Make it clearer that the private implementation details are private

Both of these modules are doc(hidden) and commented "Not public API",
but incorrect downstream code sometimes still references them. Naming
the module __private will make it more likely to be noticed in code
review.

serde/src/de/impls.rs

@@ -1906,7 +1906,7 @@ impl<'de> Deserialize<'de> for Duration {
                             b"secs" => Ok(Field::Secs),
                             b"nanos" => Ok(Field::Nanos),
                             _ => {
-                                let value = ::export::from_utf8_lossy(value);
+                                let value = ::__private::from_utf8_lossy(value);
                                 Err(Error::unknown_field(&value, FIELDS))
                             }
                         }
@@ -2214,7 +2214,7 @@ mod range {
                         b"start" => Ok(Field::Start),
                         b"end" => Ok(Field::End),
                         _ => {
-                            let value = ::export::from_utf8_lossy(value);
+                            let value = ::__private::from_utf8_lossy(value);
                             Err(Error::unknown_field(&value, FIELDS))
                         }
                     }

serde/src/integer128.rs

@@ -10,7 +10,7 @@
 /// bother with this macro and may assume support for 128-bit integers.
 ///
 /// ```edition2018
-/// # use serde::private::ser::Error;
+/// # use serde::__private::ser::Error;
 /// #
 /// # struct MySerializer;
 /// #

serde/src/lib.rs

@@ -266,13 +266,14 @@ pub use de::{Deserialize, Deserializer};
 #[doc(inline)]
 pub use ser::{Serialize, Serializer};
 
-// Generated code uses these to support no_std. Not public API.
+// Used by generated code and doc tests. Not public API.
 #[doc(hidden)]
-pub mod export;
+#[path = "private/mod.rs"]
+pub mod __private;
 
-// Helpers used by generated code and doc tests. Not public API.
-#[doc(hidden)]
-pub mod private;
+#[allow(unused_imports)]
+use self::__private as export;
+use self::__private as private;
 
 #[cfg(not(feature = "std"))]
 mod std_error;

serde/src/macros.rs

@@ -124,7 +124,7 @@ macro_rules! forward_to_deserialize_any {
 macro_rules! forward_to_deserialize_any_method {
     ($func:ident<$l:tt, $v:ident>($($arg:ident : $ty:ty),*)) => {
         #[inline]
-        fn $func<$v>(self, $($arg: $ty,)* visitor: $v) -> $crate::export::Result<$v::Value, Self::Error>
+        fn $func<$v>(self, $($arg: $ty,)* visitor: $v) -> $crate::__private::Result<$v::Value, Self::Error>
         where
             $v: $crate::de::Visitor<$l>,
         {

serde/src/private/macros.rs

@@ -37,7 +37,7 @@ macro_rules! __serialize_unimplemented {
 #[macro_export]
 macro_rules! __serialize_unimplemented_method {
     ($func:ident $(<$t:ident>)* ($($arg:ty),*) -> $ret:ident) => {
-        fn $func $(<$t: ?Sized + $crate::Serialize>)* (self $(, _: $arg)*) -> $crate::export::Result<Self::$ret, Self::Error> {
+        fn $func $(<$t: ?Sized + $crate::Serialize>)* (self $(, _: $arg)*) -> $crate::__private::Result<Self::$ret, Self::Error> {
             unimplemented!()
         }
     };

serde/src/private/mod.rs

@@ -2,3 +2,43 @@ mod macros;
 
 pub mod de;
 pub mod ser;
+
+pub use lib::clone::Clone;
+pub use lib::convert::{From, Into};
+pub use lib::default::Default;
+pub use lib::fmt::{self, Formatter};
+pub use lib::marker::PhantomData;
+pub use lib::option::Option::{self, None, Some};
+pub use lib::result::Result::{self, Err, Ok};
+
+pub use self::string::from_utf8_lossy;
+
+#[cfg(any(feature = "alloc", feature = "std"))]
+pub use lib::{ToString, Vec};
+
+#[cfg(core_try_from)]
+pub use lib::convert::TryFrom;
+
+mod string {
+    use lib::*;
+
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    pub fn from_utf8_lossy(bytes: &[u8]) -> Cow<str> {
+        String::from_utf8_lossy(bytes)
+    }
+
+    // The generated code calls this like:
+    //
+    //     let value = &_serde::__private::from_utf8_lossy(bytes);
+    //     Err(_serde::de::Error::unknown_variant(value, VARIANTS))
+    //
+    // so it is okay for the return type to be different from the std case as long
+    // as the above works.
+    #[cfg(not(any(feature = "std", feature = "alloc")))]
+    pub fn from_utf8_lossy(bytes: &[u8]) -> &str {
+        // Three unicode replacement characters if it fails. They look like a
+        // white-on-black question mark. The user will recognize it as invalid
+        // UTF-8.
+        str::from_utf8(bytes).unwrap_or("\u{fffd}\u{fffd}\u{fffd}")
+    }
+}

serde/src/ser/impossible.rs

@@ -17,7 +17,7 @@ use ser::{
 ///
 /// ```edition2018
 /// # use serde::ser::{Serializer, Impossible};
-/// # use serde::private::ser::Error;
+/// # use serde::__private::ser::Error;
 /// #
 /// # struct MySerializer;
 /// #

serde/src/ser/mod.rs

@@ -711,7 +711,7 @@ pub trait Serializer: Sized {
     ///
     /// ```edition2018
     /// # use serde::ser::{Serializer, SerializeSeq};
-    /// # use serde::private::ser::Error;
+    /// # use serde::__private::ser::Error;
     /// #
     /// # struct MySerializer;
     /// #