semantic: var hoisting

[Boshen]

Several attempts was made to pass all tests in test262 + babel + typescript regarding var hoisting in strict / non-strict mode.

Scope tree ended up with a var binding everywhere:

{
  {
    var x
  } 
}

x = 1

produces

Scope1 (ScopeFlags(StrictMode | Top)) {
  Bindings: {
    x (SymbolFlags(FunctionScopedVariable))
  }
  Scope2 (ScopeFlags(StrictMode)) {
    Bindings: {
      x (SymbolFlags(FunctionScopedVariable))
    }
    Scope3 (ScopeFlags(StrictMode)) {
      Bindings: {
        x (SymbolFlags(FunctionScopedVariable))
      }
    }
  }
}

The consequence of this is:

• unintuitive
• performance killer
• breaks mangler

  oxc/crates/oxc_mangler/src/lib.rs

  Lines 111 to 123 in 3df9e69

  	// omit var hoisting because var symbols are added to every parent scope
  	if symbol_table.get_flag(*symbol_id).is_function_scoped_declaration()
  	&& parent_bindings.is_none()
  	{
  	parent_bindings = scope_tree
  	.get_parent_id(scope_id)
  	.map(|parent_scope_id| scope_tree.get_bindings(parent_scope_id));
  	}
  	if let Some(parent_bindings) = &parent_bindings {
  	if parent_bindings.values().contains(symbol_id) {
  	continue;
  	}
  	}

To understand the test cases, try different strategies in

oxc/crates/oxc_semantic/src/binder.rs

Lines 46 to 54 in 3df9e69

	let mut var_scope_ids = vec![];
	if !builder.current_scope_flags().is_var() {
	for scope_id in builder.scope.ancestors(current_scope_id).skip(1) {
	var_scope_ids.push(scope_id);
	if builder.scope.get_flags(scope_id).is_var() {
	break;
	}
	}
	}

and run just c to see changes in snapshot.