transformer: non-type-only import is only referenced by the type, but is not eliminated.

[Dunqing]

esbuild handles this case correctly. esbuild repl

OXC Playground

In

import { T } from 'mod'
let T: T = 0;
export { T }

Out

import { T } from "mod";
let T = 0;
export { T };

Expected

let T = 0;
export { T };

[7086cmd]

May I try it? I am going to investigate this issue to enhance the understanding and functionality of oxc. Nevermind for better solutions :)

[Dunqing]

May I try it? I am going to investigate this issue to enhance the understanding and functionality of oxc. Nevermind for better solutions :)

Go for it!

[Dunqing]

I came up with a solution to this problem. We need to check if the next value binding definition has a value reference before defining it. If it doesn't, we can safely remove the import.

To make it possible, we need to add SymbolFlags in

oxc/crates/oxc_semantic/src/symbol.rs

Lines 43 to 45 in 2c431cb

	redeclarations: IndexVec<SymbolId, Option<RedeclarationId>>,
	redeclaration_spans: IndexVec<RedeclarationId, Vec<Span>>,

This could help us to find if it is a value symbol or a type symbol from redeclarations

Also, Since the transformer doesn't have a whole Semantic, so we can't get the reference's span by Semantic. So I'm going to revert #4464.

@overlookmotel Can you confirm this for me?

[overlookmotel]

I'm not familiar with the changes you've made over past few weeks to resolve TS type references. But I'd have assumed could figure out that can remove import { T } from 'mod'; without needing to figure out Span.

But, given that I don't understand how it's working now, I could well be wrong.

I don't object if you want to revert #4464. Really I didn't want to be making changes to Semantic until I've finished my "review". Mainly I made that PR just to inform that review - to research how much perf boost we can get by reducing the size of Semantic's structures.

[magic-akari]

It's hard to verify; consider this situation.

// mod.ts
export class T {}

In this situation, the value of T and its type have been exported.
I believe that only a comprehensive type analyzer like TSC can solve this problem.
Or a bundle tool like esbuild, which can analyze across files, thereby eliminating intermediate values.

[magic-akari]

Oh, I misunderstood, this is about the issue of name resolution.

[Dunqing]

After digging deeper into how TypeScript handles this, I realized that we don't need to make any changes to what I said above! Sorry to bother you @overlookmotel

[DonIsaac]

@Dunqing can we close this?