tonic_prost_build: `emit_rerun_if_changed` no longer does anything

[cmtm]

Bug Report

The Builder::emit_rerun_if_changed method in tonic-prost-build does nothing. The field is stored but never used, and the documented behavior ("Passed directly to prost_build::Config.emit_rerun_if_changed") is incorrect because prost_build::Config has no such method.

Version

0.14.4

Platform

All

Crates

tonic_prost_build

Description

The emit_rerun_if_changed field is defined in the Builder struct (lib.rs:405):

emit_rerun_if_changed: bool,

It has a setter method (lib.rs:639-642):

/// Passed directly to `prost_build::Config.emit_rerun_if_changed`.
pub fn emit_rerun_if_changed(mut self, enable: bool) -> Self {
    self.emit_rerun_if_changed = enable;
    self
}

However, searching compile_with_config (the method that actually uses the Builder fields to configure prost), the emit_rerun_if_changed field is never read or used. Every other field is passed to the prost_build::Config, but this one is simply ignored.

Furthermore, prost_build::Config does not have an emit_rerun_if_changed method. The prost-build source contains only a TODO comment about potentially adding this functionality (prost-build config.rs:1037-1041):

// TODO: This should probably emit 'rerun-if-changed=PATH' directives for cargo, however
// according to [1] if any are output then those paths replace the default crate root,
// which is undesirable. Figure out how to do it in an additive way; perhaps gcc-rs has
// this figured out.
// [1]: https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script

[pierrekin]

I attempted to fix this here: #2512

It's my first PR to the project, I hope it's an acceptable contribution.