speed up parsing for short ipv4s in std::net::Ipv4Addr::from_str

library/std/src/net/parser.rs

@@ -62,6 +62,13 @@ impl<'a> Parser<'a> {
     where
         F: FnOnce(&mut Parser<'_>) -> Option<T>,
     {
+        // don't try to parse if too short or too long
+        if self.state.len() > kind.max_string_length()
+            || self.state.len() < kind.min_string_length()
+        {
+            return Err(AddrParseError(kind));
+        }
+
         let result = inner(self);
         if self.state.is_empty() { result } else { None }.ok_or(AddrParseError(kind))
     }
@@ -285,12 +292,7 @@ impl FromStr for IpAddr {
 impl FromStr for Ipv4Addr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
-        // don't try to parse if too long
-        if s.len() > 15 {
-            Err(AddrParseError(AddrKind::Ipv4))
-        } else {
-            Parser::new(s).parse_with(|p| p.read_ipv4_addr(), AddrKind::Ipv4)
-        }
+        Parser::new(s).parse_with(|p| p.read_ipv4_addr(), AddrKind::Ipv4)
     }
 }
 
@@ -336,6 +338,26 @@ enum AddrKind {
     SocketV6,
 }
 
+impl AddrKind {
+    // the minimum length of a parsable string with this `AddrKind`
+    const fn min_string_length(&self) -> usize {
+        match self {
+            Self::Ip | Self::Ipv4 => 7,
+            // FIXME
+            _ => 0,
+        }
+    }
+
+    // the maximum length of a parsable string with this `AddrKind`
+    const fn max_string_length(&self) -> usize {
+        match self {
+            Self::Ipv4 => 15,
+            // FIXME
+            _ => usize::MAX,
+        }
+    }
+}
+
 /// An error which can be returned when parsing an IP address or a socket address.
 ///
 /// This error is used as the error type for the [`FromStr`] implementation for