rust_2018_idioms: build fails with "hidden lifetime parameters in types are deprecated"

[nickbp]

Problem

When project code configures #[deny(rust_2018_idioms)], the generated rust-protobuf code has compilation errors like the following for both 2.8.0 and master/d2a9ddb:

$ cargo build
   Compiling protobuf-busted-example v0.1.0 (/Users/nick/code/protobuf-busted-example)
error: hidden lifetime parameters in types are deprecated
  --> src/sample.rs:55:39
   |
55 |     fn merge_from(&mut self, is: &mut ::protobuf::CodedInputStream) -> ::protobuf::ProtobufResult<()> {
   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^- help: indicate the anonymous lifetime: `<'_>`
   |
note: lint level defined here
  --> src/main.rs:1:19
   |
1  | #![deny(warnings, rust_2018_idioms)]
   |                   ^^^^^^^^^^^^^^^^
   = note: #[deny(elided_lifetimes_in_paths)] implied by #[deny(rust_2018_idioms)]

error: hidden lifetime parameters in types are deprecated
   --> src/sample.rs:100:51
    |
100 |     fn write_to_with_cached_sizes(&self, os: &mut ::protobuf::CodedOutputStream) -> ::protobuf::ProtobufResult<()> {
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- help: indicate the anonymous lifetime: `<'_>`

error: hidden lifetime parameters in types are deprecated
   --> src/sample.rs:177:27
    |
177 |     fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
    |                           ^^^^^^^^^^^^^^^^^^^^^- help: indicate the anonymous lifetime: `<'_>`

error: aborting due to 3 previous errors

error: Could not compile `protobuf-busted-example`.

To learn more, run the command again with --verbose.

Environment

$ rustc --version
rustc 1.36.0 (a53f9df32 2019-07-03)

Minimal sample

Cargo.toml

Note: This sample shows master:d2a9ddb, however 2.8.0 produces the same errors.

[package]
name = "protobuf-failure-sample"
version = "0.1.0"
build = "build.rs"

[dependencies]
protobuf = { git="https://github.com/stepancheg/rust-protobuf", rev="d2a9ddb" }

[build-dependencies]
protobuf-codegen-pure = { git="https://github.com/stepancheg/rust-protobuf", rev="d2a9ddb" }

src/main.rs

#![deny(warnings, rust_2018_idioms)]
// If this is not added, the protobuf code will fail to compile:
// #![allow(elided_lifetimes_in_paths)]

use sample::SampleEnum;
mod sample;
extern crate protobuf;

fn main() {
    let foo = SampleEnum::FOO;
    println!("hello world: {:?}", foo);
}

build.rs

extern crate protobuf_codegen_pure;

fn main() {
    protobuf_codegen_pure::Args::new()
        .out_dir("src/")
        .input("proto/sample.proto")
        .include("proto")
        .run()
        .expect("protoc");
}

proto/sample.proto

syntax = "proto3";

enum SampleEnum {
  FOO = 0;
  BAR = 1;
}

message SampleMessage {
  string foo = 1;
  uint32 bar = 2;
  SampleEnum baz = 3;
}

Workarounds

1. If the generated output is manually updated to include the requested lifetime parameters, the errors go away. For example:

    fn merge_from(&mut self, is: &mut ::protobuf::CodedInputStream<'_>) -> ::protobuf::ProtobufResult<()> {

2. Alternatively, if my main.rs specifies #![allow(elided_lifetimes_in_paths)] then that also gets rid of the errors. However, just adding #![allow(elided_lifetimes_in_paths)] directly to the generated sample.rs output doesn't seem to do anything.

[stepancheg]

If the generated output is manually updated to include the requested lifetime parameters, the errors go away

Yes, I think this is the right way since this is official Rust recommendation.

AFAIU, '_ is stable since Rust 1.26 and minimum supported rust version is 1.27.

Context: this is where rust_2018_idioms specified: rust-lang/rust#52047

Patch welcome.

[stepancheg]

Reopening since it's not yet merged into stable branch.

[nickbp]

#439 above should do it for stable, assuming that v2.8 is the correct branch for stable. Let me know if it should go to a different branch instead.

[stepancheg]

In 2.8.1