Panic trying to re-define the "arguments" identifier in a function

[Razican]

When we try to define an identifier inside a function with the name arguments, this fails with a panic saying that "Identifier arguments has already been declared". You can test this with the following Test262 tests:

• test/language/eval-code/direct/func-decl-fn-body-cntns-arguments-lex-bind-declare-arguments.js
• test/language/eval-code/direct/func-decl-fn-body-cntns-arguments-lex-bind-declare-arguments-and-assign.js
• test/language/eval-code/direct/meth-fn-body-cntns-arguments-lex-bind-declare-arguments.js
• test/language/eval-code/direct/meth-fn-body-cntns-arguments-lex-bind-declare-arguments-and-assign.js
• test/language/eval-code/direct/func-expr-fn-body-cntns-arguments-lex-bind-declare-arguments.js
• test/language/eval-code/direct/func-expr-fn-body-cntns-arguments-lex-bind-declare-arguments-and-assign.js
• test/language/statements/function/arguments-with-arguments-lex.js
• test/language/expressions/function/arguments-with-arguments-lex.js

To run any of them, you just need to run:

RUST_BACKTRACE=1 cargo run --release --bin boa_tester -- run -v -s {file}

Where {file} is one of the paths in the list above.

You will get a backtrace like this one:

   0: rust_begin_unwind
             at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/panicking.rs:495:5
   1: std::panicking::begin_panic_fmt
             at /rustc/e1884a8e3c3e813aada8254edfa120e85bf5ffca/library/std/src/panicking.rs:437:5
   2: <boa::environment::function_environment_record::FunctionEnvironmentRecord as boa::environment::environment_record_trait::EnvironmentRecordTrait>::create_mutable_binding
   3: boa::environment::lexical_environment::LexicalEnvironment::create_mutable_binding
   4: <boa::syntax::ast::node::Node as boa::exec::Executable>::run
   5: <boa::syntax::ast::node::statement_list::StatementList as boa::exec::Executable>::run
   6: boa::object::gcobject::GcObject::call
   7: <boa::syntax::ast::node::call::Call as boa::exec::Executable>::run
   8: <boa::syntax::ast::node::block::Block as boa::exec::Executable>::run
   9: <boa::syntax::ast::node::Node as boa::exec::Executable>::run
  10: <boa::syntax::ast::node::statement_list::StatementList as boa::exec::Executable>::run
  11: boa::object::gcobject::GcObject::call
  12: <boa::syntax::ast::node::call::Call as boa::exec::Executable>::run
  13: <boa::syntax::ast::node::statement_list::StatementList as boa::exec::Executable>::run
  14: boa::context::Context::eval
  15: boa_tester::exec::<impl boa_tester::Test>::run_once
  16: boa_tester::exec::<impl boa_tester::Test>::run
  17: boa_tester::main

The panic would be this one:

thread 'main' panicked at 'Identifier arguments has already been declared', boa/src/environment/function_environment_record.rs:108:13

This happens because we assert here that a binding that doesn't allow re-declaration (such as a let binding) won't be able to get re-declared. The issue is that in every function, an arguments variable gets declared here and here. Even if that variable can be re-declared, the let binding cannot, and this makes it panic.

Our environment representation is not quite spec compliant, but the thing is that if a variable can be re-declared, then it should be in the new declaration when we check if that variable can be re-declared, not when declaring the variable gets declared for the first time, so this information should be persisted.

[0x7D2B]

I can fix this in #988 after #1156 (for proper environments) is merged.

[Razican]

I can fix this in #988 after #1156 (for proper environments) is merged.

Cool, let's assign this to you :) I will try to get some free time in the next days to review those PRs.