Deny-by-default lints - The rustc book

# Deny-by-default lints

These lints are all set to the 'deny' level by default.

## exceeding-bitshifts

This lint detects that a shift exceeds the type's number of bits. Some example code that triggers this lint:

    1_i32 << 32;

This will produce:

    error: bitshift exceeds the type's number of bits
     --> src/main.rs:2:5
      |
    2 |     1_i32 << 32;
      |     ^^^^^^^^^^^
      |

## invalid-type-param-default

This lint detects type parameter default erroneously allowed in invalid location. Some example code that triggers this lint:

    fn foo<T=i32>(t: T) {}

This will produce:

    error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions.
     --> src/main.rs:4:8
      |
    4 | fn foo<T=i32>(t: T) {}
      |        ^
      |
      = note: `#[deny(invalid_type_param_default)]` on by default
      = 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 #36887 <https://github.com/rust-lang/rust/issues/36887>

## missing-fragment-specifier

The missing_fragment_specifier warning is issued when an unused pattern in a `macro_rules!` macro definition has a meta-variable (e.g. `$e`) that is not followed by a fragment specifier (e.g. `:expr`).

This warning can always be fixed by removing the unused pattern in the `macro_rules!` macro definition.

## mutable-transmutes

This lint catches transmuting from `&T` to `&mut T` because it is undefined behavior. Some example code that triggers this lint:

    unsafe {
        let y = std::mem::transmute::<&i32, &mut i32>(&5);
    }

This will produce:

    error: mutating transmuted &mut T from &T may cause undefined behavior, consider instead using an UnsafeCell
     --> src/main.rs:3:17
      |
    3 |         let y = std::mem::transmute::<&i32, &mut i32>(&5);
      |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |

## no-mangle-const-items

This lint detects any `const` items with the `#[no_mangle]` attribute. Constants do not have their symbols exported, and therefore, this probably means you meant to use a `static`, not a `const`. Some example code that triggers this lint:

    #[no_mangle]
    const FOO: i32 = 5;

This will produce:

    error: const items should never be `#[no_mangle]`
     --> src/main.rs:3:1
      |
    3 | const FOO: i32 = 5;
      | -----^^^^^^^^^^^^^^
      | |
      | help: try a static value: `pub static`
      |

## overflowing-literals

This lint detects literal out of range for its type. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    let x: u8 = 1000;
    }

This will produce:

    error: literal out of range for u8
     --> src/main.rs:2:17
      |
    2 |     let x: u8 = 1000;
      |                 ^^^^
      |

## patterns-in-fns-without-body

This lint detects patterns in functions without body were that were previously erroneously allowed. Some example code that triggers this lint:

    #![allow(unused_variables)]
    fn main() {
    trait Trait {
        fn foo(mut arg: u8);
    }
    }

This will produce:

    warning: patterns aren't allowed in methods without bodies
     --> src/main.rs:2:12
      |
    2 |     fn foo(mut arg: u8);
      |            ^^^^^^^
      |
      = note: `#[warn(patterns_in_fns_without_body)]` on by default
      = 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 #35203 <https://github.com/rust-lang/rust/issues/35203>

To fix this, remove the pattern; it can be used in the implementation without being used in the definition. That is:

    #![allow(unused_variables)]
    fn main() {
    trait Trait {
        fn foo(arg: u8);
    }

    impl Trait for i32 {
        fn foo(mut arg: u8) {

        }
    }
    }

## pub-use-of-private-extern-crate

This lint detects a specific situation of re-exporting a private `extern crate`;

## unknown-crate-types

This lint detects an unknown crate type found in a `#[crate_type]` directive. Some example code that triggers this lint:

    #![crate_type="lol"]

This will produce:

    error: invalid `crate_type` value
     --> src/lib.rs:1:1
      |
    1 | #![crate_type="lol"]
      | ^^^^^^^^^^^^^^^^^^^^
      |

## const-err

This lint detects expressions that will always panic at runtime and would be an error in a `const` context.

    let _ = [0; 4][4];

This will produce:

    error: index out of bounds: the len is 4 but the index is 4
     --> src/lib.rs:1:9
      |
    1 | let _ = [0; 4][4];
      |         ^^^^^^^^^
      |

## order-dependent-trait-objects

This lint detects a trait coherency violation that would allow creating two trait impls for the same dynamic trait object involving marker traits.