The protobuf version checking is too strict

[Tim-Zhang]

What's you consideration about the limitation generated files compatible only with the same version of runtime?

People are used to import protobuf by writing protobuf = "2.8.0" in cargo.toml, and rust would use a version between 2.8.0 and 3.0.0, so the incompatible errors may occurred frequently.

It's inconvenient to regenerate codes for the minor/patch version updates, and the codes generated by protobuf-codegen 2.8.1 should be compatible with protobuf 2.8.0 if we follow the rust convention.

[stepancheg]

It's hard to:

• make API used in generated code backwards compatible
• it's hard to check that changes do not break generated code

Version 2.9 was the version where strict version checking was introduced.

However, a lot of projects broke, so I had to yank that version.

Maybe we should revert this restriction. It is not clear now.

But anyway, the best way to use rust-protobuf is to generate .rs files from .proto files from build.rs.

[Tim-Zhang]

I think we should revert the restriction and if there are any compatible problems in future versions just increase the major version.

[stepancheg]

OK, I did not intend to put version check in 2.10 (this was actually the reason why 2.9 was yanked). Going to revert it now.

[stepancheg]

Published 2.10.1