rust-clippy/README.md

#rust-clippy
[![Build Status](https://travis-ci.org/Manishearth/rust-clippy.svg?branch=master)](https://travis-ci.org/Manishearth/rust-clippy)

A collection of lints that give helpful tips to newbies and catch oversights.

##Lints
There are 50 lints included in this crate:

name                                                                                                 | default | meaning
-----------------------------------------------------------------------------------------------------|---------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
[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
[bad_bit_mask](https://github.com/Manishearth/rust-clippy/wiki#bad_bit_mask)                         | deny    | 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)
[box_vec](https://github.com/Manishearth/rust-clippy/wiki#box_vec)                                   | warn    | usage of `Box<Vec<T>>`, vector elements are already on the heap
[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`
[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`
[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`
[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`
[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)
[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()`
[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() }`
[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`)
[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
[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)
[identity_op](https://github.com/Manishearth/rust-clippy/wiki#identity_op)                           | warn    | using identity operations, e.g. `x + 0` or `y / 1`
[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`
[inline_always](https://github.com/Manishearth/rust-clippy/wiki#inline_always)                       | warn    | `#[inline(always)]` is a bad idea in most cases
[iter_next_loop](https://github.com/Manishearth/rust-clippy/wiki#iter_next_loop)                     | warn    | for-looping over `_.next()` which is probably not intended
[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()`
[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
[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 function
[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
[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 RingBuf
[match_ref_pats](https://github.com/Manishearth/rust-clippy/wiki#match_ref_pats)                     | warn    | a match has all arms prefixed with `&`; the match expression can be dereferenced instead
[modulo_one](https://github.com/Manishearth/rust-clippy/wiki#modulo_one)                             | warn    | taking a number modulo 1, which always returns 0
[mut_mut](https://github.com/Manishearth/rust-clippy/wiki#mut_mut)                                   | warn    | usage of double-mut refs, e.g. `&mut &mut ...` (either copy'n'paste error, or shows a fundamental misunderstanding of references)
[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 }`
[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
[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
[needless_return](https://github.com/Manishearth/rust-clippy/wiki#needless_return)                   | warn    | using a return statement like `return expr;` where an expression would suffice
[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
[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()`
[precedence](https://github.com/Manishearth/rust-clippy/wiki#precedence)                             | warn    | expressions where precedence may trip up the unwary reader of the source; suggests adding parentheses, e.g. `x << 2 + y` will be parsed as `x << (2 + y)`
[ptr_arg](https://github.com/Manishearth/rust-clippy/wiki#ptr_arg)                                   | allow   | fn arguments of the type `&Vec<...>` or `&String`, suggesting to use `&[...]` or `&str` instead, respectively
[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
[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`)
[result_unwrap_used](https://github.com/Manishearth/rust-clippy/wiki#result_unwrap_used)             | allow   | using `Result.unwrap()`, which might be better handled
[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`
[shadow_same](https://github.com/Manishearth/rust-clippy/wiki#shadow_same)                           | allow   | rebinding a name to itself, e.g. `let mut x = &mut x`
[shadow_unrelated](https://github.com/Manishearth/rust-clippy/wiki#shadow_unrelated)                 | warn    | The name is re-bound without even using the original value
[should_implement_trait](https://github.com/Manishearth/rust-clippy/wiki#should_implement_trait)     | warn    | defining a method that should be implementing a std trait
[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
[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()`
[string_add](https://github.com/Manishearth/rust-clippy/wiki#string_add)                             | allow   | using `x + ..` where x is a `String`; suggests using `push_str()` instead
[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
[string_to_string](https://github.com/Manishearth/rust-clippy/wiki#string_to_string)                 | warn    | calling `String.to_string()` which is a no-op
[toplevel_ref_arg](https://github.com/Manishearth/rust-clippy/wiki#toplevel_ref_arg)                 | warn    | a function argument is declared `ref` (i.e. `fn foo(ref x: u8)`, but not `fn foo((ref x, ref y): (u8, u8))`)
[type_complexity](https://github.com/Manishearth/rust-clippy/wiki#type_complexity)                   | warn    | usage of very complex types; recommends factoring out parts into `type` definitions
[unit_cmp](https://github.com/Manishearth/rust-clippy/wiki#unit_cmp)                                 | warn    | comparing unit values (which is always `true` or `false`, respectively)
[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
[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

More to come, please [file an issue](https://github.com/Manishearth/rust-clippy/issues) if you have ideas!

##Usage

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`:
```toml
[dependencies]
clippy = "*"
```

Sample `main.rs`:
```rust
#![feature(plugin)]

#![plugin(clippy)]


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

Produce this warning:
```
src/main.rs:8:5: 11:6 warning: You seem to be trying to use match for destructuring a single type. Did you mean to use `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) }
```

You can add options  to `allow`/`warn`/`deny`:
- the whole set using the `clippy` lint group (`#![deny(clippy)]`, etc)
- 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
```

##License
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.