Allowed-by-default lints - The rustc book

# Allowed-by-default lints

These lints are all set to the 'allow' level by default. As such, they won't show up unless you set them to a higher lint level with a flag or attribute.

## anonymous-parameters

This lint detects anonymous parameters. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    trait Foo {
        fn foo(usize);
    }
    }

When set to 'deny', this will produce:

    error: use of deprecated anonymous parameter
     --> src/lib.rs:5:11
      |
    5 |     fn foo(usize);
      |           ^
      |
      = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
      = note: for more information, see issue #41686 <https://github.com/rust-lang/rust/issues/41686>

This syntax is mostly a historical accident, and can be worked around quite easily:

    #![allow(unused_variables)]
    fn main() {
    trait Foo {
        fn foo(_: usize);
    }
    }

## bare-trait-object

This lint suggests using `dyn Trait` for trait objects. Some example code that triggers this lint:

    #![allow(unused_variables)]
    #![feature(dyn_trait)]

    fn main() {
    trait Trait { }

    fn takes_trait_object(_: Box<Trait>) {
    }
    }

When set to 'deny', this will produce:

    error: trait objects without an explicit `dyn` are deprecated
     --> src/lib.rs:7:30
      |
    7 | fn takes_trait_object(_: Box<Trait>) {
      |                              ^^^^^ help: use `dyn`: `dyn Trait`
      |

To fix it, do as the help message suggests:

    #![allow(unused_variables)]
    #![feature(dyn_trait)]
    #![deny(bare_trait_objects)]

    fn main() {
    trait Trait { }

    fn takes_trait_object(_: Box<dyn Trait>) {
    }
    }

## box-pointers

This lints use of the Box type. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    struct Foo {
        x: Box<isize>,
    }
    }

When set to 'deny', this will produce:

    error: type uses owned (Box type) pointers: std::boxed::Box<isize>
     --> src/lib.rs:6:5
      |
    6 |     x: Box<isize> //~ ERROR type uses owned
      |     ^^^^^^^^^^^^^
      |

This lint is mostly historical, and not particularly useful. `Box<T>` used to be built into the language, and the only way to do heap allocation. Today's Rust can call into other allocators, etc.

## elided-lifetime-in-path

This lint detects the use of hidden lifetime parameters. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    struct Foo<'a> {
        x: &'a u32
    }

    fn foo(x: &Foo) {
    }
    }

When set to 'deny', this will produce:

    error: hidden lifetime parameters are deprecated, try `Foo<'_>`
     --> src/lib.rs:5:12
      |
    5 | fn foo(x: &Foo) {
      |            ^^^
      |

Lifetime elision elides this lifetime, but that is being deprecated.

## missing-copy-implementations

This lint detects potentially-forgotten implementations of `Copy`. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    pub struct Foo {
        pub field: i32
    }
    }

When set to 'deny', this will produce:

    error: type could implement `Copy`; consider adding `impl Copy`
     --> src/main.rs:3:1
      |
    3 | / pub struct Foo { //~ ERROR type could implement `Copy`; consider adding `impl Copy`
    4 | |     pub field: i32
    5 | | }
      | |_^
      |

You can fix the lint by deriving `Copy`.

This lint is set to 'allow' because this code isn't bad; it's common to write newtypes like this specifically so that a `Copy` type is no longer `Copy`.

## missing-debug-implementations

This lint detects missing implementations of `fmt::Debug`. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    pub struct Foo;
    }

When set to 'deny', this will produce:

    error: type does not implement `fmt::Debug`; consider adding `#[derive(Debug)]` or a manual implementation
     --> src/main.rs:3:1
      |
    3 | pub struct Foo;
      | ^^^^^^^^^^^^^^^
      |

You can fix the lint by deriving `Debug`.

## missing-docs

This lint detects missing documentation for public items. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    pub fn foo() {}
    }

When set to 'deny', this will produce:

    error: missing documentation for crate
     --> src/main.rs:1:1
      |
    1 | / #![deny(missing_docs)]
    2 | |
    3 | | pub fn foo() {}
    4 | |
    5 | | fn main() {}
      | |____________^
      |

    error: missing documentation for a function
     --> src/main.rs:3:1
      |
    3 | pub fn foo() {}
      | ^^^^^^^^^^^^

To fix the lint, add documentation to all items.

