rust_2018_idioms: build warning "hidden lifetime parameters in types are deprecated"

[oherrala]

I get the following warnings when compiling my code (with 2.25.2):

303 |     fn as_ref(&self) -> ::protobuf::reflect::ReflectValueRef {
    |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected named lifetime parameter
    |
    = note: `-W elided-lifetimes-in-paths` implied by `-W rust-2018-idioms`
help: consider using the `'_` lifetime
    |
303 |     fn as_ref(&self) -> ::protobuf::reflect::ReflectValueRef<'_> {

Same kind of issue is previously reported in #435.

[stepancheg]

Where is this code located?

[oherrala]

This is the generated code:

impl ::protobuf::reflect::ProtobufValue for CollectorMessages {
    fn as_ref(&self) -> ::protobuf::reflect::ReflectValueRef {
        ::protobuf::reflect::ReflectValueRef::Message(self)
    }
}

CollectorMessages is defined in protobuf as:

message CollectorMessages {
    oneof message {
        SendObservationRequest send_observation_req = 1;
        SendObservationResponse send_observation_resp = 2;
    }
}

[stepancheg]

Did you turn on #[warn(rust_2018_idioms)] explicitly?

Because otherwise I cannot reproduce it.

[oherrala]

Yes, apparently so. I just tried turning it on to see the warnings and off to not see them. Sorry I forgot to specify what lints I had turned on (I have quite a few).

export RUSTFLAGS="-W deprecated-in-future -W noop-method-call -W future-incompatible -W nonstandard-style -W rust-2018-compatibility -W rust-2018-idioms -W rust-2021-compatibility -W unused ${RUSTFLAGS}"

[stepancheg]

If #[warn(rust_2018_idioms)] was inserted in generated code, would it be enough?

[oherrala]

I think the generated code should be modified as described by the linter:

  impl ::protobuf::reflect::ProtobufValue for CollectorMessages {
-     fn as_ref(&self) -> ::protobuf::reflect::ReflectValueRef {
+     fn as_ref(&self) -> ::protobuf::reflect::ReflectValueRef<'_> {
          ::protobuf::reflect::ReflectValueRef::Message(self)
      }
  }

[stepancheg]

Yes, I think you are right, it would be safer this way.

[stepancheg]

Interesting, current version from master does not have this warning (it generate similar code differently).

rust-protobuf v3 will be released in the following weeks. Is it critical to have this warning fixed in rust-protobuf v2?

[oherrala]

No, it's not critical. Just annoying.

And can of course be silenced with: #![allow(elided_lifetimes_in_paths)]