ipnetwork: Deserialize str instead of allocating

Currently, `ipnetwork` always allocates during deserialization. This is
unfortunate because ownership isn't needed to construct any of
`ipnetwork`'s types.

I implemented a small Visitor that opportunistically deserializes from a
str borrowed from the deserializer. For some formats, this should help
zero alloc deserialization. While some formats will still allocate and
pass the deserializer a &str, this Visitor still avoids the second alloc
from String in the original.

Author: joshuamegnauth54

ipnetwork/src/ipv4.rs

@@ -20,8 +20,7 @@ impl<'de> serde::Deserialize<'de> for Ipv4Network {
     where
         D: serde::Deserializer<'de>,
     {
-        let s = <String>::deserialize(deserializer)?;
-        Ipv4Network::from_str(&s).map_err(serde::de::Error::custom)
+        deserializer.deserialize_str(crate::serde_helpers::FromStrVisitor(core::marker::PhantomData))
     }
 }
 

ipnetwork/src/ipv6.rs

@@ -21,8 +21,7 @@ impl<'de> serde::Deserialize<'de> for Ipv6Network {
     where
         D: serde::Deserializer<'de>,
     {
-        let s = <String>::deserialize(deserializer)?;
-        Ipv6Network::from_str(&s).map_err(serde::de::Error::custom)
+        deserializer.deserialize_str(crate::serde_helpers::FromStrVisitor(core::marker::PhantomData))
     }
 }
 

ipnetwork/src/lib.rs

@@ -19,6 +19,7 @@ mod error;
 mod ipv4;
 mod ipv6;
 mod parse;
+mod serde_helpers;
 mod size;
 
 pub use crate::{
@@ -42,8 +43,7 @@ impl<'de> serde::Deserialize<'de> for IpNetwork {
     where
         D: serde::Deserializer<'de>,
     {
-        let s = <String>::deserialize(deserializer)?;
-        IpNetwork::from_str(&s).map_err(serde::de::Error::custom)
+        deserializer.deserialize_str(crate::serde_helpers::FromStrVisitor(core::marker::PhantomData))
     }
 }
 

ipnetwork/src/serde_helpers.rs

@@ -0,0 +1,36 @@
+#![cfg(feature = "serde")]
+
+//! Useful [`serde`] helpers.
+
+use core::{fmt, marker::PhantomData, str::FromStr};
+
+use serde::de::{Error as DeError, Visitor};
+
+/// Deserialize a struct implementing [`FromStr`] without allocating a temporary String.
+pub(crate) struct FromStrVisitor<F>(pub PhantomData<F>);
+
+impl<'de, F> Visitor<'de> for FromStrVisitor<F>
+where
+    F: FromStr,
+    F::Err: fmt::Display,
+{
+    type Value = F;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        write!(formatter, "a network as a string")
+    }
+
+    fn visit_str<E>(self, v: &str) -> Result<Self::Value, E>
+    where
+        E: DeError,
+    {
+        F::from_str(v).map_err(DeError::custom)
+    }
+
+    fn visit_borrowed_str<E>(self, v: &'de str) -> Result<Self::Value, E>
+    where
+        E: DeError,
+    {
+        F::from_str(v).map_err(DeError::custom)
+    }
+}