Bring net/parser.rs up to modern up to date with modern rust patterns

Made the following changes throughout the IP address parser:
- Replaced all uses of `is_some()` / `is_none()` with `?`.
- "Upgraded" loops wherever possible; ie, replace `while` with `for`, etc.
    - Removed all cases of manual index tracking / incrementing.
- Renamed several single-character variables with more expressive names.
- Replaced several manual control flow segments with equivalent adapters (such as `Option::filter`).
- Removed `read_seq_3`; replaced with simple sequences of `?`.
- Parser now reslices its state when consuming, rather than carrying a separate state and index variable.
- `read_digit` now uses `char::to_digit`.
- Removed unnecessary casts back and forth between u8 and u32
- Added comments throughout, especially in the complex IPv6 parsing logic.
- Added comprehensive local unit tests for the parser to validate these changes.

src/libstd/net/parser.rs

@@ -10,163 +10,132 @@
 
 struct Parser<'a> {
     // parsing as ASCII, so can use byte array
-    s: &'a [u8],
-    pos: usize,
+    state: &'a [u8],
 }
 
 impl<'a> Parser<'a> {
-    fn new(s: &'a str) -> Parser<'a> {
-        Parser { s: s.as_bytes(), pos: 0 }
+    fn new(input: &'a str) -> Parser<'a> {
+        Parser { state: input.as_bytes() }
     }
 
     fn is_eof(&self) -> bool {
-        self.pos == self.s.len()
+        self.state.is_empty()
     }
 
-    // Commit only if parser returns Some
-    fn read_atomically<T, F>(&mut self, cb: F) -> Option<T>
+    /// Run a parser, and restore the pre-parse state if it fails
+    fn read_atomically<T, F>(&mut self, inner: F) -> Option<T>
     where
         F: FnOnce(&mut Parser<'_>) -> Option<T>,
     {
-        let pos = self.pos;
-        let r = cb(self);
-        if r.is_none() {
-            self.pos = pos;
+        let state = self.state;
+        let result = inner(self);
+        if result.is_none() {
+            self.state = state;
         }
-        r
+        result
     }
 
-    // Commit only if parser read till EOF
-    fn read_till_eof<T, F>(&mut self, cb: F) -> Option<T>
+    /// Run a parser, but fail if the entire input wasn't consumed.
+    /// Doesn't run atomically.
+    fn read_till_eof<T, F>(&mut self, inner: F) -> Option<T>
     where
         F: FnOnce(&mut Parser<'_>) -> Option<T>,
     {
-        self.read_atomically(move |p| cb(p).filter(|_| p.is_eof()))
+        inner(self).filter(|_| self.is_eof())
     }
 
-    // Apply 3 parsers sequentially
-    fn read_seq_3<A, B, C, PA, PB, PC>(&mut self, pa: PA, pb: PB, pc: PC) -> Option<(A, B, C)>
+    /// Same as read_till_eof, but returns a Result<AddrParseError> on failure
+    fn parse_with<T, F>(&mut self, inner: F) -> Result<T, AddrParseError>
     where
-        PA: FnOnce(&mut Parser<'_>) -> Option<A>,
-        PB: FnOnce(&mut Parser<'_>) -> Option<B>,
-        PC: FnOnce(&mut Parser<'_>) -> Option<C>,
+        F: FnOnce(&mut Parser<'_>) -> Option<T>,
+    {
+        self.read_till_eof(inner).ok_or(AddrParseError(()))
+    }
+
+    /// Read the next character from the input
+    fn read_char(&mut self) -> Option<char> {
+        self.state.split_first().map(|(&b, tail)| {
+            self.state = tail;
+            b as char
+        })
+    }
+
+    /// Read the next character from the input if it matches the target
+    fn read_given_char(&mut self, target: char) -> Option<char> {
+        self.read_atomically(|p| p.read_char().filter(|&c| c == target))
+    }
+
+    /// Helper for reading separators in an indexed loop. Reads the separator
+    /// character iff index > 0, then runs the parser. When used in a loop,
+    /// the separator character will only be read on index > 0 (see
+    /// read_ipv4_addr for an example)
+    fn read_separator<T, F>(&mut self, sep: char, index: usize, inner: F) -> Option<T>
+    where
+        F: FnOnce(&mut Parser<'_>) -> Option<T>,
     {
         self.read_atomically(move |p| {
-            let a = pa(p);
-            let b = if a.is_some() { pb(p) } else { None };
-            let c = if b.is_some() { pc(p) } else { None };
-            match (a, b, c) {
-                (Some(a), Some(b), Some(c)) => Some((a, b, c)),
-                _ => None,
+            if index > 0 {
+                let _ = p.read_given_char(sep)?;
             }
+            inner(p)
         })
     }
 
