Trait rustc_std_workspace_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

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!

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;

You can also implement Copy and Clone manually:

struct MyStruct;

impl Copy for MyStruct { }

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

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,
}

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

struct PointList {
    points: Vec<Point>,
}

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,
}

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 DIR

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impl Copy for FILE

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impl Copy for fpos_t

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impl Copy for timezone

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impl Copy for fpos64_t

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impl Copy for AsciiChar

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impl Copy for rustc_std_workspace_std::cmp::Ordering

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impl Copy for Infallible

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impl Copy for rustc_std_workspace_std::fmt::Alignment

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impl Copy for ErrorKind

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impl Copy for SeekFrom

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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 Which

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impl Copy for SearchStep

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impl Copy for rustc_std_workspace_std::sync::atomic::Ordering

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impl Copy for RecvTimeoutError

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impl Copy for TryRecvError

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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 usize

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impl Copy for TimSortRun

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impl Copy for in6_addr

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impl Copy for termios2

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impl Copy for sem_t

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impl Copy for msqid_ds

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impl Copy for semid_ds

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impl Copy for sigset_t

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impl Copy for sysinfo

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impl Copy for clone_args

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impl Copy for max_align_t

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impl Copy for statvfs

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impl Copy for _libc_fpstate

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impl Copy for _libc_fpxreg

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impl Copy for _libc_xmmreg

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impl Copy for flock64

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impl Copy for flock

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impl Copy for ipc_perm

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impl Copy for mcontext_t

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impl Copy for pthread_attr_t

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impl Copy for ptrace_rseq_configuration

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impl Copy for seccomp_notif_sizes

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impl Copy for shmid_ds

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impl Copy for sigaction

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impl Copy for siginfo_t

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impl Copy for stack_t

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impl Copy for stat64

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impl Copy for stat

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impl Copy for statfs64

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impl Copy for statfs

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impl Copy for statvfs64

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impl Copy for ucontext_t

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impl Copy for user

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impl Copy for user_fpregs_struct

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impl Copy for user_regs_struct

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impl Copy for Elf32_Chdr

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impl Copy for Elf64_Chdr

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impl Copy for __c_anonymous_ptrace_syscall_info_entry

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impl Copy for __c_anonymous_ptrace_syscall_info_exit

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impl Copy for __c_anonymous_ptrace_syscall_info_seccomp

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impl Copy for __exit_status

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impl Copy for __timeval

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impl Copy for aiocb

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impl Copy for cmsghdr

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impl Copy for glob64_t

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impl Copy for mallinfo2

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impl Copy for mallinfo

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impl Copy for msghdr

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impl Copy for nl_mmap_hdr

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impl Copy for nl_mmap_req

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impl Copy for nl_pktinfo

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impl Copy for ntptimeval

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impl Copy for ptrace_peeksiginfo_args

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impl Copy for ptrace_syscall_info

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impl Copy for regex_t

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impl Copy for rtentry

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impl Copy for seminfo

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impl Copy for statx

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impl Copy for statx_timestamp

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impl Copy for termios

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impl Copy for timex

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impl Copy for utmpx

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impl Copy for open_how

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impl Copy for Elf32_Ehdr

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impl Copy for Elf32_Phdr

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impl Copy for Elf32_Shdr

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impl Copy for Elf32_Sym

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impl Copy for Elf64_Ehdr

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impl Copy for Elf64_Phdr

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impl Copy for Elf64_Shdr

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impl Copy for Elf64_Sym

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impl Copy for __c_anonymous_ifru_map

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impl Copy for __c_anonymous_sockaddr_can_j1939

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impl Copy for __c_anonymous_sockaddr_can_tp

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impl Copy for af_alg_iv

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impl Copy for arpd_request

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impl Copy for can_filter

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impl Copy for can_frame

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impl Copy for canfd_frame

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impl Copy for canxl_frame

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impl Copy for cpu_set_t

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impl Copy for dirent64

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impl Copy for dirent

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impl Copy for dl_phdr_info

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impl Copy for dqblk

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impl Copy for fanotify_event_metadata

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impl Copy for fanotify_response

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impl Copy for ff_condition_effect

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impl Copy for ff_constant_effect

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impl Copy for ff_effect

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impl Copy for ff_envelope

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impl Copy for ff_periodic_effect

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impl Copy for ff_ramp_effect

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impl Copy for ff_replay

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impl Copy for ff_rumble_effect

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impl Copy for ff_trigger

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impl Copy for file_clone_range

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impl Copy for fsid_t

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impl Copy for genlmsghdr

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impl Copy for glob_t

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impl Copy for hwtstamp_config

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impl Copy for if_nameindex

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impl Copy for ifreq

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impl Copy for in6_ifreq

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impl Copy for in6_pktinfo

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impl Copy for inotify_event

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impl Copy for input_absinfo

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impl Copy for input_event

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impl Copy for input_id

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impl Copy for input_keymap_entry

