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Discourage overuse of mem::forget #53503

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Aug 24, 2018
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Discourage overuse of mem::forget #53503

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46 changes: 7 additions & 39 deletions src/libcore/mem.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -29,13 +29,15 @@ use ops::{Deref, DerefMut, CoerceUnsized};
#[stable(feature = "rust1", since = "1.0.0")]
pub use intrinsics::transmute;

/// Leaks a value: takes ownership and "forgets" about the value **without running
/// its destructor**.
/// Takes ownership and "forgets" about the value **without running its destructor**.
///
/// Any resources the value manages, such as heap memory or a file handle, will linger
/// forever in an unreachable state.
/// forever in an unreachable state. However, it does not guarantee that pointers
/// to this memory will remain valid.
///
/// If you want to dispose of a value properly, running its destructor, see
/// * If you want to leak memory, see [`Box::leak`][leak].
/// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`][into_raw].
/// * If you want to dispose of a value properly, running its destructor, see
/// [`mem::drop`][drop].
///
/// # Safety
Expand All @@ -59,15 +61,6 @@ pub use intrinsics::transmute;
///
/// # Examples
///
/// Leak some heap memory by never deallocating it:
///
/// ```
/// use std::mem;
///
/// let heap_memory = Box::new(3);
/// mem::forget(heap_memory);
/// ```
///
/// Leak an I/O object, never closing the file:
///
/// ```no_run
Expand Down Expand Up @@ -137,38 +130,13 @@ pub use intrinsics::transmute;
/// }
/// ```
///
/// ## Use case 3
///
/// You are transferring ownership across a [FFI] boundary to code written in
/// another language. You need to `forget` the value on the Rust side because Rust
/// code is no longer responsible for it.
///
/// ```no_run
/// use std::mem;
///
/// extern "C" {
/// fn my_c_function(x: *const u32);
/// }
///
/// let x: Box<u32> = Box::new(3);
///
/// // Transfer ownership into C code.
/// unsafe {
/// my_c_function(&*x);
/// }
/// mem::forget(x);
/// ```
///
/// In this case, C code must call back into Rust to free the object. Calling C's `free`
/// function on a [`Box`][box] is *not* safe! Also, `Box` provides an [`into_raw`][into_raw]
/// method which is the preferred way to do this in practice.
///
/// [drop]: fn.drop.html
/// [uninit]: fn.uninitialized.html
/// [clone]: ../clone/trait.Clone.html
/// [swap]: fn.swap.html
/// [FFI]: ../../book/first-edition/ffi.html
/// [box]: ../../std/boxed/struct.Box.html
/// [leak]: ../../std/boxed/struct.Box.html#method.leak
/// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw
/// [ub]: ../../reference/behavior-considered-undefined.html
#[inline]
Expand Down