Compile protoc from source rather than bundled binaries

See tokio-rs#575.

prost-build/Cargo.toml

@@ -19,6 +19,7 @@ itertools = "0.10"
 log = "0.4"
 multimap = { version = "0.8", default-features = false }
 petgraph = { version = "0.6", default-features = false }
+protobuf-src = "1.0.1"
 prost = { version = "0.9.0", path = "..", default-features = false }
 prost-types = { version = "0.9.0", path = "../prost-types", default-features = false }
 tempfile = "3"

prost-build/src/lib.rs

@@ -76,38 +76,6 @@
 //!
 //! That's it! Run `cargo doc` to see documentation for the generated code. The full
 //! example project can be found on [GitHub](https://github.com/danburkert/snazzy).
-//!
-//! ## Sourcing `protoc`
-//!
-//! `prost-build` depends on the Protocol Buffers compiler, `protoc`, to parse `.proto` files into
-//! a representation that can be transformed into Rust. If set, `prost-build` uses the `PROTOC` and
-//! `PROTOC_INCLUDE` environment variables for locating `protoc` and the Protobuf includes
-//! directory. For example, on a macOS system where Protobuf is installed with Homebrew, set the
-//! environment to:
-//!
-//! ```bash
-//! PROTOC=/usr/local/bin/protoc
-//! PROTOC_INCLUDE=/usr/local/include
-//! ```
-//!
-//! and in a typical Linux installation:
-//!
-//! ```bash
-//! PROTOC=/usr/bin/protoc
-//! PROTOC_INCLUDE=/usr/include
-//! ```
-//!
-//! If `PROTOC` is not found in the environment, then a pre-compiled `protoc` binary bundled in the
-//! prost-build crate is used. Pre-compiled `protoc` binaries exist for Linux (non-musl), macOS,
-//! and Windows systems. If no pre-compiled `protoc` is available for the host platform, then the
-//! `protoc` or `protoc.exe` binary on the `PATH` is used. If `protoc` is not available in any of
-//! these fallback locations, then the build fails.
-//!
-//! If `PROTOC_INCLUDE` is not found in the environment, then the Protobuf include directory
-//! bundled in the prost-build crate is be used.
-//!
-//! To force `prost-build` to use the `protoc` on the `PATH`, add `PROTOC=protoc` to the
-//! environment.
 
 mod ast;
 mod code_generator;
@@ -802,7 +770,7 @@ impl Config {
         };
 
         if !self.skip_protoc_run {
-            let mut cmd = Command::new(protoc());
+            let mut cmd = Command::new(protobuf_src::protoc());
             cmd.arg("--include_imports")
                 .arg("--include_source_info")
                 .arg("-o")
@@ -814,7 +782,7 @@ impl Config {
 
             // Set the protoc include after the user includes in case the user wants to
             // override one of the built-in .protos.
-            cmd.arg("-I").arg(protoc_include());
+            cmd.arg("-I").arg(protobuf_src::include());
 
             for arg in &self.protoc_args {
                 cmd.arg(arg);
@@ -1077,22 +1045,6 @@ pub fn compile_protos(protos: &[impl AsRef<Path>], includes: &[impl AsRef<Path>]
     Config::new().compile_protos(protos, includes)
 }
 
-/// Returns the path to the `protoc` binary.
-pub fn protoc() -> PathBuf {
-    match env::var_os("PROTOC") {
-        Some(protoc) => PathBuf::from(protoc),
-        None => PathBuf::from(env!("PROTOC")),
-    }
-}
-
-/// Returns the path to the Protobuf include directory.
-pub fn protoc_include() -> PathBuf {
-    match env::var_os("PROTOC_INCLUDE") {
-        Some(include) => PathBuf::from(include),
-        None => PathBuf::from(env!("PROTOC_INCLUDE")),
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;