Trait std::marker::Copy

1.0.0 · source ·
pub trait Copy: Clone { }
Expand description

Types whose values can be duplicated simply by copying bits.

By default, variable bindings have ‘move semantics.’ In other words:

#[derive(Debug)]
struct Foo;

let x = Foo;

let y = x;

// `x` has moved into `y`, and so cannot be used

// println!("{x:?}"); // error: use of moved value
Run

However, if a type implements Copy, it instead has ‘copy semantics’:

// We can derive a `Copy` implementation. `Clone` is also required, as it's
// a supertrait of `Copy`.
#[derive(Debug, Copy, Clone)]
struct Foo;

let x = Foo;

let y = x;

// `y` is a copy of `x`

println!("{x:?}"); // A-OK!
Run

It’s important to note that in these two examples, the only difference is whether you are allowed to access x after the assignment. Under the hood, both a copy and a move can result in bits being copied in memory, although this is sometimes optimized away.

How can I implement Copy?

There are two ways to implement Copy on your type. The simplest is to use derive:

#[derive(Copy, Clone)]
struct MyStruct;
Run

You can also implement Copy and Clone manually:

struct MyStruct;

impl Copy for MyStruct { }

impl Clone for MyStruct {
    fn clone(&self) -> MyStruct {
        *self
    }
}
Run

There is a small difference between the two: the derive strategy will also place a Copy bound on type parameters, which isn’t always desired.

What’s the difference between Copy and Clone?

Copies happen implicitly, for example as part of an assignment y = x. The behavior of Copy is not overloadable; it is always a simple bit-wise copy.

Cloning is an explicit action, x.clone(). The implementation of Clone can provide any type-specific behavior necessary to duplicate values safely. For example, the implementation of Clone for String needs to copy the pointed-to string buffer in the heap. A simple bitwise copy of String values would merely copy the pointer, leading to a double free down the line. For this reason, String is Clone but not Copy.

Clone is a supertrait of Copy, so everything which is Copy must also implement Clone. If a type is Copy then its Clone implementation only needs to return *self (see the example above).

When can my type be Copy?

A type can implement Copy if all of its components implement Copy. For example, this struct can be Copy:

#[derive(Copy, Clone)]
struct Point {
   x: i32,
   y: i32,
}
Run

A struct can be Copy, and i32 is Copy, therefore Point is eligible to be Copy. By contrast, consider

struct PointList {
    points: Vec<Point>,
}
Run

The struct PointList cannot implement Copy, because Vec<T> is not Copy. If we attempt to derive a Copy implementation, we’ll get an error:

the trait `Copy` cannot be implemented for this type; field `points` does not implement `Copy`

Shared references (&T) are also Copy, so a type can be Copy, even when it holds shared references of types T that are not Copy. Consider the following struct, which can implement Copy, because it only holds a shared reference to our non-Copy type PointList from above:

#[derive(Copy, Clone)]
struct PointListWrapper<'a> {
    point_list_ref: &'a PointList,
}
Run

When can’t my type be Copy?

Some types can’t be copied safely. For example, copying &mut T would create an aliased mutable reference. Copying String would duplicate responsibility for managing the String’s buffer, leading to a double free.

Generalizing the latter case, any type implementing Drop can’t be Copy, because it’s managing some resource besides its own size_of::<T> bytes.

If you try to implement Copy on a struct or enum containing non-Copy data, you will get the error E0204.

When should my type be Copy?

Generally speaking, if your type can implement Copy, it should. Keep in mind, though, that implementing Copy is part of the public API of your type. If the type might become non-Copy in the future, it could be prudent to omit the Copy implementation now, to avoid a breaking API change.

Additional implementors

In addition to the implementors listed below, the following types also implement Copy:

  • Function item types (i.e., the distinct types defined for each function)
  • Function pointer types (e.g., fn() -> i32)
  • Closure types, if they capture no value from the environment or if all such captured values implement Copy themselves. Note that variables captured by shared reference always implement Copy (even if the referent doesn’t), while variables captured by mutable reference never implement Copy.

Implementors§

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impl Copy for AsciiChar

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impl Copy for StackFrame

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impl Copy for PrintFmt

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impl Copy for std::cmp::Ordering

1.34.0 · source§

impl Copy for Infallible

1.28.0 · source§

impl Copy for std::fmt::Alignment

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impl Copy for SeekFrom

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impl Copy for ErrorKind

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impl Copy for IpAddr

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impl Copy for Ipv6MulticastScope

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impl Copy for Shutdown

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impl Copy for SocketAddr

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impl Copy for FpCategory

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impl Copy for BacktraceStyle

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impl Copy for State

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impl Copy for Which

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impl Copy for SearchStep

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impl Copy for std::sync::atomic::Ordering

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impl Copy for Selected

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impl Copy for RecvTimeoutError

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impl Copy for TryRecvError

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impl Copy for ReparsePoint

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impl Copy for bool

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impl Copy for char

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impl Copy for f32

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impl Copy for f64

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impl Copy for i8

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impl Copy for i16

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impl Copy for i32

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impl Copy for i64

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impl Copy for i128

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impl Copy for isize

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impl Copy for !

