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    // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Basic functions for dealing with memory
    //!
    //! This module contains functions for querying the size and alignment of
    //! types, initializing and manipulating memory.

    #![stable(feature = "rust1", since = "1.0.0")]

    use marker::Sized;
    use intrinsics;
    use ptr;

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use intrinsics::transmute;

    /// Leaks a value into the void, consuming ownership and never running its
    /// destructor.
    ///
    /// This function will take ownership of its argument, but is distinct from the
    /// `mem::drop` function in that it **does not run the destructor**, leaking the
    /// value and any resources that it owns.
    ///
    /// There's only a few reasons to use this function. They mainly come
    /// up in unsafe code or FFI code.
    ///
    /// * You have an uninitialized value, perhaps for performance reasons, and
    ///   need to prevent the destructor from running on it.
    /// * You have two copies of a value (like when writing something like
    ///   [`mem::swap`][swap]), but need the destructor to only run once to
    ///   prevent a double `free`.
    /// * Transferring resources across [FFI][ffi] boundaries.
    ///
    /// [swap]: fn.swap.html
    /// [ffi]: ../../book/ffi.html
    ///
    /// # Safety
    ///
    /// This function is not marked as `unsafe` as Rust does not guarantee that the
    /// `Drop` implementation for a value will always run. Note, however, that
    /// leaking resources such as memory or I/O objects is likely not desired, so
    /// this function is only recommended for specialized use cases.
    ///
    /// The safety of this function implies that when writing `unsafe` code
    /// yourself care must be taken when leveraging a destructor that is required to
    /// run to preserve memory safety. There are known situations where the
    /// destructor may not run (such as if ownership of the object with the
    /// destructor is returned) which must be taken into account.
    ///
    /// # Other forms of Leakage
    ///
    /// It's important to point out that this function is not the only method by
    /// which a value can be leaked in safe Rust code. Other known sources of
    /// leakage are:
    ///
    /// * `Rc` and `Arc` cycles
    /// * `mpsc::{Sender, Receiver}` cycles (they use `Arc` internally)
    /// * Panicking destructors are likely to leak local resources
    ///
    /// # Example
    ///
    /// Leak some heap memory by never deallocating it:
    ///
    /// ```rust
    /// use std::mem;
    ///
    /// let heap_memory = Box::new(3);
    /// mem::forget(heap_memory);
    /// ```
    ///
    /// Leak an I/O object, never closing the file:
    ///
    /// ```rust,no_run
    /// use std::mem;
    /// use std::fs::File;
    ///
    /// let file = File::open("foo.txt").unwrap();
    /// mem::forget(file);
    /// ```
    ///
    /// The `mem::swap` function uses `mem::forget` to good effect:
    ///
    /// ```rust
    /// use std::mem;
    /// use std::ptr;
    ///
    /// # #[allow(dead_code)]
    /// fn swap<T>(x: &mut T, y: &mut T) {
    ///     unsafe {
    ///         // Give ourselves some scratch space to work with
    ///         let mut t: T = mem::uninitialized();
    ///
    ///         // Perform the swap, `&mut` pointers never alias
    ///         ptr::copy_nonoverlapping(&*x, &mut t, 1);
    ///         ptr::copy_nonoverlapping(&*y, x, 1);
    ///         ptr::copy_nonoverlapping(&t, y, 1);
    ///
    ///         // y and t now point to the same thing, but we need to completely
    ///         // forget `t` because we do not want to run the destructor for `T`
    ///         // on its value, which is still owned somewhere outside this function.
    ///         mem::forget(t);
    ///     }
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn forget<T>(t: T) {
        unsafe { intrinsics::forget(t) }
    }

    /// Returns the size of a type in bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::size_of::<i32>());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn size_of<T>() -> usize {
        unsafe { intrinsics::size_of::<T>() }
    }

    /// Returns the size of the given value in bytes.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::size_of_val(&5i32));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
        unsafe { intrinsics::size_of_val(val) }
    }

    /// Returns the ABI-required minimum alignment of a type
    ///
    /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(deprecated)]
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::min_align_of::<i32>());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
    pub fn min_align_of<T>() -> usize {
        unsafe { intrinsics::min_align_of::<T>() }
    }