-    // Read next char
-    fn read_char(&mut self) -> Option<char> {
-        if self.is_eof() {
-            None
-        } else {
-            let r = self.s[self.pos] as char;
-            self.pos += 1;
-            Some(r)
-        }
+    // Read a single digit in the given radix. For instance, 0-9 in radix 10;
+    // 0-9A-F in radix 16.
+    fn read_digit(&mut self, radix: u32) -> Option<u32> {
+        self.read_atomically(move |p| p.read_char()?.to_digit(radix))
     }
 
-    // Return char and advance iff next char is equal to requested
-    fn read_given_char(&mut self, c: char) -> Option<char> {
-        self.read_atomically(|p| match p.read_char() {
-            Some(next) if next == c => Some(next),
-            _ => None,
+    // Read a number off the front of the input in the given radix, stopping
+    // at the first non-digit character or eof. Fails if the number has more
+    // digits than max_digits, or the value is >= upto, or if there is no number.
+    fn read_number(&mut self, radix: u32, max_digits: u32, upto: u32) -> Option<u32> {
+        self.read_atomically(move |p| {
+            let mut result = 0;
+            let mut digit_count = 0;
+
+            while let Some(digit) = p.read_digit(radix) {
+                result = (result * radix) + digit;
+                digit_count += 1;
+                if digit_count > max_digits || result >= upto {
+                    return None;
+                }
+            }
+
+            if digit_count == 0 { None } else { Some(result) }
         })
     }
 
-    // Read digit
-    fn read_digit(&mut self, radix: u8) -> Option<u8> {
-        fn parse_digit(c: char, radix: u8) -> Option<u8> {
-            let c = c as u8;
-            // assuming radix is either 10 or 16
-            if c >= b'0' && c <= b'9' {
-                Some(c - b'0')
-            } else if radix > 10 && c >= b'a' && c < b'a' + (radix - 10) {
-                Some(c - b'a' + 10)
-            } else if radix > 10 && c >= b'A' && c < b'A' + (radix - 10) {
-                Some(c - b'A' + 10)
-            } else {
-                None
-            }
-        }
-
-        self.read_atomically(|p| p.read_char().and_then(|c| parse_digit(c, radix)))
-    }
-
-    fn read_number_impl(&mut self, radix: u8, max_digits: u32, upto: u32) -> Option<u32> {
-        let mut r = 0;
-        let mut digit_count = 0;
-        loop {
-            match self.read_digit(radix) {
-                Some(d) => {
-                    r = r * (radix as u32) + (d as u32);
-                    digit_count += 1;
-                    if digit_count > max_digits || r >= upto {
-                        return None;
-                    }
-                }
-                None => {
-                    if digit_count == 0 {
-                        return None;
-                    } else {
-                        return Some(r);
-                    }
-                }
-            };
-        }
-    }
-
-    // Read number, failing if max_digits of number value exceeded
-    fn read_number(&mut self, radix: u8, max_digits: u32, upto: u32) -> Option<u32> {
-        self.read_atomically(|p| p.read_number_impl(radix, max_digits, upto))
-    }
-
-    fn read_ipv4_addr_impl(&mut self) -> Option<Ipv4Addr> {
-        let mut bs = [0; 4];
-        let mut i = 0;
-        while i < 4 {
-            if i != 0 && self.read_given_char('.').is_none() {
-                return None;
-            }
-
-            bs[i] = self.read_number(10, 3, 0x100).map(|n| n as u8)?;
-            i += 1;
-        }
-        Some(Ipv4Addr::new(bs[0], bs[1], bs[2], bs[3]))
-    }
-
-    // Read IPv4 address
+    /// Read an IPv4 address
     fn read_ipv4_addr(&mut self) -> Option<Ipv4Addr> {
-        self.read_atomically(|p| p.read_ipv4_addr_impl())
+        self.read_atomically(|p| {
+            let mut groups = [0; 4];
+
+            for (i, slot) in groups.iter_mut().enumerate() {
+                *slot = p.read_separator('.', i, |p| p.read_number(10, 3, 0x100))? as u8;
+            }
+
+            Some(groups.into())
+        })
     }
 