## single-use-lifetimes

This lint detects lifetimes that are only used once. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    struct Foo<'x> {
        x: &'x u32
    }
    }

When set to 'deny', this will produce:

    error: lifetime name `'x` only used once
     --> src/main.rs:3:12
      |
    3 | struct Foo<'x> {
      |            ^^
      |

## trivial-casts

This lint detects trivial casts which could be removed. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    let x: &u32 = &42;
    let _ = x as *const u32;
    }

When set to 'deny', this will produce:

    error: trivial cast: `&u32` as `*const u32`. Cast can be replaced by coercion, this might require type ascription or a temporary variable
     --> src/main.rs:5:13
      |
    5 |     let _ = x as *const u32;
      |             ^^^^^^^^^^^^^^^
      |
    note: lint level defined here
     --> src/main.rs:1:9
      |
    1 | #![deny(trivial_casts)]
      |         ^^^^^^^^^^^^^

## trivial-numeric-casts

This lint detects trivial casts of numeric types which could be removed. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    let x = 42i32 as i32;
    }

When set to 'deny', this will produce:

    error: trivial numeric cast: `i32` as `i32`. Cast can be replaced by coercion, this might require type ascription or a temporary variable
     --> src/main.rs:4:13
      |
    4 |     let x = 42i32 as i32;
      |             ^^^^^^^^^^^^
      |

## unreachable-pub

This lint triggers for `pub` items not reachable from the crate root. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    mod foo {
        pub mod bar {

        }
    }
    }

When set to 'deny', this will produce:

    error: unreachable `pub` item
     --> src/main.rs:4:5
      |
    4 |     pub mod bar {
      |     ---^^^^^^^^
      |     |
      |     help: consider restricting its visibility: `pub(crate)`
      |

## unsafe-code

This lint catches usage of `unsafe` code. Some example code that triggers this lint:

    fn main() {
        unsafe {

        }
    }

When set to 'deny', this will produce:

    error: usage of an `unsafe` block
     --> src/main.rs:4:5
      |
    4 | /     unsafe {
    5 | |
    6 | |     }
      | |_____^
      |

## unstable-features

This lint is deprecated and no longer used.

## unused-extern-crates

This lint guards against `extern crate` items that are never used. Some example code that triggers this lint:

    extern crate semver;

When set to 'deny', this will produce:

    error: unused extern crate
     --> src/main.rs:3:1
      |
    3 | extern crate semver;
      | ^^^^^^^^^^^^^^^^^^^^
      |

## unused-import-braces

This lint catches unnecessary braces around an imported item. Some example code that triggers this lint:

    use test::{A};

    pub mod test {
        pub struct A;
    }
    fn main() {}

When set to 'deny', this will produce:

    error: braces around A is unnecessary
     --> src/main.rs:3:1
      |
    3 | use test::{A};
      | ^^^^^^^^^^^^^^
      |

To fix it, `use test::A;`

## unused-qualifications

This lint detects unnecessarily qualified names. Some example code that triggers this lint:

    mod foo {
        pub fn bar() {}
    }

    fn main() {
        use foo::bar;
        foo::bar();
    }

When set to 'deny', this will produce:

    error: unnecessary qualification
     --> src/main.rs:9:5
      |
    9 |     foo::bar();
      |     ^^^^^^^^
      |

You can call `bar()` directly, without the `foo::`.

## unused-results

This lint checks for the unused result of an expression in a statement. Some example code that triggers this lint:

    fn foo<T>() -> T { panic!() }

    fn main() {
        foo::<usize>();
    }

When set to 'deny', this will produce:

    error: unused result
     --> src/main.rs:6:5
      |
    6 |     foo::<usize>();
      |     ^^^^^^^^^^^^^^^
      |

## variant-size-differences

This lint detects enums with widely varying variant sizes. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    enum En {
        V0(u8),
        VBig([u8; 1024]),
    }
    }

When set to 'deny', this will produce:

    error: enum variant is more than three times larger (1024 bytes) than the next largest
     --> src/main.rs:5:5
      |
    5 |     VBig([u8; 1024]),   //~ ERROR variant is more than three times larger
      |     ^^^^^^^^^^^^^^^^
      |