build: allow disabling snapshots for dev (#20048)

Closes #19399 (running without snapshots at all was suggested as an
alternative solution).

Adds a `__runtime_js_sources` pseudo-private feature to load extension
JS sources at runtime for faster development, instead of building and
loading snapshots or embedding sources in the binary. Will only work in
a development environment obviously.

Try running `cargo test --features __runtime_js_sources
integration::node_unit_tests::os_test`. Then break some behaviour in
`ext/node/polyfills/os.ts` e.g. make `function cpus() {}` return an
empty array, and run it again. Fix and then run again. No more build
time in between.

1 files changed, 7 insertions, 0 deletions

diff --git a/ext/node/build.rs b/ext/node/build.rs
new file mode 100644
index 000000000..778f2da17
--- /dev/null
+++ b/ext/node/build.rs
@@ -0,0 +1,7 @@
+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+
+use std::env;
+
+fn main() {
+  println!("cargo:rustc-env=TARGET={}", env::var("TARGET").unwrap());
+}