Restore DNS resolution functionality from url 1.x

[SimonSapin]

This is an alternative to #533.

I feel that an inherent method is more appropriate than an impl of the ToSocketAddrs trait: it is not the URL as a whole that is converted, only parts of it.

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This change is 

[SimonSapin]

r=nox per IRC discussion.

@benesch what do you think of this API?

[benesch]

I feel that an inherent method is more appropriate than an impl of the ToSocketAddrs trait: it is not the URL as a whole that is converted, only parts of it.

Perhaps you have a more nuanced understanding of what it means to implement ToSocketAddrs than I do. The Rust docs say ToSocketAddrs is "a trait for objects which can be converted or resolved to one or more SocketAddr values", which seems applicable enough to Url. To be honest, I'm not sure I see how a method named socket_addrs on Url is better at conveying that only part of the URL is used in the conversion.

I'd also like to point out that, in #533, implementing ToSocketAddrs on Url itself isn't strictly required. We could just restore a slightly tweaked with_default_port method instead:

+    pub fn with_default_port<F>(&self, f: F) -> io::Result<(&str, u16)>
+    where
+        F: FnOnce(&Url) -> Result<u16, ()>,
+    {
+        Ok((self
+                .host_str()
+                .ok_or(())
+                .or_else(|()| io_error("URL has no host"))?,
+            self
+                .port_or_known_default()
+                .ok_or(())
+                .or_else(|()| f(self))
+                .or_else(|()| io_error("URL has no port number"))?,
+        ))
+    }
+

As I mentioned in #533, this approach has the definite defect that it may cause an already-parsed IP address to be reparsed, a defect which is not present in this PR. (Although the extra vec allocation might mean that it's a wash.) But it has the upside that a lot of code written against 1.0 that used with_default_port would just work with this implementation with no code changes.

But! I'm happy enough with the approach in this PR! I'm quite glad I was able to convince you to restore this functionality at all. :)

[SimonSapin]

It’s a bit hand-wavy, I interpret “objects which can be converted or resolved to” as the object is something that can be converted, whereas I view URLs as having components that can be converted. They also have other components that are not relevant to that conversion.

@bors-servo r=nox

[bors-servo]

📌 Commit 2f199ff has been approved by nox

[bors-servo]

⌛ Testing commit 2f199ff with merge a93e8cb...

[bors-servo]

☀️ Test successful - checks-travis
Approved by: nox
Pushing a93e8cb to master...

[SimonSapin]

This is in https://crates.io/crates/url/2.1.0