Fix documentation on no-std targets

    error[E0463]: can't find crate for `std`
      |
      = note: the `thumbv7m-none-eabi` target may not support the standard library
      = note: `std` is required by `dyn_clone` because it does not declare `#![no_std]`
      = help: consider building the standard library from source with `cargo build -Zbuild-std`

    error: cannot find macro `assert_eq` in this scope
       --> src/lib.rs:144:9
        |
    144 |         assert_eq!(*data_ptr as *const (), t as *const T as *const ());
        |         ^^^^^^^^^
        |
    help: consider importing this macro
        |
    116 + use core::assert_eq;
        |

    error[E0405]: cannot find trait `Clone` in this scope
       --> src/lib.rs:110:13
        |
    110 |     impl<T: Clone> Sealed for T {}
        |             ^^^^^ not found in this scope
        |
    help: consider importing one of these items
        |
    109 +     use core::clone::Clone;
        |
    109 +     use crate::__private::Clone;
        |

    error[E0405]: cannot find trait `Clone` in this scope
       --> src/lib.rs:112:13
        |
    112 |     impl<T: Clone> Sealed for [T] {}
        |             ^^^^^ not found in this scope
        |
    help: consider importing one of these items
        |
    109 +     use core::clone::Clone;
        |
    109 +     use crate::__private::Clone;
        |

    error[E0405]: cannot find trait `Sized` in this scope
       --> src/lib.rs:139:9
        |
    139 |     T: ?Sized + DynClone,
        |         ^^^^^ not found in this scope
        |
    help: consider importing this trait
        |
    116 + use core::marker::Sized;
        |

    error[E0405]: cannot find trait `Sized` in this scope
       --> src/lib.rs:153:9
        |
    153 |     T: ?Sized + DynClone,
        |         ^^^^^ not found in this scope
        |
    help: consider importing this trait
        |
    116 + use core::marker::Sized;
        |

    error[E0405]: cannot find trait `Sized` in this scope
       --> src/lib.rs:179:9
        |
    179 |     T: ?Sized + DynClone,
        |         ^^^^^ not found in this scope
        |
    help: consider importing this trait
        |
    116 + use core::marker::Sized;
        |

    error[E0405]: cannot find trait `Clone` in this scope
       --> src/lib.rs:192:8
        |
    192 |     T: Clone,
        |        ^^^^^ not found in this scope
        |
    help: consider importing one of these items
        |
    116 + use core::clone::Clone;
        |
    116 + use crate::__private::Clone;
        |

    error[E0405]: cannot find trait `Clone` in this scope
       --> src/lib.rs:207:8
        |
    207 |     T: Clone,
        |        ^^^^^ not found in this scope
        |
    help: consider importing one of these items
        |
    116 + use core::clone::Clone;
        |
    116 + use crate::__private::Clone;
        |

    warning: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
       --> src/lib.rs:139:8
        |
    139 |     T: ?Sized + DynClone,
        |        ^^^^^^

    warning: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
       --> src/lib.rs:153:8
        |
    153 |     T: ?Sized + DynClone,
        |        ^^^^^^

    warning: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
       --> src/lib.rs:179:8
        |
    179 |     T: ?Sized + DynClone,
        |        ^^^^^^

    Some errors have detailed explanations: E0405, E0463.
    For more information about an error, try `rustc --explain E0405`.
    warning: `dyn-clone` (lib doc) generated 3 warnings
    error: could not document `dyn-clone`

src/lib.rs

@@ -81,7 +81,7 @@
 //! ```
 
 #![doc(html_root_url = "https://docs.rs/dyn_clone/1.0.14")]
-#![cfg_attr(not(doc), no_std)]
+#![no_std]
 #![allow(
     clippy::missing_panics_doc,
     clippy::needless_doctest_main,
@@ -90,6 +90,9 @@
 
 extern crate alloc;
 
+#[cfg(doc)]
+extern crate core as std;
+
 #[macro_use]
 mod macros;