Svelte 5: Compiler creates unused variables

[KevsRepos]

Describe the bug

You can "confuse" the compiler by creating indentifiers that the compiler wanted to create itself. This causing the compiler to create more variables than necessary. Notice in the repl that there is count_const_1 and count_const_2, even though count_const_2 is never used.

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAA22OwQoDIQxEf0Vy3WJ7FrfQ76ilFDcLghtF49Ky-O_V7aE99JaZN5Nkg9l5zKCuG9BjQVBwiREOwK_YRV7RMzadQ0m2Ozrb5CILG4jxyaOBJUzFo4GzIcMeOyrEYhQnQ_r4iXdG-jvznhmGbv-U7m1r_lfddlyllO2XdtDNDidQnArWW30DTRUNSMYAAAA=

Logs

No response

System Info

System:
    OS: Linux 6.5 Pop!_OS 22.04 LTS
    CPU: (12) x64 12th Gen Intel(R) Core(TM) i5-1235U
    Memory: 4.49 GB / 15.32 GB
    Container: Yes
    Shell: 5.1.16 - /bin/bash
  Binaries:
    Node: 19.3.0 - ~/.nvm/versions/node/v19.3.0/bin/node
    npm: 9.2.0 - ~/.nvm/versions/node/v19.3.0/bin/npm
  Browsers:
    Chrome: 118.0.5993.70
    Firefox: 118.0.2

Severity

annoyance

[dummdidumm]

While it's true the variables are unused, bundlers are smart enough to dead-code-eliminate this, so I'm not sure how much time we should invest into not generating these.

[Rich-Harris]

I don't know, something is fishy here. Why is that declaration being generated twice? To be honest I'm not sure why we're creating a local copy of count inside the component at all. It all seems off

[Rich-Harris]

ok yeah, definitely fishy. this code is suspicious as all hell

1. why 'static through the lifetime of the template'?
2. checking state.init for a name + '_const' declaration is just about the hackiest approach to this we could dream up. this code should be nuked

svelte/packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

Lines 1540 to 1564 in b966470

	// When we combine expressions as part of a single template element, we might
	// be referencing variables that can be mutated, but are not actually state.
	// In order to prevent this undesired behavior, we need ensure we cache the
	// latest value we have of that variable before we process the template, enforcing
	// the value remains static through the lifetime of the template.
	if (binding !== null && binding.kind === 'normal' && binding.mutated) {
	let has_already_cached = false;
	// Check if we already create a const of this expression
	for (let node of state.init) {
	if (
	node.type === 'VariableDeclaration' &&
	node.declarations[0].id.type === 'Identifier' &&
	node.declarations[0].id.name === name + '_const'
	) {
	has_already_cached = true;
	expression = b.id(name + '_const');
	break;
	}
	}
	if (!has_already_cached) {
	const tmp_id = scope.generate(name + '_const');
	state.init.push(b.const(tmp_id, expression));
	expression = b.id(tmp_id);
	}
	}