WIP: Non-blocking Rust futures running on local thread

.gitignore

@@ -12,3 +12,4 @@ pkgs/create-neon/create-neon-manual-test-project
 test/cli/lib
 npm-debug.log
 rls*.log
+.vscode

crates/neon/Cargo.toml

@@ -35,6 +35,7 @@ doc-comment = { version = "0.3.3", optional = true }
 send_wrapper = "0.6.0"
 serde = { version = "1.0.197", optional = true }
 serde_json = { version = "1.0.114", optional = true }
+futures = { version = "0.3", optional = true }
 
 [dependencies.tokio]
 version = "1.34.0"
@@ -57,6 +58,8 @@ external-buffers = []
 # https://github.com/neon-bindings/rfcs/pull/46
 futures = ["tokio"]
 
+async_local = ["dep:futures"]
+
 # Enable low-level system APIs. The `sys` API allows augmenting the Neon API
 # from external crates.
 sys = []

crates/neon/src/async_local/executor/enter.rs

@@ -0,0 +1,53 @@
+use std::cell::Cell;
+use std::fmt;
+
+std::thread_local!(static ENTERED: Cell<bool> = Cell::new(false));
+
+pub struct Enter {
+  _priv: (),
+}
+
+pub struct EnterError {
+  _priv: (),
+}
+
+impl fmt::Debug for EnterError {
+  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    f.debug_struct("EnterError").finish()
+  }
+}
+
+impl fmt::Display for EnterError {
+  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    write!(f, "an execution scope has already been entered")
+  }
+}
+
+impl std::error::Error for EnterError {}
+
+pub fn enter() -> Result<Enter, EnterError> {
+  ENTERED.with(|c| {
+    if c.get() {
+      Err(EnterError { _priv: () })
+    } else {
+      c.set(true);
+
+      Ok(Enter { _priv: () })
+    }
+  })
+}
+
+impl fmt::Debug for Enter {
+  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    f.debug_struct("Enter").finish()
+  }
+}
+
+impl Drop for Enter {
+  fn drop(&mut self) {
+    ENTERED.with(|c| {
+      assert!(c.get());
+      c.set(false);
+    });
+  }
+}