Add support for returning references in Dart (#402)

* Add skip_if_unsupported; use to enable borrow-returning feature test

* Track ownership of opaques in dart

* Add support for return-of-reference to dart

Makefile.toml

@@ -202,6 +202,7 @@ exec --fail-on-error npm run test
 [tasks.test-dart-example]
 category = "Tests"
 script_runner = "@duckscript"
+dependencies = ["build-example"]
 script = '''
 exit_on_error true
 cd example/dart/
@@ -212,6 +213,7 @@ exec --fail-on-error dart --enable-experiment=native-assets test
 [tasks.test-dart-feature]
 category = "Tests"
 script_runner = "@duckscript"
+dependencies = ["build-feature"]
 script = '''
 exit_on_error true
 cd feature_tests/dart/

core/src/ast/attrs.rs

@@ -11,13 +11,19 @@ use syn::{Attribute, Ident, LitStr, Meta, Token};
 pub struct Attrs {
  pub cfg: Vec<Attribute>,
  pub attrs: Vec<DiplomatBackendAttr>,
+ /// AST backends only. For using features that may panic AST backends, like returning references.
+ ///
+ /// This isn't a regular attribute since AST backends do not handle regular attributes. Do not use
+ /// in HIR backends,
+ pub skip_if_unsupported: bool,
 }
 
 impl Attrs {
  fn add_attr(&mut self, attr: Attr) {
  match attr {
  Attr::Cfg(attr) => self.cfg.push(attr),
- Attr::DiplomatBackendAttr(attr) => self.attrs.push(attr),
+ Attr::DiplomatBackend(attr) => self.attrs.push(attr),
+ Attr::SkipIfUnsupported => self.skip_if_unsupported = true,
  }
  }
 
@@ -45,21 +51,25 @@ impl From<&[Attribute]> for Attrs {
 
 enum Attr {
  Cfg(Attribute),
- DiplomatBackendAttr(DiplomatBackendAttr),
+ DiplomatBackend(DiplomatBackendAttr),
+ SkipIfUnsupported,
  // More goes here
 }
 
 fn syn_attr_to_ast_attr(attrs: &[Attribute]) -> impl Iterator<Item = Attr> + '_ {
  let cfg_path: syn::Path = syn::parse_str("cfg").unwrap();
  let dattr_path: syn::Path = syn::parse_str("diplomat::attr").unwrap();
+ let skipast: syn::Path = syn::parse_str("diplomat::skip_if_unsupported").unwrap();
  attrs.iter().filter_map(move |a| {
  if a.path() == &cfg_path {
  Some(Attr::Cfg(a.clone()))
  } else if a.path() == &dattr_path {
- Some(Attr::DiplomatBackendAttr(
+ Some(Attr::DiplomatBackend(
  a.parse_args()
  .expect("Failed to parse malformed diplomat::attr"),
  ))
+ } else if a.path() == &skipast {
+ Some(Attr::SkipIfUnsupported)
  } else {
  None
  }