-    fn read_ipv6_addr_impl(&mut self) -> Option<Ipv6Addr> {
-        fn ipv6_addr_from_head_tail(head: &[u16], tail: &[u16]) -> Ipv6Addr {
-            assert!(head.len() + tail.len() <= 8);
-            let mut gs = [0; 8];
-            gs[..head.len()].copy_from_slice(head);
-            gs[(8 - tail.len())..8].copy_from_slice(tail);
-            Ipv6Addr::new(gs[0], gs[1], gs[2], gs[3], gs[4], gs[5], gs[6], gs[7])
-        }
+    /// Read an IPV6 Address
+    fn read_ipv6_addr(&mut self) -> Option<Ipv6Addr> {
+        /// Read a chunk of an ipv6 address into `groups`. Returns the number
+        /// of groups read, along with a bool indicating if an embedded
+        /// trailing ipv4 address was read. Specifically, read a series of
+        /// colon-separated ipv6 groups (0x0000 - 0xFFFF), with an optional
+        /// trailing embedded ipv4 address.
+        fn read_groups(p: &mut Parser<'_>, groups: &mut [u16]) -> (usize, bool) {
+            let limit = groups.len();
 
-        fn read_groups(p: &mut Parser<'_>, groups: &mut [u16; 8], limit: usize) -> (usize, bool) {
-            let mut i = 0;
-            while i < limit {
+            for (i, slot) in groups.iter_mut().enumerate() {
+                // Try to read a trailing embedded ipv4 address. There must be
+                // at least two groups left.
                 if i < limit - 1 {
-                    let ipv4 = p.read_atomically(|p| {
-                        if i == 0 || p.read_given_char(':').is_some() {
-                            p.read_ipv4_addr()
-                        } else {
-                            None
-                        }
-                    });
+                    let ipv4 = p.read_separator(':', i, |p| p.read_ipv4_addr());
+
                     if let Some(v4_addr) = ipv4 {
                         let octets = v4_addr.octets();
                         groups[i + 0] = ((octets[0] as u16) << 8) | (octets[1] as u16);
@@ -175,83 +144,85 @@ fn ipv6_addr_from_head_tail(head: &[u16], tail: &[u16]) -> Ipv6Addr {
                     }
                 }
 
-                let group = p.read_atomically(|p| {
-                    if i == 0 || p.read_given_char(':').is_some() {
-                        p.read_number(16, 4, 0x10000).map(|n| n as u16)
-                    } else {
-                        None
-                    }
-                });
+                let group = p.read_separator(':', i, |p| p.read_number(16, 4, 0x10000));
+
                 match group {
-                    Some(g) => groups[i] = g,
+                    Some(g) => *slot = g as u16,
                     None => return (i, false),
                 }
-                i += 1;
             }
-            (i, false)
+            (groups.len(), false)
         }
 
