transformer: incorrect semantic check

[Dunqing]

I saw nearly all tests have a Symbols mismatch after transform error. This error is thrown here

oxc/crates/oxc_semantic/src/post_transform_checker.rs

Lines 129 to 132 in 081e2a3

	if self.previous_collect.symbol_ids.len() != current_collect.symbol_ids.len() {
	self.errors.push(OxcDiagnostic::error("Symbols mismatch after transform"));
	return;
	}

After looking into it, what happened here, I found a bug in PostTransformChecker. From the code above, it can be seen that this is due to the unequal length of semantic symbols in the two different phases.

The previous_collect is collected in after_semantic and the current_collect is collected in after_transform. This is the cause of the problem. After the transformation, the Semantic will definitely change.

So we shouldn't use after_semantic's semantic to compare with after_transform's semantic. Instead, we should rebuild the semantic after the transformation, and compare it to after_transform's semantic

[Dunqing]

We may need to solve this problem as soon as possible. Since we can't catch valid any regression in semantic changes, I am a bit afraid to refactor the transformer 🥲

[overlookmotel]

I'm going to look at this tomorrow. Will try to wrap it up as quickly as possible.

[Boshen]

@Dunqing Can you give us a test case? Ideally what's wrong and the expected result?

[Dunqing]

In the following test used nullish-coalescing-operators plugin

Input

// nullish-coalescing/transform-in-function/input.js
function foo(opts) {
  var foo = opts.foo ?? "default";
}

Output

// nullish-coalescing/transform-in-function/output.js
function foo(opts) {
  var _opts$foo;
  var foo = (_opts$foo = opts.foo) !== null && _opts$foo !== void 0 ? _opts$foo : "default";
}

The previous_collect.symbol_ids have 3 symbols, They are foo(function name), opts, and foo(var foo).
The current_collect.symbol_ids have 4 symbols, They are foo(function name), opts, _opts$foo, and foo(var foo).

Both of the different phases' symbols are correct. The root cause is we use after_semantic's Semantic to compare with after_transform's Semantic.

[overlookmotel]

Fixed in #5035.