fix: correct start of {:else if} and {:else}

[baseballyama]

Svelte 5 rewrite

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---

Currently, the AST node for {:else if} and {:else} has its start pointing to the start of the body. However, it should point to the start of the block.

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAE2WQu27DMAxFf4VQl7h1amd1HkDHDl2KbnUHQaYTAhJlSFSQwvC_FzKcPtJBD-rcyytpVD1ZjKp5HxVrh6pRT8OgSiWfQy7iGa2gKlX0KZh8sosm0CCHllvpExshz3BEedXceffMsnLEJTh9KWDMIgBHDHt40XJ6NEg2C4rtgvTlinrrfVhl38ICSgr8G87bMAetCriHrIZ17l_Aw7xsoareTpj7kksOKAJejE2RzgiaO5AMia-QeIE5cspTHsZzFDA-scD-7-PWm7qETT3fkXfVz2fweEf94jlAPfca53LKQceAWjCAnDRD3fLYoI0I35bdf4vFGG_0t5IMKuqnllWpnO-oJ-xUIyHh9DF9AVb2PN_dAQAA

With the PR, AST will be like this.

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[dummdidumm]

Why the change? What's currently there mimics what you get from the Svelte 4 AST - if you paste the code into the Svelte 4 REPL you get the same AST as on Svelte 5 preview.

If there is a good reason to change this we need to make sure to adjust the position to the start of the body when converting to the legacy AST.

[baseballyama]

On the AST, even though {#if} and {#else if} both use IfBlock, the start character of {#if} becomes start, while for {#else if}, the consequent part becomes start. This discrepancy should be corrected.

I have checked this in Svelte 4 now, and I think this should fix in Svelte 4 as well, I think. If there is an issue in some Svelte ecosystem, I can implement converting logic for the legacy AST.

[baseballyama]

Why the change?

I am currently working on component-level tree shaking and needed this.
Now start of IfBlock is different in {#if} and {#else if}, so it's difficult to know the actual start of the block when I walk AST.

[dummdidumm]

I'm probably ok with changing the behavior for the new AST, but we absolutely should keep it as-is for the legacy AST. I'm pretty sure there's logic in language tools / prettier that relies on the start position being that way.

[Rich-Harris]

I kinda wonder if we should change the shape of the AST in this case rather than just moving the start, because no start position is exactly correct (though I do think the start in this PR makes more sense than the status quo, aside from any questions about whether it's a worthwhile change).

Something like this:

export interface IfBlock extends BaseNode {
  type: 'IfBlock';
-  elseif: boolean;
  test: Expression;
  consequent: Fragment;
-  alternate: Fragment | null;
+  alternate: ElseIfBlock | Fragment | null;
}

+export interface ElseIfBlock extends IfBlock {
+  type: 'ElseIfBlock'
+}

[baseballyama]

It may be noted that if ElseIfBlock is used, the shape will be slightly different from the JavaScript AST.
(acorn and other parsers are using IfStatement for else if.)

In any case, it is strange that the start of the IfBlock is a larger number than the start of the test node, so it is better to fix this in some way.
In the meantime, I will add the conversion code to legacy AST.

[baseballyama]

@dummdidumm @Rich-Harris

In my personal conclusion, I believe it’s better to use IfBlock and just modify the start property, rather than creating a separate ElseIfBlock.

Reason 1: It’s preferable to keep the structure consistent with the JavaScript AST. Different AST structures for JavaScript and Svelte IF statements can cause confusion. It’s better if only the loc property differs slightly.

Reason 2: DRY principle regression.
There are 14 logic locations related to IfBlock, with most of the processing being common between IfBlock and ElseIfBlock. Separating them could increase the risk of implementation errors due to duplicate code.