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impl Copy for input_mask

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impl Copy for itimerspec

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impl Copy for j1939_filter

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impl Copy for mntent

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impl Copy for mq_attr

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impl Copy for msginfo

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impl Copy for nlattr

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impl Copy for nlmsgerr

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impl Copy for nlmsghdr

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impl Copy for packet_mreq

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impl Copy for passwd

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impl Copy for posix_spawn_file_actions_t

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impl Copy for posix_spawnattr_t

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impl Copy for pthread_barrier_t

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impl Copy for pthread_barrierattr_t

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impl Copy for pthread_cond_t

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impl Copy for pthread_condattr_t

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impl Copy for pthread_mutex_t

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impl Copy for pthread_mutexattr_t

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impl Copy for pthread_rwlock_t

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impl Copy for pthread_rwlockattr_t

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impl Copy for regmatch_t

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impl Copy for rlimit64

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impl Copy for sctp_authinfo

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impl Copy for sctp_initmsg

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impl Copy for sctp_nxtinfo

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impl Copy for sctp_prinfo

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impl Copy for sctp_rcvinfo

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impl Copy for sctp_sndinfo

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impl Copy for sctp_sndrcvinfo

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impl Copy for seccomp_data

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impl Copy for sembuf

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impl Copy for signalfd_siginfo

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impl Copy for sock_extended_err

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impl Copy for sock_filter

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impl Copy for sock_fprog

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impl Copy for sock_txtime

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impl Copy for sockaddr_alg

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impl Copy for sockaddr_can

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impl Copy for sockaddr_nl

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impl Copy for sockaddr_vm

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impl Copy for spwd

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impl Copy for ucred

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impl Copy for uinput_abs_setup

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impl Copy for uinput_ff_erase

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impl Copy for uinput_ff_upload

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impl Copy for uinput_setup

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impl Copy for uinput_user_dev

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impl Copy for Dl_info

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impl Copy for addrinfo

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impl Copy for arphdr

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impl Copy for arpreq

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impl Copy for arpreq_old

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impl Copy for epoll_event

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impl Copy for fd_set

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impl Copy for ifaddrs

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impl Copy for in6_rtmsg

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impl Copy for in_addr

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impl Copy for in_pktinfo

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impl Copy for ip_mreq

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impl Copy for ip_mreq_source

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impl Copy for ip_mreqn

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impl Copy for lconv

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impl Copy for mmsghdr

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impl Copy for sched_param

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impl Copy for sigevent

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impl Copy for sockaddr

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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 sockaddr_ll

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impl Copy for sockaddr_storage

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impl Copy for sockaddr_un

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impl Copy for tm

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impl Copy for utsname

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impl Copy for group

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impl Copy for hostent

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impl Copy for iovec

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impl Copy for ipv6_mreq

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impl Copy for itimerval

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impl Copy for linger

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impl Copy for pollfd

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impl Copy for protoent

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impl Copy for rlimit

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impl Copy for rusage

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impl Copy for servent

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impl Copy for sigval

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impl Copy for timespec

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impl Copy for timeval

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impl Copy for tms

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impl Copy for utimbuf

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impl Copy for winsize

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impl Copy for AllocError

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impl Copy for Global

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impl Copy for Layout

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impl Copy for System

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impl Copy for TypeId

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impl Copy for CpuidResult

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impl Copy for __m128

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impl Copy for __m128bh

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impl Copy for __m128d

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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

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impl Copy for __m512

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impl Copy for __m512bh

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impl Copy for __m512d

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impl Copy for __m512i

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impl Copy for TryFromSliceError

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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 FileTimes

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impl Copy for 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

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impl Copy for NonZeroI16

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impl Copy for NonZeroI32

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impl Copy for NonZeroI64

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impl Copy for NonZeroI128

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impl Copy for NonZeroIsize

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impl Copy for NonZeroU8

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impl Copy for NonZeroU16

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impl Copy for NonZeroU32

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impl Copy for NonZeroU64

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impl Copy for NonZeroU128

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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 UCred

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impl Copy for ExitCode

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impl Copy for ExitStatus

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impl Copy for ExitStatusError

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impl Copy for rustc_std_workspace_std::ptr::Alignment

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impl Copy for Utf8Error

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impl Copy for RecvError

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impl Copy for WaitTimeoutResult

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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 Instant

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impl Copy for SystemTime

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impl Copy for PhantomPinned

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impl Copy for __c_anonymous_ptrace_syscall_info_data

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impl Copy for __c_anonymous_ifr_ifru

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impl Copy for __c_anonymous_sockaddr_can_can_addr

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impl Copy for _Unwind_Action

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impl Copy for _Unwind_Reason_Code

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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 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, 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<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<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 TrySendError<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!

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

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impl<T> Copy for Discriminant<T>

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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 SendError<T>where T: Copy,

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