_used](https://github.com/Manishearth/rust-clippy/wiki#result_unwrap_used)                       | allow   | using `Result.unwrap()`, which might be better handled
  91 [reverse_range_loop](https://github.com/Manishearth/rust-clippy/wiki#reverse_range_loop)                       | warn    | Iterating over an empty range, such as `10..0` or `5..5`
  92 [search_is_some](https://github.com/Manishearth/rust-clippy/wiki#search_is_some)                               | warn    | using an iterator search followed by `is_some()`, which is more succinctly expressed as a call to `any()`
  93 [shadow_reuse](https://github.com/Manishearth/rust-clippy/wiki#shadow_reuse)                                   | allow   | rebinding a name to an expression that re-uses the original value, e.g. `let x = x + 1`
  94 [shadow_same](https://github.com/Manishearth/rust-clippy/wiki#shadow_same)                                     | allow   | rebinding a name to itself, e.g. `let mut x = &mut x`
  95 [shadow_unrelated](https://github.com/Manishearth/rust-clippy/wiki#shadow_unrelated)                           | allow   | The name is re-bound without even using the original value
  96 [should_implement_trait](https://github.com/Manishearth/rust-clippy/wiki#should_implement_trait)               | warn    | defining a method that should be implementing a std trait
  97 [single_match](https://github.com/Manishearth/rust-clippy/wiki#single_match)                                   | warn    | a match statement with a single nontrivial arm (i.e, where the other arm is `_ => {}`) is used; recommends `if let` instead
  98 [single_match_else](https://github.com/Manishearth/rust-clippy/wiki#single_match_else)                         | allow   | a match statement with a two arms where the second arm's pattern is a wildcard; recommends `if let` instead
  99 [str_to_string](https://github.com/Manishearth/rust-clippy/wiki#str_to_string)                                 | warn    | using `to_string()` on a str, which should be `to_owned()`
 100 [string_add](https://github.com/Manishearth/rust-clippy/wiki#string_add)                                       | allow   | using `x + ..` where x is a `String`; suggests using `push_str()` instead
 101 [string_add_assign](https://github.com/Manishearth/rust-clippy/wiki#string_add_assign)                         | allow   | using `x = x + ..` where x is a `String`; suggests using `push_str()` instead
 102 [string_lit_as_bytes](https://github.com/Manishearth/rust-clippy/wiki#string_lit_as_bytes)                     | warn    | calling `as_bytes` on a string literal; suggests using a byte string literal instead
 103 [string_to_string](https://github.com/Manishearth/rust-clippy/wiki#string_to_string)                           | warn    | calling `String::to_string` which is inefficient
 104 [temporary_assignment](https://github.com/Manishearth/rust-clippy/wiki#temporary_assignment)                   | warn    | assignments to temporaries
 105 [toplevel_ref_arg](https://github.com/Manishearth/rust-clippy/wiki#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 `&`.
 106 [type_complexity](https://github.com/Manishearth/rust-clippy/wiki#type_complexity)                             | warn    | usage of very complex types; recommends factoring out parts into `type` definitions
 107 [unicode_not_nfc](https://github.com/Manishearth/rust-clippy/wiki#unicode_not_nfc)                             | allow   | using a unicode literal not in NFC normal form (see http://www.unicode.org/reports/tr15/ for further information)
 108 [unit_cmp](https://github.com/Manishearth/rust-clippy/wiki#unit_cmp)                                           | warn    | comparing unit values (which is always `true` or `false`, respectively)
 109 [unnecessary_mut_passed](https://github.com/Manishearth/rust-clippy/wiki#unnecessary_mut_passed)               | warn    | an argument is passed as a mutable reference although the function/method only demands an immutable reference
 110 [unneeded_field_pattern](https://github.com/Manishearth/rust-clippy/wiki#unneeded_field_pattern)               | warn    | Struct fields are bound to a wildcard instead of using `..`
 111 [unstable_as_mut_slice](https://github.com/Manishearth/rust-clippy/wiki#unstable_as_mut_slice)                 | warn    | as_mut_slice is not stable and can be replaced by &mut v[..]see https://github.com/rust-lang/rust/issues/27729
 112 [unstable_as_slice](https://github.com/Manishearth/rust-clippy/wiki#unstable_as_slice)                         | warn    | as_slice is not stable and can be replaced by & v[..]see https://github.com/rust-lang/rust/issues/27729
