Write index routes as index.html with build.format: 'file'

[matthewp]

Changes

• Previously we ignored whether a route was an index route (aka index.astro or index.md) when writing files with build.format: 'file'. This is because the logic only looked at the pathname and didn't know it was an index route.
• This was unexpected given the nature of file-based routing. The fix is to keep track of index routes in routing and use that to write out the correct file.
• Fixes Index pages appear to be invalid according to web standards (i.e. Apache MultiViews and others) #6947

Testing

• Test case added

Docs

• Astro 4.0 breaking changes tracker docs#5390 (comment)

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[bluwy]

I think the previous behaviour is expected to me:

• format: 'file' forces foo/index.html to foo.html
• format: 'directory' forces foo.html to foo/index.html

The "forcing" style intentionally doesn't respect how you declare your files. With this PR, it also slightly conflicts with our recommendation here:

To prevent inconsistencies with trailing slash behaviour in dev, you can restrict the trailingSlash option to 'always' or 'never' depending on your build format:

• directory - Set trailingSlash: 'always'
• file - Set trailingSlash: 'never'

---

Practically speaking though, I prefer that Astro doesn't force a certain build format style and always respect how the files were declared. This PR seems to make build.format: 'file' that way, but maybe it makes more sense if it's build.format: 'preserve' instead 🤔

[MoustaphaDev]

Nit: Maybe we could omit the check for an empty pathname since, in this scenario, it's guaranteed to represent an index route, isn't it?

[matthewp]

@bluwy I, good point. I'll move this back into draft for discussion then.

[matthewp]

@bluwy what would you think about adding this as format: 'preserve', keeping 'file' as it currently is, and then deprecating 'file' (remove in the next major)?

[bluwy]

That makes sense to me 👍 aside from this part:

and then deprecating 'file' (remove in the next major)?

I think it might still be worth keeping it. Currently it seems like there's a symmetry with trailingSlash that:

build.format	trailingSlash
file	never
directory	always
preserve	ignore

So if we plan on keeping trailingSlash: 'never', it might make sense to also keep build.format: 'file'. The confusing thing is that we have two options that are now even similar 😅 The last time I tracked this down lead me to #1197 but I don't understand why there's two options.

[matthewp]

We're probably going to do this in a minor release as the discussion above about making this a new format means it will not be a breaking change. So deprioritizing, but keeping on my personal board.

[matthewp]

Closing the PR, but will reopen later with new implementation.