multiaddr: feature gate url dependency (#1843)

* multiaddr: feature gate url dependency

Make it possible to exclude url and its dependencies
from the dependency tree, by feature gating the
from_url* functions in the public API.

* Document feature gating of multiaddr::from_url*

misc/multiaddr/src/from_url.rs

@@ -7,6 +7,9 @@ use std::{error, fmt, iter, net::IpAddr};
 /// generated multiaddress. This includes a username, password, path (if not supported by the
 /// multiaddr), and query string.
 ///
+/// This function is only present if the `url` feature is enabled, and it is
+/// enabled by default.
+///
 /// The supported URL schemes are:
 ///
 /// - `ws://example.com/`
@@ -31,6 +34,9 @@ pub fn from_url(url: &str) -> std::result::Result<Multiaddr, FromUrlErr> {
 /// This function is similar to [`from_url`], except that we don't return an error if some
 /// information in the URL cannot be retain in the generated multiaddres.
 ///
+/// This function is only present if the `url` feature is enabled, and it is
+/// enabled by default.
+///
 /// # Example
 ///
 /// ```

misc/multiaddr/src/lib.rs

@@ -5,6 +5,8 @@ pub use multihash;
 mod protocol;
 mod onion_addr;
 mod errors;
+
+#[cfg(feature = "url")]
 mod from_url;
 
 use serde::{
@@ -25,10 +27,12 @@ use std::{
     sync::Arc
 };
 pub use self::errors::{Result, Error};
-pub use self::from_url::{FromUrlErr, from_url, from_url_lossy};
 pub use self::protocol::Protocol;
 pub use self::onion_addr::Onion3Addr;
 
+#[cfg(feature = "url")]
+pub use self::from_url::{FromUrlErr, from_url, from_url_lossy};
+
 static_assertions::const_assert! {
     // This check is most certainly overkill right now, but done here
     // anyway to ensure the `as u64` casts in this crate are safe.