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, 9 insertions, 5 deletions

diff --git a/runtime/js.rs b/runtime/js.rs
index def2724ce..f942493a3 100644
--- a/runtime/js.rs
+++ b/runtime/js.rs
@@ -1,17 +1,21 @@
 // Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
-#[cfg(not(feature = "dont_create_runtime_snapshot"))]
 use deno_core::Snapshot;
-#[cfg(not(feature = "dont_create_runtime_snapshot"))]
 use log::debug;
 
 #[cfg(not(feature = "dont_create_runtime_snapshot"))]
 static RUNTIME_SNAPSHOT: &[u8] =
   include_bytes!(concat!(env!("OUT_DIR"), "/RUNTIME_SNAPSHOT.bin"));
 
-#[cfg(not(feature = "dont_create_runtime_snapshot"))]
-pub fn deno_isolate_init() -> Snapshot {
+pub fn deno_isolate_init() -> Option<Snapshot> {
   debug!("Deno isolate init with snapshots.");
-  Snapshot::Static(RUNTIME_SNAPSHOT)
+  #[cfg(not(feature = "dont_create_runtime_snapshot"))]
+  {
+    Some(Snapshot::Static(RUNTIME_SNAPSHOT))
+  }
+  #[cfg(feature = "dont_create_runtime_snapshot")]
+  {
+    None
+  }
 }
 
 #[cfg(not(feature = "include_js_files_for_snapshotting"))]