    /// Returns the ABI-required minimum alignment of the type of the value that `val` points to
    ///
    /// # Examples
    ///
    /// ```
    /// # #![allow(deprecated)]
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::min_align_of_val(&5i32));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
    pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
        unsafe { intrinsics::min_align_of_val(val) }
    }

    /// Returns the alignment in memory for a type.
    ///
    /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::align_of::<i32>());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn align_of<T>() -> usize {
        unsafe { intrinsics::min_align_of::<T>() }
    }

    /// Returns the ABI-required minimum alignment of the type of the value that `val` points to
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::align_of_val(&5i32));
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
        unsafe { intrinsics::min_align_of_val(val) }
    }

    /// Creates a value initialized to zero.
    ///
    /// This function is similar to allocating space for a local variable and zeroing it out (an unsafe
    /// operation).
    ///
    /// Care must be taken when using this function, if the type `T` has a destructor and the value
    /// falls out of scope (due to unwinding or returning) before being initialized, then the
    /// destructor will run on zeroed data, likely leading to crashes.
    ///
    /// This is useful for FFI functions sometimes, but should generally be avoided.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// let x: i32 = unsafe { mem::zeroed() };
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn zeroed<T>() -> T {
        intrinsics::init()
    }

    /// Creates a value initialized to an unspecified series of bytes.
    ///
    /// The byte sequence usually indicates that the value at the memory
    /// in question has been dropped. Thus, *if* T carries a drop flag,
    /// any associated destructor will not be run when the value falls out
    /// of scope.
    ///
    /// Some code at one time used the `zeroed` function above to
    /// accomplish this goal.
    ///
    /// This function is expected to be deprecated with the transition
    /// to non-zeroing drop.
    #[inline]
    #[unstable(feature = "filling_drop", issue = "5016")]
    pub unsafe fn dropped<T>() -> T {
        #[inline(always)]
        unsafe fn dropped_impl<T>() -> T { intrinsics::init_dropped() }

        dropped_impl()
    }

    /// Bypasses Rust's normal memory-initialization checks by pretending to
    /// produce a value of type T, while doing nothing at all.
    ///
    /// **This is incredibly dangerous, and should not be done lightly. Deeply
    /// consider initializing your memory with a default value instead.**
    ///
    /// This is useful for FFI functions and initializing arrays sometimes,
    /// but should generally be avoided.
    ///
    /// # Undefined Behavior
    ///
    /// It is Undefined Behavior to read uninitialized memory. Even just an
    /// uninitialized boolean. For instance, if you branch on the value of such
    /// a boolean your program may take one, both, or neither of the branches.
    ///
    /// Note that this often also includes *writing* to the uninitialized value.
    /// Rust believes the value is initialized, and will therefore try to Drop
    /// the uninitialized value and its fields if you try to overwrite the memory
    /// in a normal manner. The only way to safely initialize an arbitrary
    /// uninitialized value is with one of the `ptr` functions: `write`, `copy`, or
    /// `copy_nonoverlapping`. This isn't necessary if `T` is a primitive
    /// or otherwise only contains types that don't implement Drop.
    ///
    /// If this value *does* need some kind of Drop, it must be initialized before
    /// it goes out of scope (and therefore would be dropped). Note that this
    /// includes a `panic` occurring and unwinding the stack suddenly.
    ///
    /// # Examples
    ///
    /// Here's how to safely initialize an array of `Vec`s.
    ///
    /// ```
    /// use std::mem;
    /// use std::ptr;
    ///
    /// // Only declare the array. This safely leaves it
    /// // uninitialized in a way that Rust will track for us.
    /// // However we can't initialize it element-by-element
    /// // safely, and we can't use the `[value; 1000]`
    /// // constructor because it only works with `Copy` data.
    /// let mut data: [Vec<u32>; 1000];
    ///
    /// unsafe {
    ///     // So we need to do this to initialize it.
    ///     data = mem::uninitialized();
    ///
    ///     // DANGER ZONE: if anything panics or otherwise
    ///     // incorrectly reads the array here, we will have
    ///     // Undefined Behavior.
    ///
    ///     // It's ok to mutably iterate the data, since this
    ///     // doesn't involve reading it at all.
    ///     // (ptr and len are statically known for arrays)
    ///     for elem in &mut data[..] {
    ///         // *elem = Vec::new() would try to drop the
    ///         // uninitialized memory at `elem` -- bad!
    ///         //
    ///         // Vec::new doesn't allocate or do really
    ///         // anything. It's only safe to call here
    ///         // because we know it won't panic.
    ///         ptr::write(elem, Vec::new());
    ///     }
    ///
    ///     // SAFE ZONE: everything is initialized.
    /// }
    ///
    /// println!("{:?}", &data[0]);
    /// ```
    ///
    /// This example emphasizes exactly how delicate and dangerous doing this is.
    /// Note that the `vec!` macro *does* let you initialize every element with a
    /// value that is only `Clone`, so the following is semantically equivalent and
    /// vastly less dangerous, as long as you can live with an extra heap
    /// allocation:
    ///
    /// ```
    /// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000];
    /// println!("{:?}", &data[0]);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn uninitialized<T>() -> T {
        intrinsics::uninit()
    }

