Scopes: how to encode unavailable bindings

[hbenl]

The scopes proposal adds information about scopes and their variables and bindings to the sourcemap. But some bindings will be unavailable, i.e. for some original variables there will be no expression that can be evaluated to get the variable's current value. We could encode this by using -1 as the index in the names array for these bindings but this clashes with our use of relative indices into the names array. We use relative indices in the hopes of reducing the encoded scope's size because the encoding of relative indices will hopefully be smaller on average than that of absolute indices. But if some of the indices will be -1 then using relative indices will probably not reduce the size of the encoded scope.

[szuend]

To summarize the possible solutions that were discussed on the call 2024-02-29:

• Use an additional bitfield to indicate which of the live ranges indicate an "unavailable" range and the expression index must be ignored.
• Introduce one more character on top of " and , to signal undefined or null.
• (from @jridgewell in chat) Add an empty string to names and point to that.

[hbenl]

Add an empty string to names and point to that.

That would leave us with the same problem: if, for example, you added the empty string at the start of names then unavailable bindings would all be encoded with 0 instead of -1 - using relative indices will still not reduce the size of the encoded scope.