Compile-time url parsing 'url!' macro

[frewsxcv]

Inspired by sfackler/rust-postgres-macros which is a compile time SQL parser

There are ton of instances in servo, where we do something like Url::parse("http://mozilla.com").unwrap(). It'd be great if we could do that validation at compile time with a macro (e.g. url!("http://mozilla.com")).

I have no idea how feasible this is, just throwing around the idea.

[SimonSapin]

Sounds feasible with a plugin, maybe not specific to URLs. Anything that implements FromStr?

[KiChjang]

I wonder whether regex would be enough to parse URLs? The language of URLs seem regular enough to me (I may be wrong though).

[SimonSapin]

@KiChjang A regex may(?) be able to separate the components of an URL, but the parser as specified in https://url.spec.whatwg.org/#url-parsing does more stuff along the way such as percent-decoding, IDNA, resolving relative URLs, normalizing (anti)slashes, collapsing . and .. path components, etc. Anyway, this is unrelated to this issue.

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Making a plugin generic over T: FromStr wouldn’t work, the plugin needs to know at its own compile-time which crates it should depend on.

Another approach might be to skip string parsing entirely. All fields of Url are public, so it can be constructed directly as a struct literal.

[SimonSapin]

Larger explanation of the above, copied from IRC:

• SimonSapin: You can make a compiler plugin (syntax extension) so that url!("http://example") runs Url::parse() at compile time and expands the result to a struct literal. Then I was thinking, why not generalize this to anything that implements FromStr?
• SimonSapin: like from_str!(::url::Url, "http://example")
• SimonSapin: the problem is that by the time such a plugin would see the ::url::Url path, it’s already compiled and can not add to itself a new dependency to the url crate

[frewsxcv]

@SimonSapin @KiChjang just to prevent duplicate efforts, are either of you working on this currently?

[KiChjang]

No I'm not working on it at all. You can assign the issue to yourself.

[frewsxcv]

Opened a PR #137

[nox]

#137

[DenisGorbachev]

There's a url-macro crate that provides url! macro for compile-time validation