drop.rs.html -- source
Description: Source to the Rust file `library/core/src/ops/drop.rs`.
Keywords: rust, rustlang, rust-lang

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    /// Custom code within the destructor.
    ///
    /// When a value is no longer needed, Rust will run a "destructor" on that value.
    /// The most common way that a value is no longer needed is when it goes out of
    /// scope. Destructors may still run in other circumstances, but we're going to
    /// focus on scope for the examples here. To learn about some of those other cases,
    /// please see [the reference] section on destructors.
    ///
    /// [the reference]: https://doc.rust-lang.org/reference/destructors.html
    ///
    /// This destructor consists of two components:
    /// - A call to `Drop::drop` for that value, if this special `Drop` trait is implemented for its type.
    /// - The automatically generated "drop glue" which recursively calls the destructors
    ///     of the all fields of this value.
    ///
    /// As Rust automatically calls the destructors of all contained fields,
    /// you don't have to implement `Drop` in most cases. But there are some cases where
    /// it is useful, for example for types which directly manage a resource.
    /// That resource may be memory, it may be a file descriptor, it may be a network socket.
    /// Once a value of that type is no longer going to be used, it should "clean up" its
    /// resource by freeing the memory or closing the file or socket. This is
    /// the job of a destructor, and therefore the job of `Drop::drop`.
    ///
    /// ## Examples
    ///
    /// To see destructors in action, let's take a look at the following program:
    ///
    /// ```rust
    /// struct HasDrop;
    ///
    /// impl Drop for HasDrop {
    ///     fn drop(&mut self) {
    ///         println!("Dropping HasDrop!");
    ///     }
    /// }
    ///
    /// struct HasTwoDrops {
    ///     one: HasDrop,
    ///     two: HasDrop,
    /// }
    ///
    /// impl Drop for HasTwoDrops {
    ///     fn drop(&mut self) {
    ///         println!("Dropping HasTwoDrops!");
    ///     }
    /// }
    ///
    /// fn main() {
    ///     let _x = HasTwoDrops { one: HasDrop, two: HasDrop };
    ///     println!("Running!");
    /// }
    /// ```
    ///
    /// Rust will first call `Drop::drop` for `_x` and then for both `_x.one` and `_x.two`,
    /// meaning that running this will print
    ///
    /// ```text
    /// Running!
    /// Dropping HasTwoDrops!
    /// Dropping HasDrop!
    /// Dropping HasDrop!
    /// ```
    ///
    /// Even if we remove the implementation of `Drop` for `HasTwoDrop`, the destructors of its fields are still called.
    /// This would result in
    ///
    /// ```test
    /// Running!
    /// Dropping HasDrop!
    /// Dropping HasDrop!
    /// ```
    ///
    /// ## You cannot call `Drop::drop` yourself
    ///
    /// Because `Drop::drop` is used to clean up a value, it may be dangerous to use this value after
    /// the method has been called. As `Drop::drop` does not take ownership of its input,
    /// Rust prevents misuse by not allowing you to call `Drop::drop` directly.
    ///
    /// In other words, if you tried to explicitly call `Drop::drop` in the above example, you'd get a compiler error.
    ///
    /// If you'd like explicitly call the destructor of a value, [`mem::drop`] can be used instead.
    ///
    /// [`mem::drop`]: drop
    ///
    /// ## Drop order
    ///
    /// Which of our two `HasDrop` drops first, though? For structs, it's the same
    /// order that they're declared: first `one`, then `two`. If you'd like to try
    /// this yourself, you can modify `HasDrop` above to contain some data, like an
    /// integer, and then use it in the `println!` inside of `Drop`. This behavior is
    /// guaranteed by the language.
    ///
    /// Unlike for structs, local variables are dropped in reverse order:
    ///
    /// ```rust
    /// struct Foo;
    ///
    /// impl Drop for Foo {
    ///     fn drop(&mut self) {
    ///         println!("Dropping Foo!")
    ///     }
    /// }
    ///
    /// struct Bar;
    ///
    /// impl Drop for Bar {
    ///     fn drop(&mut self) {
    ///         println!("Dropping Bar!")
    ///     }
    /// }
    ///
    /// fn main() {
    ///     let _foo = Foo;
    ///     let _bar = Bar;
    /// }
    /// ```
    ///
    /// This will print
    ///
    /// ```text
    /// Dropping Bar!
    /// Dropping Foo!
    /// ```
    ///
    /// Please see [the reference] for the full rules.
    ///
    /// [the reference]: https://doc.rust-lang.org/reference/destructors.html
    ///
    /// ## `Copy` and `Drop` are exclusive
    ///
    /// You cannot implement both [`Copy`] and `Drop` on the same type. Types that
    /// are `Copy` get implicitly duplicated by the compiler, making it very
    /// hard to predict when, and how often destructors will be executed. As such,
    /// these types cannot have destructors.
    #[lang = "drop"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Drop {
        /// Executes the destructor for this type.
        ///
        /// This method is called implicitly when the value goes out of scope,
        /// and cannot be called explicitly (this is compiler error [E0040]).
        /// However, the [`mem::drop`] function in the prelude can be
        /// used to call the argument's `Drop` implementation.
        ///
        /// When this method has been called, `self` has not yet been deallocated.
        /// That only happens after the method is over.
        /// If this wasn't the case, `self` would be a dangling reference.
        ///
        /// # Panics
        ///
        /// Given that a [`panic!`] will call `drop` as it unwinds, any [`panic!`]
        /// in a `drop` implementation will likely abort.
        ///
        /// Note that even if this panics, the value is considered to be dropped;
        /// you must not cause `drop` to be called again. This is normally automatically
        /// handled by the compiler, but when using unsafe code, can sometimes occur
        /// unintentionally, particularly when using [`ptr::drop_in_place`].
        ///
        /// [E0040]: ../../error-index.html#E0040
        /// [`panic!`]: crate::panic!
        /// [`mem::drop`]: drop
        /// [`ptr::drop_in_place`]: crate::ptr::drop_in_place
        #[stable(feature = "rust1", since = "1.0.0")]
        fn drop(&mut self);
    }