Trying to improve doctests with cfg doctest

Unfortunately, `#[cfg(doctest)]` does not seem to work:
rust-lang/rust#67295

proto/doctest_pb.proto

@@ -0,0 +1,6 @@
+// Messages used in doctests
+
+syntax = "proto3";
+
+message MyMessage {
+}

protobuf/regenerate.sh

@@ -38,9 +38,15 @@ protoc \
     -I../proto \
     ../proto/google/protobuf/*.proto \
     ../proto/google/protobuf/compiler/* \
-    ../proto/rustproto.proto
+    ../proto/rustproto.proto \
+    ../proto/doctest_pb.proto \
 
-mv tmp-generated/descriptor.rs tmp-generated/plugin.rs tmp-generated/rustproto.rs src/
+mv \
+    tmp-generated/descriptor.rs \
+    tmp-generated/plugin.rs \
+    tmp-generated/rustproto.rs \
+    tmp-generated/doctest_pb.rs \
+    src/
 mv tmp-generated/*.rs src/well_known_types/
 (
     cd src/well_known_types

protobuf/src/doctest_pb.rs

@@ -0,0 +1,147 @@
+// This file is generated by rust-protobuf 3.0.0-pre. Do not edit
+// .proto file is parsed by protoc --rust-out=...
+// @generated
+
+// https://github.com/rust-lang/rust-clippy/issues/702
+#![allow(unknown_lints)]
+#![allow(clippy::all)]
+
+#![cfg_attr(rustfmt, rustfmt_skip)]
+
+#![allow(box_pointers)]
+#![allow(dead_code)]
+#![allow(missing_docs)]
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+#![allow(non_upper_case_globals)]
+#![allow(trivial_casts)]
+#![allow(unused_imports)]
+#![allow(unused_results)]
+
+//! Generated file from `doctest_pb.proto`
+
+#[derive(PartialEq,Clone,Default)]
+#[cfg_attr(serde, derive(Serialize, Deserialize))]
+pub struct MyMessage {
+    // special fields
+    #[cfg_attr(serde, serde(skip))]
+    pub unknown_fields: crate::UnknownFields,
+    #[cfg_attr(serde, serde(skip))]
+    pub cached_size: crate::rt::CachedSize,
+}
+
+impl<'a> ::std::default::Default for &'a MyMessage {
+    fn default() -> &'a MyMessage {
+        <MyMessage as crate::Message>::default_instance()
+    }
+}
+
+impl MyMessage {
+    pub fn new() -> MyMessage {
+        ::std::default::Default::default()
+    }
+}
+
+impl crate::Message for MyMessage {
+    fn is_initialized(&self) -> bool {
+        true
+    }
+
+    fn merge_from(&mut self, is: &mut crate::CodedInputStream<'_>) -> crate::ProtobufResult<()> {
+        while !is.eof()? {
+            let (field_number, wire_type) = is.read_tag_unpack()?;
+            match field_number {
+                _ => {
+                    crate::rt::read_unknown_or_skip_group(field_number, wire_type, is, self.mut_unknown_fields())?;
+                },
+            };
+        }
+        ::std::result::Result::Ok(())
+    }
+
+    // Compute sizes of nested messages
+    #[allow(unused_variables)]
+    fn compute_size(&self) -> u32 {
+        let mut my_size = 0;
+        my_size += crate::rt::unknown_fields_size(self.get_unknown_fields());
+        self.cached_size.set(my_size);
+        my_size
+    }
+
+    fn write_to_with_cached_sizes(&self, os: &mut crate::CodedOutputStream<'_>) -> crate::ProtobufResult<()> {
+        os.write_unknown_fields(self.get_unknown_fields())?;
+        ::std::result::Result::Ok(())
+    }
+
+    fn get_cached_size(&self) -> u32 {
+        self.cached_size.get()
+    }
+
+    fn get_unknown_fields(&self) -> &crate::UnknownFields {
+        &self.unknown_fields
+    }
+
+    fn mut_unknown_fields(&mut self) -> &mut crate::UnknownFields {
+        &mut self.unknown_fields
+    }
+
+    fn descriptor(&self) -> &'static crate::reflect::MessageDescriptor {
+        Self::descriptor_static()
+    }
+
+    fn new() -> MyMessage {
+        MyMessage::new()
+    }
+
+    fn descriptor_static() -> &'static crate::reflect::MessageDescriptor {
+        static descriptor: crate::rt::Lazy<crate::reflect::MessageDescriptor> = crate::rt::Lazy::INIT;
+        descriptor.get(|| {
+            let fields = ::std::vec::Vec::new();
+            crate::reflect::MessageDescriptor::new::<MyMessage>(
+                "MyMessage",
+                fields,
+                file_descriptor_proto()
+            )
+        })
+    }
+
+    fn default_instance() -> &'static MyMessage {
+        static instance: crate::rt::Lazy<MyMessage> = crate::rt::Lazy::INIT;
+        instance.get(MyMessage::new)
+    }
+}
+
+impl crate::Clear for MyMessage {
+    fn clear(&mut self) {
+        self.unknown_fields.clear();
+    }
+}
+
+impl ::std::fmt::Debug for MyMessage {
+    fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+        crate::text_format::fmt(self, f)
+    }
+}
+
+impl crate::reflect::ProtobufValue for MyMessage {
+}
+
+static file_descriptor_proto_data: &'static [u8] = b"\
+    \n\x10doctest_pb.proto\"\x0b\n\tMyMessageJG\n\x06\x12\x04\x02\0\x05\x01\
+    \n%\n\x01\x0c\x12\x03\x02\0\x122\x1b\x20Messages\x20used\x20in\x20doctes\
+    ts\n\n\n\n\x02\x04\0\x12\x04\x04\0\x05\x01\n\n\n\x03\x04\0\x01\x12\x03\
+    \x04\x08\x11b\x06proto3\
+";
+
+static file_descriptor_proto_lazy: crate::rt::Lazy<crate::descriptor::FileDescriptorProto> = crate::rt::Lazy::INIT;
+
+fn parse_descriptor_proto() -> crate::descriptor::FileDescriptorProto {
+    crate::parse_from_bytes(file_descriptor_proto_data).unwrap()
+}
+
+/// `FileDescriptorProto` object which was a source for this generated file
+pub fn file_descriptor_proto() -> &'static crate::descriptor::FileDescriptorProto {
+    file_descriptor_proto_lazy.get(|| {
+        parse_descriptor_proto()
+    })
+}

protobuf/src/lib.rs

@@ -80,6 +80,10 @@ mod misc;
 mod buf_read_iter;
 mod buf_read_or_reader;
 
+// TODO: does not work: https://github.com/rust-lang/rust/issues/67295
+#[cfg(doctest)]
+mod doctest_pb;
+
 /// This symbol is in generated `version.rs`, include here for IDE
 #[cfg(never)]
 pub const VERSION: &str = "";