    /// Swap the values at two mutable locations of the same type, without deinitializing or copying
    /// either one.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// let x = &mut 5;
    /// let y = &mut 42;
    ///
    /// mem::swap(x, y);
    ///
    /// assert_eq!(42, *x);
    /// assert_eq!(5, *y);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn swap<T>(x: &mut T, y: &mut T) {
        unsafe {
            // Give ourselves some scratch space to work with
            let mut t: T = uninitialized();

            // Perform the swap, `&mut` pointers never alias
            ptr::copy_nonoverlapping(&*x, &mut t, 1);
            ptr::copy_nonoverlapping(&*y, x, 1);
            ptr::copy_nonoverlapping(&t, y, 1);

            // y and t now point to the same thing, but we need to completely
            // forget `t` because we do not want to run the destructor for `T`
            // on its value, which is still owned somewhere outside this function.
            forget(t);
        }
    }

    /// Replaces the value at a mutable location with a new one, returning the old value, without
    /// deinitializing or copying either one.
    ///
    /// This is primarily used for transferring and swapping ownership of a value in a mutable
    /// location.
    ///
    /// # Examples
    ///
    /// A simple example:
    ///
    /// ```
    /// use std::mem;
    ///
    /// let mut v: Vec<i32> = Vec::new();
    ///
    /// mem::replace(&mut v, Vec::new());
    /// ```
    ///
    /// This function allows consumption of one field of a struct by replacing it with another value.
    /// The normal approach doesn't always work:
    ///
    /// ```rust,ignore
    /// struct Buffer<T> { buf: Vec<T> }
    ///
    /// impl<T> Buffer<T> {
    ///     fn get_and_reset(&mut self) -> Vec<T> {
    ///         // error: cannot move out of dereference of `&mut`-pointer
    ///         let buf = self.buf;
    ///         self.buf = Vec::new();
    ///         buf
    ///     }
    /// }
    /// ```
    ///
    /// Note that `T` does not necessarily implement `Clone`, so it can't even clone and reset
    /// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
    /// `self`, allowing it to be returned:
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// use std::mem;
    /// # struct Buffer<T> { buf: Vec<T> }
    /// impl<T> Buffer<T> {
    ///     fn get_and_reset(&mut self) -> Vec<T> {
    ///         mem::replace(&mut self.buf, Vec::new())
    ///     }
    /// }
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn replace<T>(dest: &mut T, mut src: T) -> T {
        swap(dest, &mut src);
        src
    }

