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, 11 insertions, 3 deletions

diff --git a/cli/js.rs b/cli/js.rs
index 6a312a206..f87bf6d73 100644
--- a/cli/js.rs
+++ b/cli/js.rs
@@ -3,12 +3,20 @@
 use deno_core::Snapshot;
 use log::debug;
 
+#[cfg(not(feature = "__runtime_js_sources"))]
 static CLI_SNAPSHOT: &[u8] =
   include_bytes!(concat!(env!("OUT_DIR"), "/CLI_SNAPSHOT.bin"));
 
-pub fn deno_isolate_init() -> Snapshot {
+pub fn deno_isolate_init() -> Option<Snapshot> {
   debug!("Deno isolate init with snapshots.");
-  Snapshot::Static(CLI_SNAPSHOT)
+  #[cfg(not(feature = "__runtime_js_sources"))]
+  {
+    Some(Snapshot::Static(CLI_SNAPSHOT))
+  }
+  #[cfg(feature = "__runtime_js_sources")]
+  {
+    None
+  }
 }
 
 #[cfg(test)]
@@ -18,7 +26,7 @@ mod tests {
   #[test]
   fn runtime_snapshot() {
     let mut js_runtime = deno_core::JsRuntime::new(deno_core::RuntimeOptions {
-      startup_snapshot: Some(deno_isolate_init()),
+      startup_snapshot: deno_isolate_init(),
       ..Default::default()
     });
     js_runtime