cargo: build in Cargo's out dir if DENO_BUILD_PATH is not set

Plus some minor improvements and clean-ups:

* Resolve DENO_BUILD_PATH to an absolute path if necessary.
* Rename DENO_BUILD_PATH to GN_OUT_DIR in places where it is supposed to
  be set by the build system (and not a user configuration variable).
* Output Cargo `rerun-if-*-changed` instructions first, so even if the
  build itself fails, configuration changes will still trigger a re-run
  of build.rs.
* Remove TODOs that are impossible.
* Re-run build.rs when the flatbuffer definition file changes.

1 files changed, 6 insertions, 6 deletions

diff --git a/build_extra/rust/run.py b/build_extra/rust/run.py
index 0b1fa3dc9..3a86c423c 100644
--- a/build_extra/rust/run.py
+++ b/build_extra/rust/run.py
@@ -1,13 +1,13 @@
 #!/usr/bin/env python
-# This file just executes its arguments, except that also adds OUT_DIR to the
+# This file just executes its arguments, except that also adds GN_OUT_DIR to the
 # environ. This is for compatibility with cargo.
 import subprocess
 import sys
 import os
 
-# TODO This is for src/msg.rs to know where to find msg_generated.rs
-# In the future we should use OUT_DIR here.
-os.environ["DENO_BUILD_PATH"] = os.path.abspath(".")
-assert os.path.isdir(os.environ["DENO_BUILD_PATH"])
+# This is for src/msg.rs to know where to find msg_generated.rs.
+# When building with Cargo this variable is set by build.rs.
+os.environ["GN_OUT_DIR"] = os.path.abspath(".")
+assert os.path.isdir(os.environ["GN_OUT_DIR"])
 
-sys.exit(subprocess.call(sys.argv[1:], env=os.environ))
+sys.exit(subprocess.call(sys.argv[1:]))