    /// Disposes of a value.
    ///
    /// While this does call the argument's implementation of `Drop`, it will not
    /// release any borrows, as borrows are based on lexical scope.
    ///
    /// This effectively does nothing for
    /// [types which implement `Copy`](../../book/ownership.html#copy-types),
    /// e.g. integers. Such values are copied and _then_ moved into the function,
    /// so the value persists after this function call.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let v = vec![1, 2, 3];
    ///
    /// drop(v); // explicitly drop the vector
    /// ```
    ///
    /// Borrows are based on lexical scope, so this produces an error:
    ///
    /// ```ignore
    /// let mut v = vec![1, 2, 3];
    /// let x = &v[0];
    ///
    /// drop(x); // explicitly drop the reference, but the borrow still exists
    ///
    /// v.push(4); // error: cannot borrow `v` as mutable because it is also
    ///            // borrowed as immutable
    /// ```
    ///
    /// An inner scope is needed to fix this:
    ///
    /// ```
    /// let mut v = vec![1, 2, 3];
    ///
    /// {
    ///     let x = &v[0];
    ///
    ///     drop(x); // this is now redundant, as `x` is going out of scope anyway
    /// }
    ///
    /// v.push(4); // no problems
    /// ```
    ///
    /// Since `RefCell` enforces the borrow rules at runtime, `drop()` can
    /// seemingly release a borrow of one:
    ///
    /// ```
    /// use std::cell::RefCell;
    ///
    /// let x = RefCell::new(1);
    ///
    /// let mut mutable_borrow = x.borrow_mut();
    /// *mutable_borrow = 1;
    ///
    /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
    ///
    /// let borrow = x.borrow();
    /// println!("{}", *borrow);
    /// ```
    ///
    /// Integers and other types implementing `Copy` are unaffected by `drop()`
    ///
    /// ```
    /// #[derive(Copy, Clone)]
    /// struct Foo(u8);
    ///
    /// let x = 1;
    /// let y = Foo(2);
    /// drop(x); // a copy of `x` is moved and dropped
    /// drop(y); // a copy of `y` is moved and dropped
    ///
    /// println!("x: {}, y: {}", x, y.0); // still available
    /// ```
    ///
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn drop<T>(_x: T) { }

    macro_rules! repeat_u8_as_u32 {
        ($name:expr) => { (($name as u32) << 24 |
                           ($name as u32) << 16 |
                           ($name as u32) <<  8 |
                           ($name as u32)) }
    }
    macro_rules! repeat_u8_as_u64 {
        ($name:expr) => { ((repeat_u8_as_u32!($name) as u64) << 32 |
                           (repeat_u8_as_u32!($name) as u64)) }
    }

    // NOTE: Keep synchronized with values used in librustc_trans::trans::adt.
    //
    // In particular, the POST_DROP_U8 marker must never equal the
    // DTOR_NEEDED_U8 marker.
    //
    // For a while pnkfelix was using 0xc1 here.
    // But having the sign bit set is a pain, so 0x1d is probably better.
    //
    // And of course, 0x00 brings back the old world of zero'ing on drop.
    #[unstable(feature = "filling_drop", issue = "5016")]
    #[allow(missing_docs)]
    pub const POST_DROP_U8: u8 = 0x1d;
    #[unstable(feature = "filling_drop", issue = "5016")]
    #[allow(missing_docs)]
    pub const POST_DROP_U32: u32 = repeat_u8_as_u32!(POST_DROP_U8);
    #[unstable(feature = "filling_drop", issue = "5016")]
    #[allow(missing_docs)]
    pub const POST_DROP_U64: u64 = repeat_u8_as_u64!(POST_DROP_U8);

    #[cfg(target_pointer_width = "32")]
    #[unstable(feature = "filling_drop", issue = "5016")]
    #[allow(missing_docs)]
    pub const POST_DROP_USIZE: usize = POST_DROP_U32 as usize;
    #[cfg(target_pointer_width = "64")]
    #[unstable(feature = "filling_drop", issue = "5016")]
    #[allow(missing_docs)]
    pub const POST_DROP_USIZE: usize = POST_DROP_U64 as usize;

    /// Interprets `src` as `&U`, and then reads `src` without moving the contained
    /// value.
    ///
    /// This function will unsafely assume the pointer `src` is valid for
    /// `sizeof(U)` bytes by transmuting `&T` to `&U` and then reading the `&U`. It
    /// will also unsafely create a copy of the contained value instead of moving
    /// out of `src`.
    ///
    /// It is not a compile-time error if `T` and `U` have different sizes, but it
    /// is highly encouraged to only invoke this function where `T` and `U` have the
    /// same size. This function triggers undefined behavior if `U` is larger than
    /// `T`.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// let one = unsafe { mem::transmute_copy(&1) };
    ///
    /// assert_eq!(1, one);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
        // FIXME(#23542) Replace with type ascription.
        #![allow(trivial_casts)]
        ptr::read(src as *const T as *const U)
    }

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