 113 [unused_collect](https://github.com/Manishearth/rust-clippy/wiki#unused_collect)                               | warn    | `collect()`ing an iterator without using the result; this is usually better written as a for loop
 114 [unused_lifetimes](https://github.com/Manishearth/rust-clippy/wiki#unused_lifetimes)                           | warn    | unused lifetimes in function definitions
 115 [used_underscore_binding](https://github.com/Manishearth/rust-clippy/wiki#used_underscore_binding)             | warn    | using a binding which is prefixed with an underscore
 116 [useless_transmute](https://github.com/Manishearth/rust-clippy/wiki#useless_transmute)                         | warn    | transmutes that have the same to and from types
 117 [useless_vec](https://github.com/Manishearth/rust-clippy/wiki#useless_vec)                                     | warn    | useless `vec!`
 118 [while_let_loop](https://github.com/Manishearth/rust-clippy/wiki#while_let_loop)                               | warn    | `loop { if let { ... } else break }` can be written as a `while let` loop
 119 [while_let_on_iterator](https://github.com/Manishearth/rust-clippy/wiki#while_let_on_iterator)                 | warn    | using a while-let loop instead of a for loop on an iterator
 120 [wrong_pub_self_convention](https://github.com/Manishearth/rust-clippy/wiki#wrong_pub_self_convention)         | allow   | defining a public method named with an established prefix (like "into_") that takes `self` with the wrong convention
 121 [wrong_self_convention](https://github.com/Manishearth/rust-clippy/wiki#wrong_self_convention)                 | warn    | defining a method named with an established prefix (like "into_") that takes `self` with the wrong convention
 122 [zero_divided_by_zero](https://github.com/Manishearth/rust-clippy/wiki#zero_divided_by_zero)                   | warn    | usage of `0.0 / 0.0` to obtain NaN instead of std::f32::NaN or std::f64::NaN
 123 [zero_width_space](https://github.com/Manishearth/rust-clippy/wiki#zero_width_space)                           | deny    | using a zero-width space in a string literal, which is confusing
 124
 125 More to come, please [file an issue](https://github.com/Manishearth/rust-clippy/issues) if you have ideas!
 126
 127 ##Usage
 128
 129 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.
 130
 131 Add in your `Cargo.toml`:
 132 ```toml
 133 [dependencies]
 134 clippy = "*"
 135 ```
 136
 137 You may also use [`cargo clippy`](https://github.com/arcnmx/cargo-clippy), a custom cargo subcommand that runs clippy on a given project.
 138
 139 Sample `main.rs`:
 140 ```rust
 141 #![feature(plugin)]
 142
 143 #![plugin(clippy)]
 144
 145
 146 fn main(){
 147     let x = Some(1u8);
 148     match x {
 149         Some(y) => println!("{:?}", y),
 150         _ => ()
 151     }
 152 }
 153 ```
 154
 155 Produces this warning:
 156 ```
 157 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
 158 src/main.rs:8     match x {
 159 src/main.rs:9         Some(y) => println!("{:?}", y),
 160 src/main.rs:10         _ => ()
 161 src/main.rs:11     }
 162 src/main.rs:8:5: 11:6 help: Try
 163 if let Some(y) = x { println!("{:?}", y) }
 164 ```
 165
 166 You can add options  to `allow`/`warn`/`deny`:
 167 - the whole set of `Warn` lints using the `clippy` lint group (`#![deny(clippy)]`)
 168 - 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.
 169 - only some lints (`#![deny(single_match, box_vec)]`, etc)
 170 - `allow`/`warn`/`deny` can be limited to a single function or module using `#[allow(...)]`, etc
 171
 172 Note: `deny` produces errors instead of warnings
 173
 174 To have cargo compile your crate with clippy without needing `#![plugin(clippy)]`
 175 in your code, you can use:
 176
 177 ```
 178 cargo rustc -- -L /path/to/clippy_so -Z extra-plugins=clippy
 179 ```
 180
 181 *[Note](https://github.com/Manishearth/rust-clippy/wiki#a-word-of-warning):* Be sure that clippy was compiled with the same version of rustc that cargo invokes here!
 182
 183 If you want to make clippy an optional dependency, you can do the following:
 184
 185 In your `Cargo.toml`:
 186 ```toml
 187 [dependencies]
 188 clippy = {version = "*", optional = true}
 189
 190 [features]
 191 default=[]
 192 ```
 193
 194 And, in your `main.rs` or `lib.rs`:
 195
 196 ```rust
 197 #![cfg_attr(feature="clippy", feature(plugin))]
 198
 199 #![cfg_attr(feature="clippy", plugin(clippy))]
 200 ```
 201
 202 ##License
 203 Licensed under [MPL](https://www.mozilla.org/MPL/2.0/). If you're having issues with the license, let me know and I'll try to change it to something more permissive.