Create an internal prelude

When building with `rustc-dep-of-std`, we don't get the core types
imported by default (`Clone`, `Copy`, `Option`). In order to avoid
needing to import these individually, introduce a prelude that includes
them, along with commonly used C numeric types.

This allows cleaning up some of the `use` statements.

(backport <#4161>)
(cherry picked from commit 30bc78b)

src/lib.rs

@@ -33,35 +33,10 @@ mod macros;
 cfg_if! {
     if #[cfg(feature = "rustc-dep-of-std")] {
         extern crate rustc_std_workspace_core as core;
-        #[allow(unused_imports)]
-        use core::iter;
-        #[allow(unused_imports)]
-        use core::ops;
-        #[allow(unused_imports)]
-        use core::option;
     }
 }
 
-#[doc(hidden)]
-#[allow(unused_imports)]
-use core::clone::Clone;
-#[allow(unused_imports)]
-use core::ffi;
 pub use core::ffi::c_void;
-#[allow(unused_imports)]
-use core::fmt;
-#[allow(unused_imports)]
-use core::hash;
-#[doc(hidden)]
-#[allow(unused_imports)]
-use core::marker::{Copy, Send, Sync};
-#[allow(unused_imports)]
-use core::mem;
-#[allow(unused_imports)]
-use core::num;
-#[doc(hidden)]
-#[allow(unused_imports)]
-use core::option::Option;
 
 cfg_if! {
     if #[cfg(windows)] {
@@ -70,78 +45,104 @@ cfg_if! {
 
         mod windows;
         pub use crate::windows::*;
+
+        prelude!();
     } else if #[cfg(target_os = "fuchsia")] {
         mod fixed_width_ints;
         pub use crate::fixed_width_ints::*;
 
         mod fuchsia;
         pub use crate::fuchsia::*;
+
+        prelude!();
     } else if #[cfg(target_os = "switch")] {
         mod fixed_width_ints;
         pub use fixed_width_ints::*;
 
         mod switch;
         pub use switch::*;
+
+        prelude!();
     } else if #[cfg(target_os = "psp")] {
         mod fixed_width_ints;
         pub use crate::fixed_width_ints::*;
 
         mod psp;
         pub use crate::psp::*;
+
+        prelude!();
     } else if #[cfg(target_os = "vxworks")] {
         mod fixed_width_ints;
         pub use crate::fixed_width_ints::*;
 
         mod vxworks;
         pub use crate::vxworks::*;
+
+        prelude!();
     } else if #[cfg(target_os = "solid_asp3")] {
         mod fixed_width_ints;
         pub use crate::fixed_width_ints::*;
 
         mod solid;
         pub use crate::solid::*;
+
+        prelude!();
     } else if #[cfg(unix)] {
         mod fixed_width_ints;
         pub use crate::fixed_width_ints::*;
 
         mod unix;
         pub use crate::unix::*;
+
+        prelude!();
     } else if #[cfg(target_os = "hermit")] {
         mod fixed_width_ints;
         pub use crate::fixed_width_ints::*;
 
         mod hermit;
         pub use crate::hermit::*;
+
+        prelude!();
     } else if #[cfg(target_os = "teeos")] {
         mod fixed_width_ints;
         pub use fixed_width_ints::*;
 
         mod teeos;
         pub use teeos::*;
+
+        prelude!();
     } else if #[cfg(target_os = "trusty")] {
         mod fixed_width_ints;
         pub use crate::fixed_width_ints::*;
 
         mod trusty;
         pub use crate::trusty::*;
+
+        prelude!();
     } else if #[cfg(all(target_env = "sgx", target_vendor = "fortanix"))] {
         mod fixed_width_ints;
         pub use crate::fixed_width_ints::*;
 
         mod sgx;
         pub use crate::sgx::*;
+
+        prelude!();
     } else if #[cfg(any(target_env = "wasi", target_os = "wasi"))] {
         mod fixed_width_ints;
         pub use crate::fixed_width_ints::*;
 
         mod wasi;
         pub use crate::wasi::*;
+
+        prelude!();
     } else if #[cfg(target_os = "xous")] {
         mod fixed_width_ints;
         pub use crate::fixed_width_ints::*;
 
         mod xous;
         pub use crate::xous::*;
+
+        prelude!();
     } else {
         // non-supported targets: empty...
     }

src/macros.rs

@@ -61,6 +61,33 @@ macro_rules! cfg_if {
     };
 }
 
+/// Create an internal crate prelude with `core` reexports and common types.
+macro_rules! prelude {
+    () => {
+        /// Frequently-used types that are available on all platforms
+        ///
+        /// We need to reexport the core types so this works with `rust-dep-of-std`.
+        mod prelude {
+            // Exports from `core`
+            #[allow(unused_imports)]
+            pub(crate) use core::clone::Clone;
+            #[allow(unused_imports)]
+            pub(crate) use core::marker::{Copy, Send, Sync};
+            #[allow(unused_imports)]
+            pub(crate) use core::option::Option;
+            #[allow(unused_imports)]
+            pub(crate) use core::{fmt, hash, iter, mem};
+
+            // Commonly used types defined in this crate
+            #[allow(unused_imports)]
+            pub(crate) use crate::{
+                c_char, c_double, c_float, c_int, c_long, c_longlong, c_short, c_uchar, c_uint,
+                c_ulong, c_ulonglong, c_ushort, c_void, intptr_t, size_t, ssize_t, uintptr_t,
+            };
+        }
+    };
+}
+
 /// Implement `Clone` and `Copy` for a struct, as well as `Debug`, `Eq`, `Hash`, and
 /// `PartialEq` if the `extra_traits` feature is enabled.
 ///