From 67944f298c1fb64f69766fc6ee0269ea6b1efb75 Mon Sep 17 00:00:00 2001
From: Bert Belder <bertbelder@gmail.com>
Date: Thu, 1 Nov 2018 02:29:39 +0100
Subject: [PATCH] cargo: build only necessary Ninja targets when invoked by RLS

---
 BUILD.gn |  7 +++++++
 build.rs | 21 ++++++++++++++++++++-
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/BUILD.gn b/BUILD.gn
index 0399098932..9e6c7abd05 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -18,6 +18,13 @@ group("default") {
   ]
 }
 
+# Set of targets that need to be built for `cargo check` to succeed.
+group("cargo_check_deps") {
+  deps = [
+    ":msg_rs",
+  ]
+}
+
 config("deno_config") {
   include_dirs = [ "third_party/v8" ]  # This allows us to v8/src/base/ libraries.
   configs = [ "third_party/v8:external_config" ]
diff --git a/build.rs b/build.rs
index 489f9f9d26..351a00cf73 100644
--- a/build.rs
+++ b/build.rs
@@ -7,12 +7,31 @@
 // TODO Combine DENO_BUILD_PATH and OUT_DIR.
 
 use std::env;
+use std::path::PathBuf;
 use std::process::Command;
 
 fn main() {
   let mode = env::var("PROFILE").unwrap();
   let deno_build_path = env::var("DENO_BUILD_PATH").unwrap();
 
+  // Detect if we're being invoked by the rust language server (RLS).
+  // Unfortunately we can't detect whether we're being run by `cargo check`.
+  let check_only = env::var_os("CARGO")
+    .map(PathBuf::from)
+    .as_ref()
+    .and_then(|p| p.file_stem())
+    .and_then(|f| f.to_str())
+    .map(|s| s.starts_with("rls"))
+    .unwrap_or(false);
+
+  // If we're being invoked by the RLS, build only the targets that are needed
+  // for `cargo check` to succeed.
+  let gn_target = if check_only {
+    "cargo_check_deps"
+  } else {
+    "deno_deps"
+  };
+
   let status = Command::new("python")
     .env("DENO_BUILD_PATH", &deno_build_path)
     .env("DENO_BUILD_MODE", &mode)
@@ -31,7 +50,7 @@ fn main() {
     .env("DENO_BUILD_PATH", &deno_build_path)
     .env("DENO_BUILD_MODE", &mode)
     .arg("./tools/build.py")
-    .arg("deno_deps")
+    .arg(gn_target)
     .arg("-v")
     .status()
     .expect("build.py failed");