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impl Copy for u8

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impl Copy for u16

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impl Copy for u32

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impl Copy for u64

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impl Copy for u128

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impl Copy for ()

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impl Copy for usize

1.27.0 · source§

impl Copy for CpuidResult

1.27.0 · source§

impl Copy for __m128

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impl Copy for __m128bh

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impl Copy for __m128d

1.27.0 · source§

impl Copy for __m128i

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impl Copy for __m256

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impl Copy for __m256bh

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impl Copy for __m256d

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impl Copy for __m256i

1.73.0-nightly · source§

impl Copy for __m512

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impl Copy for __m512bh

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impl Copy for __m512d

1.73.0-nightly · source§

impl Copy for __m512i

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impl Copy for TimSortRun

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impl Copy for AllocError

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impl Copy for Global

1.28.0 · source§

impl Copy for Layout

1.28.0 · source§

impl Copy for System

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impl Copy for TypeId

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impl Copy for TryFromSliceError

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impl Copy for Frame

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impl Copy for ADDRESS64

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impl Copy for std::backtrace_rs::windows::CONTEXT

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impl Copy for std::backtrace_rs::windows::FLOATING_SAVE_AREA

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impl Copy for IMAGEHLP_LINEW64

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impl Copy for KDHELP64

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impl Copy for std::backtrace_rs::windows::M128A

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impl Copy for MODULEENTRY32W

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impl Copy for STACKFRAME64

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impl Copy for STACKFRAME_EX

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impl Copy for SYMBOL_INFOW

1.34.0 · source§

impl Copy for CharTryFromError

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impl Copy for TryFromCharError

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impl Copy for Error

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impl Copy for std::fs::FileTimes

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impl Copy for std::fs::FileType

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impl Copy for Empty

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impl Copy for Sink

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impl Copy for Assume

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impl Copy for Ipv4Addr

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impl Copy for Ipv6Addr

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impl Copy for SocketAddrV4

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impl Copy for SocketAddrV6

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impl Copy for NonZeroI8

1.34.0 · source§

impl Copy for NonZeroI16

1.34.0 · source§

impl Copy for NonZeroI32

1.34.0 · source§

impl Copy for NonZeroI64

1.34.0 · source§

impl Copy for NonZeroI128

1.34.0 · source§

impl Copy for NonZeroIsize

1.28.0 · source§

impl Copy for NonZeroU8

1.28.0 · source§

impl Copy for NonZeroU16

1.28.0 · source§

impl Copy for NonZeroU32

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impl Copy for NonZeroU64

1.28.0 · source§

impl Copy for NonZeroU128

1.28.0 · source§

impl Copy for NonZeroUsize

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impl Copy for TryFromIntError

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impl Copy for RangeFull

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impl Copy for std::process::ExitCode

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impl Copy for std::process::ExitStatus

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impl Copy for std::process::ExitStatusError

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impl Copy for std::ptr::Alignment

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impl Copy for Utf8Error

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impl Copy for WaitTimeoutResult

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impl Copy for Operation

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impl Copy for RecvError

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impl Copy for in6_addr

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impl Copy for in_addr

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impl Copy for sockaddr_in6

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impl Copy for sockaddr_in

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impl Copy for ADDRINFOA

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impl Copy for ARM64_NT_NEON128_0

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impl Copy for BY_HANDLE_FILE_INFORMATION

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impl Copy for CONSOLE_READCONSOLE_CONTROL

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impl Copy for std::sys::windows::c::windows_sys::CONTEXT

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impl Copy for CONTEXT_0_0

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impl Copy for EXCEPTION_RECORD

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impl Copy for FD_SET

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impl Copy for FILETIME

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impl Copy for FILE_ATTRIBUTE_TAG_INFO

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impl Copy for FILE_BASIC_INFO

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impl Copy for FILE_DISPOSITION_INFO

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impl Copy for FILE_DISPOSITION_INFO_EX

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impl Copy for FILE_END_OF_FILE_INFO

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impl Copy for FILE_ID_BOTH_DIR_INFO

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impl Copy for FILE_STANDARD_INFO

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impl Copy for std::sys::windows::c::windows_sys::FLOATING_SAVE_AREA

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impl Copy for GUID

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impl Copy for IN6_ADDR

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impl Copy for IN_ADDR

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impl Copy for IN_ADDR_0_0

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impl Copy for IN_ADDR_0_1

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impl Copy for IO_STATUS_BLOCK

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impl Copy for IPV6_MREQ

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impl Copy for IP_MREQ

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impl Copy for LINGER

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impl Copy for std::sys::windows::c::windows_sys::M128A

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impl Copy for OBJECT_ATTRIBUTES

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impl Copy for OVERLAPPED

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impl Copy for OVERLAPPED_0_0

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impl Copy for PROCESS_INFORMATION

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impl Copy for RTL_CONDITION_VARIABLE

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impl Copy for RTL_SRWLOCK

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impl Copy for SECURITY_ATTRIBUTES