-        let mut head = [0; 8];
-        let (head_size, head_ipv4) = read_groups(self, &mut head, 8);
+        self.read_atomically(|p| {
+            // Read the front part of the address; either the whole thing, or up
+            // to the first ::
+            let mut head = [0; 8];
+            let (head_size, head_ipv4) = read_groups(p, &mut head);
 
-        if head_size == 8 {
-            return Some(Ipv6Addr::new(
-                head[0], head[1], head[2], head[3], head[4], head[5], head[6], head[7],
-            ));
-        }
+            if head_size == 8 {
+                return Some(head.into());
+            }
 
-        // IPv4 part is not allowed before `::`
-        if head_ipv4 {
-            return None;
-        }
+            // IPv4 part is not allowed before `::`
+            if head_ipv4 {
+                return None;
+            }
 
-        // read `::` if previous code parsed less than 8 groups
-        if self.read_given_char(':').is_none() || self.read_given_char(':').is_none() {
-            return None;
-        }
+            // read `::` if previous code parsed less than 8 groups
+            // `::` indicates one or more groups of 16 bits of zeros
+            let _ = p.read_given_char(':')?;
+            let _ = p.read_given_char(':')?;
 
-        let mut tail = [0; 8];
-        // `::` indicates one or more groups of 16 bits of zeros
-        let limit = 8 - (head_size + 1);
-        let (tail_size, _) = read_groups(self, &mut tail, limit);
-        Some(ipv6_addr_from_head_tail(&head[..head_size], &tail[..tail_size]))
-    }
-
-    fn read_ipv6_addr(&mut self) -> Option<Ipv6Addr> {
-        self.read_atomically(|p| p.read_ipv6_addr_impl())
+            // Read the back part of the address. The :: must contain at least one
+            // set of zeroes, so our max length is 7.
+            let mut tail = [0; 7];
+            let limit = 8 - (head_size + 1);
+            let (tail_size, _) = read_groups(p, &mut tail[..limit]);
+
+            // Concat the head and tail of the IP address
+            head[(8 - tail_size)..8].copy_from_slice(&tail[..tail_size]);
+
+            Some(head.into())
+        })
     }
 
+    /// Read an IP Address, either IPV4 or IPV6.
     fn read_ip_addr(&mut self) -> Option<IpAddr> {
-        self.read_ipv4_addr().map(IpAddr::V4).or_else(|| self.read_ipv6_addr().map(IpAddr::V6))
+        self.read_ipv4_addr().map(IpAddr::V4).or_else(move || self.read_ipv6_addr().map(IpAddr::V6))
     }
 
+    /// Read a : followed by a port in base 10
+    fn read_port(&mut self) -> Option<u16> {
+        self.read_atomically(|p| {
+            let _ = p.read_given_char(':')?;
+            let port = p.read_number(10, 5, 0x10000)?;
+            Some(port as u16)
+        })
+    }
+
+    /// Read an IPV4 address with a port
     fn read_socket_addr_v4(&mut self) -> Option<SocketAddrV4> {
-        let ip_addr = |p: &mut Parser<'_>| p.read_ipv4_addr();
-        let colon = |p: &mut Parser<'_>| p.read_given_char(':');
-        let port = |p: &mut Parser<'_>| p.read_number(10, 5, 0x10000).map(|n| n as u16);
-
-        self.read_seq_3(ip_addr, colon, port).map(|t| {
-            let (ip, _, port): (Ipv4Addr, char, u16) = t;
-            SocketAddrV4::new(ip, port)
+        self.read_atomically(|p| {
+            let ip = p.read_ipv4_addr()?;
+            let port = p.read_port()?;
+            Some(SocketAddrV4::new(ip, port))
         })
     }
 
