Rust: automatically source the matching protoc-prebuilt in build.rs (at least opt-in behavior).

[HellButcher]

Hi,
the version-check in the rust protobuf_codegen crate seams very strict (Equality).

protobuf/rust/release_crates/protobuf_codegen/src/lib.rs

Line 157 in f37611e

if protoc_version != expected_protoc_version {

The static assertion inserted into the generated code seams more reasonable:

protobuf/rust/internal.rs

Line 94 in f37611e

const fn are_versions_compatible(gencode: &str, runtime: &str) -> bool {

The strict version check enforces a tight coupling between the versions used inside the crate, and the version of protoc on the system.

maybe also provide a way to automaticaly download the right protoc version. for example using the protoc-prebuilt crate.

[honglooker]

cc @esrauchg

[esrauchg]

Thanks for kicking the tires on the Rust Protobuf implementation!

the version-check in the rust protobuf_codegen crate seams very strict (Equality).

This strictness is temporary and we intend to relax it. We're still doing some final breaking changes to try to improve the developer ergonomics before we finally stabilize it (including to traits, etc). This has the effect that the gencode is incompatible across versions.

Note that C++Proto is intended to live under the 'exact match rule' forever by contrast (https://protobuf.dev/support/cross-version-runtime-guarantee/#:~:text=is%20not%20supported.-,C%2B%2B%20and%20Rust%2Dspecific%20Guarantees,-Protobuf%20C%2B%2B%20and)

We're aiming at burning down the last ergonomics concerns so that we can drop the dash and consider it stable, and from there we'll officially support skewed gencode in the direction of "older gencode using a later runtime" and relax the check you identified. We're targeting ~next month for that stabilization, but still discussing with gRPC folks if they would prefer we do a clean "stabilized release candidate" style release or not.

maybe also provide a way to automaticaly download the right protoc version. for example using the protoc-prebuilt crate.

We also have looked at this and really want this as well, but are worried about supply chain concerns (or even perceptions of them) and so don't have a clear decision. I'm going to retitle the issue to be about this latter enhancement, since the former one is already decided as "will happen when we drop the dash"

Thanks!