Make all scopes unconditional

[overlookmotel]

Almost all nodes which can hold a scope always hold a scope. But a small handful only create a scope if required:

• ForStatement
• ForInStatement
• ForOfStatement
• CatchClause

I propose that we make all scopes unconditional i.e. all these nodes always have a scope.

My rationale is:

• Simplicity - especially in transformer where scope-related logic is already complicated.
• In most JS code, these nodes will create a scope anyway (for (let item of arr) is more common than for (item of arr), and catch (err) { is much more common than catch {).
• Resolving references against a scope which has no bindings is very cheap.
• These are not the most common constructs (vs if or function), so even if it wasn't so cheap, a few extra unnecessary scopes would not hurt perf much anyway.

If I remember right, Dunqing was in favour of this a while back, but I resisted. At that point I didn't realise how few nodes had conditional scopes, so I thought the perf upside of eliding them was much bigger than it is.

[Dunqing]

• Simplicity - especially in transformer where scope-related logic is already complicated.

Yes, I agreed

[Dunqing]

We need to remove these conditional in SemanticBuilder

oxc/crates/oxc_semantic/src/builder.rs

Lines 946 to 950 in 4e1f4ab

	let is_lexical_declaration =
	stmt.init.as_ref().is_some_and(ForStatementInit::is_lexical_declaration);
	if is_lexical_declaration {
	self.enter_scope(ScopeFlags::empty(), &stmt.scope_id);
	}

[overlookmotel]

Glad you agree!

We would also need to remove the #[scope(if(...))] attrs from AST (and remove support for that attr from ast_tools and from Traverse codegen).

[overlookmotel]

@Boshen Would you be happy with this change?

[Dunqing]

@Boshen Would you be happy with this change?

I bet @Boshen totally agree this, he likes the standard way to handle scopes 😀

[Boshen]

[overlookmotel]

What does a mouse dressed as a detective mean??!

[Boshen]

What does a mouse dressed as a detective mean??!

:shipit: https://www.quora.com/On-GitHub-what-is-the-significance-of-the-Ship-It-squirrel

Sorry I'm old.

[overlookmotel]

Not as old as me! But thanks for explaining.

[overlookmotel]

Implemented in #5114 and preceding PRs.