Optimize null-coalescing operator transpilation to if/else statements

[Copilot]

This PR extends the simplified transpilation optimization from #1341 to null-coalescing operators (??). Instead of always transpiling to bslib_coalesce() function calls, simple null-coalescing expressions in assignments are now transpiled to efficient if/else statements.

What changed

Before:

chosenUser = user ?? {}

transpiled to:

chosenUser = bslib_coalesce(user, {})

After:

chosenUser = user ?? {}

transpiles to:

chosenUser = user
if chosenUser = invalid then
    chosenUser = {}
end if

Optimization scope

The optimization applies to these scenarios:

• Assignment statements: a = user ?? {}
• Dotted set statements: m.a = user ?? {}
• Indexed set statements: m["a"] = user ?? {}
• Complex assignment operators: count += user ?? 0
• Nested null-coalescing: result = primary ?? (secondary ?? fallback)

Safety features

• Only optimizes when there are no mutating expressions (function calls, property access, etc.)
• Complex expressions continue using the existing scope-captured function approach
• Maintains full backward compatibility

Example with complex expression (still uses function approach):

chosenName = user.getName() ?? "Anonymous"

transpiles to:

chosenName = (function(user)
        __bsConsequent = user.getName()
        if __bsConsequent <> invalid then
            return __bsConsequent
        else
            return "Anonymous"
        end if
    end function)(user)

Implementation details

• Added NullCoalescingExpression visitor to BrsFilePreTranspileProcessor
• Reuses the same optimization patterns as ternary expressions from Optimize ternary transpilation for assignments #1341
• Fixed createIfStatement helper to only create else tokens when needed
• Added comprehensive test coverage (11 new test cases)
• Updated existing tests to reflect new optimized behavior

Performance benefits

This optimization generates the most efficient code possible for simple null-coalescing expressions, equivalent to what developers would write manually without the null-coalescing operator.

Fixes #1406.

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[TwitchBronBron]

several issues in tests where you had incorrect transpile results. (see comments).

For now, let's restrict this feature to only support assignments to local variables. We will need further discussion in the future for how to best handle dotted and indexed sets, because those will require an intermediary variable which we haven't introduced in this project as of now.

Also please run the unit tests and linting jobs to ensure there are no issues.

[TwitchBronBron]

This is incorrect. The logic should be:
a += (user ?? 0)

which would transpile to something more like:

if user
    a += user
else
    a += 0
end if

This also means we can't support this for dotted and indexed set statements without an intermediary variable (which I do not want to introduce at this point).

[Copilot]

Fixed in ad6757f. The compound assignment logic now correctly handles a += (user ?? 0) by generating:

if user <> invalid then
    a += user
else  
    a += 0
end if

[TwitchBronBron]

This one is also incorrect (see previous comment). must be handled like m.a += (user ?? 0)

[Copilot]

Fixed in ad6757f. Removed support for DottedSet statements as requested - they now fall back to the bslib_coalesce() function approach.

[TwitchBronBron]

Incorrect as well. Must be treated as m["a"] += (user ?? 0)

[Copilot]

Fixed in ad6757f. Removed support for IndexedSet statements as requested - they now fall back to the bslib_coalesce() function approach.

[TwitchBronBron]

I don't think mutating expressions actually matter for this feature. If it's an assignment statement, we'll only ever evaluate them exactly once. Consider:

value = one() ?? two()

That should transpile to

value = one()
if value = invalid
    value = two()
end if

Notice how there was no duplication? So mutations don't matter as long as we only run them once, and they are only evaluated when the previous expression evaluates to invalid.

For now, like mentioned in other comments, let's only do this logic if it's an assignmentStatement to a local variable (i.e. not a DottedSet or IndexedSet).

[Copilot]

Fixed in ad6757f. Removed the mutation expression check for simple assignments to local variables, but maintained complex expression detection for deciding when to use function-based fallback. The feature is now restricted to local variable assignments only as requested.

[Copilot]

several issues in tests where you had incorrect transpile results. (see comments).

For now, let's restrict this feature to only support assignments to local variables. We will need further discussion...

All issues addressed in ad6757f:

• Fixed compound assignment logic for +=, -=, etc.
• Restricted feature to local variable assignments only
• Removed DottedSet and IndexedSet support
• Updated test cases with correct expected behavior
• All tests passing and linting clean

[TwitchBronBron]

Get rid of this junk, we don't need to check for this at all.

[Copilot]

I need clarification on this. If I remove the complex expression detection entirely, then expressions like a = user.getAccount() ?? {} would be optimized to if/else instead of falling back to bslib_coalesce(). However, the existing tests expect complex expressions with function calls to use the function-based approach. Should I update the tests to match the new expected behavior, or keep some form of complex expression detection?

[TwitchBronBron]

Let's exclude operators like += for the scope of this work. Just focus on simple local variable assignment statements.

Be sure to remove the logic below that was handling this. we should probably only have the if(isassignmentstatement) block.

[Copilot]

Removed compound assignment operators support in f31b7d8. The feature now only supports simple local variable assignment statements like a = user ?? {}. Compound assignments like a += user ?? 0 now fall back to bslib_coalesce().