+    /// Read an IPV6 address with a port
     fn read_socket_addr_v6(&mut self) -> Option<SocketAddrV6> {
-        let ip_addr = |p: &mut Parser<'_>| {
-            let open_br = |p: &mut Parser<'_>| p.read_given_char('[');
-            let ip_addr = |p: &mut Parser<'_>| p.read_ipv6_addr();
-            let clos_br = |p: &mut Parser<'_>| p.read_given_char(']');
-            p.read_seq_3(open_br, ip_addr, clos_br).map(|t| t.1)
-        };
-        let colon = |p: &mut Parser<'_>| p.read_given_char(':');
-        let port = |p: &mut Parser<'_>| p.read_number(10, 5, 0x10000).map(|n| n as u16);
+        self.read_atomically(|p| {
+            let _ = p.read_given_char('[')?;
+            let ip = p.read_ipv6_addr()?;
+            let _ = p.read_given_char(']')?;
 
-        self.read_seq_3(ip_addr, colon, port).map(|t| {
-            let (ip, _, port): (Ipv6Addr, char, u16) = t;
-            SocketAddrV6::new(ip, port, 0, 0)
+            let port = p.read_port()?;
+            Some(SocketAddrV6::new(ip, port, 0, 0))
         })
     }
 
+    /// Read an IP address with a port
     fn read_socket_addr(&mut self) -> Option<SocketAddr> {
         self.read_socket_addr_v4()
             .map(SocketAddr::V4)
@@ -263,10 +234,7 @@ fn read_socket_addr(&mut self) -> Option<SocketAddr> {
 impl FromStr for IpAddr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<IpAddr, AddrParseError> {
-        match Parser::new(s).read_till_eof(|p| p.read_ip_addr()) {
-            Some(s) => Ok(s),
-            None => Err(AddrParseError(())),
-        }
+        Parser::new(s).parse_with(|p| p.read_ip_addr())
     }
 }
 
@@ -274,10 +242,7 @@ fn from_str(s: &str) -> Result<IpAddr, AddrParseError> {
 impl FromStr for Ipv4Addr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
-        match Parser::new(s).read_till_eof(|p| p.read_ipv4_addr()) {
-            Some(s) => Ok(s),
-            None => Err(AddrParseError(())),
-        }
+        Parser::new(s).parse_with(|p| p.read_ipv4_addr())
     }
 }
 
@@ -285,10 +250,7 @@ fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
 impl FromStr for Ipv6Addr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<Ipv6Addr, AddrParseError> {
-        match Parser::new(s).read_till_eof(|p| p.read_ipv6_addr()) {
-            Some(s) => Ok(s),
-            None => Err(AddrParseError(())),
-        }
+        Parser::new(s).parse_with(|p| p.read_ipv6_addr())
     }
 }
 
@@ -296,10 +258,7 @@ fn from_str(s: &str) -> Result<Ipv6Addr, AddrParseError> {
 impl FromStr for SocketAddrV4 {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddrV4, AddrParseError> {
-        match Parser::new(s).read_till_eof(|p| p.read_socket_addr_v4()) {
-            Some(s) => Ok(s),
-            None => Err(AddrParseError(())),
-        }
+        Parser::new(s).parse_with(|p| p.read_socket_addr_v4())
     }
 }
 
@@ -307,10 +266,7 @@ fn from_str(s: &str) -> Result<SocketAddrV4, AddrParseError> {
 impl FromStr for SocketAddrV6 {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddrV6, AddrParseError> {
-        match Parser::new(s).read_till_eof(|p| p.read_socket_addr_v6()) {
-            Some(s) => Ok(s),
-            None => Err(AddrParseError(())),
-        }
+        Parser::new(s).parse_with(|p| p.read_socket_addr_v6())
     }
 }
 
@@ -318,10 +274,7 @@ fn from_str(s: &str) -> Result<SocketAddrV6, AddrParseError> {
 impl FromStr for SocketAddr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddr, AddrParseError> {
-        match Parser::new(s).read_till_eof(|p| p.read_socket_addr()) {
-            Some(s) => Ok(s),
-            None => Err(AddrParseError(())),
-        }
+        Parser::new(s).parse_with(|p| p.read_socket_addr())
     }
 }
 
