Block scope not being used on variable allocation

[jasonwilliams]

When we step into a new function, we create a new_function_environment https://github.com/jasonwilliams/boa/blob/master/src/lib/exec.rs#L110

Because we don't do the same for blocks (if,while etc) any let values we create in blocks are added to the current environment, which will be the outer function most likely. This means the following JS works..

function outer() {
  var foo = "foo";
  if (true) {
    let bar = "bar";
  }

  return bar; // "bar"
}

What does the specification say?

When a Block or CaseBlock is evaluated a new declarative Environment Record is created and bindings for each block scoped variable, constant, function, or class declared in the block are instantiated in the Environment Record.

https://tc39.es/ecma262/#sec-blockdeclarationinstantiation

high level task

The process here is to create a new_declarative_environment when we hit a block and set that to the current environment, popping it off when we reach the end of our block. A potential bug this will cause is putting var declarations into the same scope, so we need to make sure vars still go into the function scope above.

low level

• new_declaration_environment is already exposed here. Bring it into exec.rs so that it can be used similar to how new_function_environment is used.
• lexical_environment.rs will need a way of setting bindings to the nearest function environment, rather than the current environment (to support var)