Collecting valid identifiers for a given block

[grubburg]

I am trying to use swc to collect valid, in-scope identifiers for a given block in a JS program.
Initially I thought I may be able to use collect_decls_with_context from swc_ecma_utils, but this doesn't seem to achieve exactly what I'm looking for.
e.g. for the following example:

{
    var x = 1;
    {
        var y = 2;
    }
}

I want to be able to determine that the outer block has x in scope, and the inner block has both x and y in scope.
collect_decls_with_context appears to return the opposite result.

Is there a canonical way to do what I am trying to do with swc?

Thanks in advance