Generated files are now compatible only with the same version

I. e. if `protbuf-codegen=10.20.30` was used to generate `.rs`
files, these files will be only compatible with `protobuf=10.20.30`.

Author: stepancheg

protobuf-codegen/src/lib.rs

@@ -170,6 +170,13 @@ fn gen_file(
 
         w.write_line("");
         w.write_line(&format!("//! Generated file from `{}`", file.get_name()));
+        if customize.inside_protobuf != Some(true) {
+            w.write_line("");
+            w.write_line("/// Generated files are compatible only with the same version");
+            w.write_line("/// of protobuf runtime.");
+            w.write_line(&format!("const _PROTOBUF_VERSION_CHECK: () = {}::{};",
+                protobuf_crate_path(&customize), protobuf::VERSION_IDENT));
+        }
 
         for message in &scope.get_messages() {
             // ignore map entries, because they are not used in map fields

protobuf/src/lib.rs

@@ -85,5 +85,5 @@ pub const VERSION: &str = "";
 /// This symbol is in generated `version.rs`, include here for IDE
 #[cfg(never)]
 #[doc(hidden)]
-pub const VERSION_SYMBOL: &str = "";
+pub const VERSION_IDENT: &str = "";
 include!(concat!(env!("OUT_DIR"), "/version.rs"));