@@ -376,3 +329,146 @@ fn description(&self) -> &str {
         "invalid IP address syntax"
     }
 }
+
+#[cfg(test)]
+mod tests {
+    // FIXME: These tests are all excellent candidates for AFL fuzz testing
+    use crate::net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr, SocketAddrV4, SocketAddrV6};
+    use crate::str::FromStr;
+
+    const PORT: u16 = 8080;
+
+    const IPV4: Ipv4Addr = Ipv4Addr::new(192, 168, 0, 1);
+    const IPV4_STR: &str = "192.168.0.1";
+    const IPV4_STR_PORT: &str = "192.168.0.1:8080";
+
+    const IPV6: Ipv6Addr = Ipv6Addr::new(0x2001, 0xdb8, 0, 0, 0, 0, 0xc0a8, 0x1);
+    const IPV6_STR_FULL: &str = "2001:db8:0:0:0:0:c0a8:1";
+    const IPV6_STR_COMPRESS: &str = "2001:db8::c0a8:1";
+    const IPV6_STR_V4: &str = "2001:db8::192.168.0.1";
+    const IPV6_STR_PORT: &str = "[2001:db8::c0a8:1]:8080";
+
+    #[test]
+    fn parse_ipv4() {
+        let result: Ipv4Addr = IPV4_STR.parse().unwrap();
+        assert_eq!(result, IPV4);
+
+        assert!(Ipv4Addr::from_str(IPV4_STR_PORT).is_err());
+        assert!(Ipv4Addr::from_str(IPV6_STR_FULL).is_err());
+        assert!(Ipv4Addr::from_str(IPV6_STR_COMPRESS).is_err());
+        assert!(Ipv4Addr::from_str(IPV6_STR_V4).is_err());
+        assert!(Ipv4Addr::from_str(IPV6_STR_PORT).is_err());
+    }
+
+    #[test]
+    fn parse_ipv6() {
+        let result: Ipv6Addr = IPV6_STR_FULL.parse().unwrap();
+        assert_eq!(result, IPV6);
+
+        let result: Ipv6Addr = IPV6_STR_COMPRESS.parse().unwrap();
+        assert_eq!(result, IPV6);
+
+        let result: Ipv6Addr = IPV6_STR_V4.parse().unwrap();
+        assert_eq!(result, IPV6);
+
+        assert!(Ipv6Addr::from_str(IPV4_STR).is_err());
+        assert!(Ipv6Addr::from_str(IPV4_STR_PORT).is_err());
+        assert!(Ipv6Addr::from_str(IPV6_STR_PORT).is_err());
+    }
+
+    #[test]
+    fn parse_ip() {
+        let result: IpAddr = IPV4_STR.parse().unwrap();
+        assert_eq!(result, IpAddr::from(IPV4));
+
+        let result: IpAddr = IPV6_STR_FULL.parse().unwrap();
+        assert_eq!(result, IpAddr::from(IPV6));
+
+        let result: IpAddr = IPV6_STR_COMPRESS.parse().unwrap();
+        assert_eq!(result, IpAddr::from(IPV6));
+
+        let result: IpAddr = IPV6_STR_V4.parse().unwrap();
+        assert_eq!(result, IpAddr::from(IPV6));
+
+        assert!(IpAddr::from_str(IPV4_STR_PORT).is_err());
+        assert!(IpAddr::from_str(IPV6_STR_PORT).is_err());
+    }
+
+    #[test]
+    fn parse_socket_v4() {
+        let result: SocketAddrV4 = IPV4_STR_PORT.parse().unwrap();
+        assert_eq!(result, SocketAddrV4::new(IPV4, PORT));
+
+        assert!(SocketAddrV4::from_str(IPV4_STR).is_err());
+        assert!(SocketAddrV4::from_str(IPV6_STR_FULL).is_err());
+        assert!(SocketAddrV4::from_str(IPV6_STR_COMPRESS).is_err());
