[Routing][Twig] allow generation of request-path-relative URLs

[Tobion]

At the moment the Routing component only supports absolute and domain-relative URLs, e.g.
http://example.org/user-slug/article-slug/comments and
/user-slug/article-slug/comments.

But I find it very useful to specify path-relative URLs that depend on the current request path. Lets say the client requests http://example.org/user-slug/article-slug/ and I want to generate a link to the comments of the same article.
Currently the resulting link would again duplicate the user-slug and article-slug -> /user-slug/article-slug/comments.
But it could be simplified to comments or ./comments.

One additional feature that could be implemented is a new twig function (besides url and path) lets call it subpath.
It fetches the variables of the current request, e.g. user-slug and article-slug internally and then generates such a relative URL. The advantage is you don't need to specify the variables of the current request again.
So simply {{ subpath('article_comments') }} is enough for the URL ./comments. But you can optionally overwrite some request variables with {{ subpath('article_comments', {'article': 'different-article-slug'} ) }} which would generate ../different-article-slug/comments (i.e. same user, but different article).

Thinking of it, the twig function is also a quite useful without the path-relative URLs. So these are basically two feature requests.
I already got an idea how to implement this but would like to get your feedback whether one of these features is also useful to others.

[fabpot]

That would completely defeat the routing system main goal, which is to abstract to real URLs from the user perspective. So, definitely -1. By the way, you can already do that with the help of the various Request methods.

[Tobion]

Pardon, you must have misunderstood something. It still abstracts the real URLs as you only specify the route name as before.
Please reopen.

[linniksa]

@Tobion
What reason of generate path-relative URLs ? reduce the length of links?

[Tobion]

Yes, imagine you are at user/myusername/articles/ and list all articles of a user there, which can be many, you will each time have a url link like user/myusername/articles/myarticle1, user/myusername/articles/myarticle2, ...
But you could reduce it to myarticle1, myarticle2, ...
This would save pagesize -> bandwidth (costs) and impvove faster page loads -> better user experience. This might be exaggerated, but you get the point.
In my case I also have many semantic annotations in the background (RDFa), that reference resources which are also identified with URIs. These will be reduced, too.

[AurelC2G]

@Tobion Even if you only specify the route name, you still assume that article_comments is in a subpath of article_show. But for all you know, those two could yield two completely different paths. What would be the generated subpath then ? Something like ../../../foo/bar/user-slug/article-slug/comments ? I think I am with Fabien on that one.

[Tobion]

Of couse, if the parent doesn't match, the relative link needs to travel upwards.
If you know beforehand that this is the case you probably won't use this function. But even when you do, it still works.

[Tobion]

One more use case: If you want to generate static documentation that can be downloaded as an HTML archive, your only option is to use path-relative links from one file to another. At the moment symfony cannot be used for that.