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impl Copy for SOCKADDR

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impl Copy for STARTUPINFOW

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impl Copy for SYSTEM_INFO

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impl Copy for SYSTEM_INFO_0_0

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impl Copy for TIMEVAL

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impl Copy for UNICODE_STRING

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impl Copy for WIN32_FIND_DATAW

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impl Copy for WSABUF

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impl Copy for WSADATA

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impl Copy for WSAPROTOCOLCHAIN

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impl Copy for WSAPROTOCOL_INFOW

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impl Copy for XSAVE_FORMAT

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impl Copy for Module

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impl Copy for std::sys::windows::fs::FileTimes

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impl Copy for std::sys::windows::fs::FileType

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impl Copy for std::sys::windows::process::ExitCode

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impl Copy for std::sys::windows::process::ExitStatus

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impl Copy for std::sys::windows::process::ExitStatusError

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impl Copy for std::sys::windows::time::Instant

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impl Copy for std::sys::windows::time::SystemTime

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impl Copy for CodePoint

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impl Copy for RawWakerVTable

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impl Copy for AccessError

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impl Copy for ThreadId

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impl Copy for Duration

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impl Copy for std::time::Instant

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impl Copy for std::time::SystemTime

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impl Copy for PhantomPinned

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impl Copy for ARM64_NT_NEON128

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impl Copy for CONTEXT_0

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impl Copy for IN6_ADDR_0

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impl Copy for IN_ADDR_0

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impl Copy for IO_STATUS_BLOCK_0

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impl Copy for OVERLAPPED_0

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impl Copy for RTL_RUN_ONCE

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impl Copy for SYSTEM_INFO_0

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impl<'a> Copy for Component<'a>

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impl<'a> Copy for Prefix<'a>

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impl<'a> Copy for Arguments<'a>

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impl<'a> Copy for std::io::IoSlice<'a>

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impl<'a> Copy for Location<'a>

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impl<'a> Copy for Ancestors<'a>

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impl<'a> Copy for PrefixComponent<'a>

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impl<'a> Copy for EHContext<'a>

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impl<'a> Copy for std::sys::windows::io::IoSlice<'a>

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impl<'a, T, const N: usize> Copy for ArrayWindows<'a, T, N>where T: Copy + 'a,

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impl<'fd> Copy for BorrowedFd<'fd>

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impl<'handle> Copy for BorrowedHandle<'handle>

Available on Windows only.
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impl<'socket> Copy for BorrowedSocket<'socket>

Available on Windows only.
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impl<B, C> Copy for ControlFlow<B, C>where B: Copy, C: Copy,

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impl<Dyn> Copy for DynMetadata<Dyn>where Dyn: ?Sized,

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impl<F> Copy for RepeatWith<F>where F: Copy,

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impl<Idx> Copy for RangeTo<Idx>where Idx: Copy,

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impl<Idx> Copy for RangeToInclusive<Idx>where Idx: Copy,

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impl<P> Copy for Pin<P>where P: Copy,

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impl<Ret, T> Copy for fn (T₁, T₂, …, Tₙ) -> Ret

This trait is implemented on function pointers with any number of arguments.

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impl<T> Copy for Bound<T>where T: Copy,

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impl<T> Copy for Option<T>where T: Copy,

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impl<T> Copy for Poll<T>where T: Copy,

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impl<T> Copy for *const Twhere T: ?Sized,

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impl<T> Copy for *mut Twhere T: ?Sized,

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impl<T> Copy for &Twhere T: ?Sized,

Shared references can be copied, but mutable references cannot!

1.19.0 · source§

impl<T> Copy for Reverse<T>where T: Copy,

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impl<T> Copy for Discriminant<T>

1.20.0 · source§

impl<T> Copy for ManuallyDrop<T>where T: Copy + ?Sized,

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impl<T> Copy for Saturating<T>where T: Copy,

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impl<T> Copy for Wrapping<T>where T: Copy,

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impl<T> Copy for NonNull<T>where T: ?Sized,

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impl<T> Copy for PhantomData<T>where T: ?Sized,

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impl<T> Copy for MaybeUninit<T>where T: Copy,

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impl<T, E> Copy for Result<T, E>where T: Copy, E: Copy,

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impl<T, const LANES: usize> Copy for Mask<T, LANES>where T: MaskElement, LaneCount<LANES>: SupportedLaneCount,

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impl<T, const N: usize> Copy for [T; N]where T: Copy,

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impl<T, const N: usize> Copy for Simd<T, N>where LaneCount<N>: SupportedLaneCount, T: SimdElement,

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impl<T: Copy + ?Sized> Copy for Align8<T>

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impl<T: Copy> Copy for SendTimeoutError<T>

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impl<T: Copy> Copy for TrySendError<T>

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impl<T: Copy> Copy for (T₁, T₂, …, Tₙ)

This trait is implemented on arbitrary-length tuples.

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impl<T: Copy> Copy for CachePadded<T>

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impl<T: Copy> Copy for SendError<T>

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impl<Y, R> Copy for GeneratorState<Y, R>where Y: Copy, R: Copy,