+        assert!(SocketAddrV4::from_str(IPV6_STR_V4).is_err());
+        assert!(SocketAddrV4::from_str(IPV6_STR_PORT).is_err());
+    }
+
+    #[test]
+    fn parse_socket_v6() {
+        let result: SocketAddrV6 = IPV6_STR_PORT.parse().unwrap();
+        assert_eq!(result, SocketAddrV6::new(IPV6, PORT, 0, 0));
+
+        assert!(SocketAddrV6::from_str(IPV4_STR).is_err());
+        assert!(SocketAddrV6::from_str(IPV4_STR_PORT).is_err());
+        assert!(SocketAddrV6::from_str(IPV6_STR_FULL).is_err());
+        assert!(SocketAddrV6::from_str(IPV6_STR_COMPRESS).is_err());
+        assert!(SocketAddrV6::from_str(IPV6_STR_V4).is_err());
+    }
+
+    #[test]
+    fn parse_socket() {
+        let result: SocketAddr = IPV4_STR_PORT.parse().unwrap();
+        assert_eq!(result, SocketAddr::from((IPV4, PORT)));
+
+        let result: SocketAddr = IPV6_STR_PORT.parse().unwrap();
+        assert_eq!(result, SocketAddr::from((IPV6, PORT)));
+
+        assert!(SocketAddr::from_str(IPV4_STR).is_err());
+        assert!(SocketAddr::from_str(IPV6_STR_FULL).is_err());
+        assert!(SocketAddr::from_str(IPV6_STR_COMPRESS).is_err());
+        assert!(SocketAddr::from_str(IPV6_STR_V4).is_err());
+    }
+
+    #[test]
+    fn ipv6_corner_cases() {
+        let result: Ipv6Addr = "1::".parse().unwrap();
+        assert_eq!(result, Ipv6Addr::new(1, 0, 0, 0, 0, 0, 0, 0));
+
+        let result: Ipv6Addr = "1:1::".parse().unwrap();
+        assert_eq!(result, Ipv6Addr::new(1, 1, 0, 0, 0, 0, 0, 0));
+
+        let result: Ipv6Addr = "::1".parse().unwrap();
+        assert_eq!(result, Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1));
+
+        let result: Ipv6Addr = "::1:1".parse().unwrap();
+        assert_eq!(result, Ipv6Addr::new(0, 0, 0, 0, 0, 0, 1, 1));
+
+        let result: Ipv6Addr = "::".parse().unwrap();
+        assert_eq!(result, Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 0));
+
+        let result: Ipv6Addr = "::192.168.0.1".parse().unwrap();
+        assert_eq!(result, Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0xc0a8, 0x1));
+
+        let result: Ipv6Addr = "::1:192.168.0.1".parse().unwrap();
+        assert_eq!(result, Ipv6Addr::new(0, 0, 0, 0, 0, 1, 0xc0a8, 0x1));
+
+        let result: Ipv6Addr = "1:1:1:1:1:1:192.168.0.1".parse().unwrap();
+        assert_eq!(result, Ipv6Addr::new(1, 1, 1, 1, 1, 1, 0xc0a8, 0x1));
+    }
+
+    // Things that might not seem like failures but are
+    #[test]
+    fn ipv6_corner_failures() {
+        // No IP address before the ::
+        assert!(Ipv6Addr::from_str("1:192.168.0.1::").is_err());
+
+        // :: must have at least 1 set of zeroes
+        assert!(Ipv6Addr::from_str("1:1:1:1::1:1:1:1").is_err());
+
+        // Need brackets for a port
+        assert!(SocketAddrV6::from_str("1:1:1:1:1:1:1:1:8080").is_err());
+    }
+}