capnpc: add option to build capnp exe from src

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "capnpc/capnproto"]
+	path = capnpc/capnproto
+	url = https://github.com/capnproto/capnproto.git

capnpc/Cargo.toml

@@ -13,6 +13,9 @@ readme = "README.md"
 
 keywords = ["encoding", "protocol", "serialization"]
 
+[features]
+build-capnp = []
+
 [lib]
 
 name = "capnpc"
@@ -37,3 +40,7 @@ path = "../capnp"
 # Fortunately, this is pretty easy, and is just an internal implementation detail.
 default-features=false
 features=[]
+
+[build-dependencies]
+cmake = "0.1"
+which = "4.2"

capnpc/build.rs

@@ -0,0 +1,14 @@
+#[cfg(feature = "build-capnp")]
+fn main() {
+    cmake::build("capnproto");
+}
+
+#[cfg(not(feature = "build-capnp"))]
+fn main() {
+    if !which::which("capnp").is_ok() {
+        panic!(
+            "capnp executable not found. install it with your package manager or enable the \
+            \"build-capnp\" feature to build it from source"
+        );
+    }
+}

capnpc/src/lib.rs

@@ -66,6 +66,19 @@ mod pointer_constants;
 
 use std::path::{Path, PathBuf};
 
+#[cfg(feature = "build-capnp")]
+fn capnp_exe() -> PathBuf {
+    let mut p = PathBuf::from(env!("OUT_DIR"));
+    p.push("bin");
+    p.push("capnp");
+    p
+}
+
+#[cfg(not(feature = "build-capnp"))]
+fn capnp_exe() -> PathBuf {
+    PathBuf::from("capnp")
+}
+
 // Copied from capnp/src/lib.rs, where this conversion lives behind the "std" feature flag,
 // which we don't want to depend on here.
 pub(crate) fn convert_io_err(err: std::io::Error) -> capnp::Error {
@@ -218,7 +231,7 @@ impl CompilerCommand {
         let mut command = if let Some(executable) = &self.executable_path {
             ::std::process::Command::new(executable)
         } else {
-            ::std::process::Command::new("capnp")
+            ::std::process::Command::new(&capnp_exe())
         };
 
         command.arg("compile").arg("-o").arg("-");