RFC: Move std::net::IpAddr types into core::net.

text/2832-core-net-types.md

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+- Feature Name: `core_net_types`
+- Start Date: 2019-12-06
+- RFC PR: [rust-lang/rfcs#2832](https://github.com/rust-lang/rfcs/pull/2832)
+- Rust Issue: [rust-lang/rust#108443](https://github.com/rust-lang/rust/issues/108443)
+
+# Summary
+[summary]: #summary
+
+Make the `IpAddr`, `Ipv4Addr`, `Ipv6Addr`, `SocketAddr`, `SocketAddrV4`,
+`SocketAddrV6`, `Ipv6MulticastScope` and `AddrParseError` types available in `no_std`
+contexts by moving them into a `core::net` module.
+
+# Motivation
+[motivation]: #motivation
+
+The motivation here is to provide common types for both `no_std` and `std`
+targets which in turn will ease the creation of libraries based around IP
+addresses. Embedded IoT development is one area where this will be beneficial.
+IP addresses are portable across all platforms and have no external
+dependencies which is in line with the definition of the core library.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The `core::net::IpAddr`, `core::net::Ipv4Addr`, `core::net::Ipv6Addr`,
+`core::net::SocketAddr`, `core::net::SocketAddrV4`, `core::net::SocketAddrV6`,
+`core::net::Ipv6MulticastScope` and `core::net::AddrParseError` types are
+available in `no_std` contexts.
+
+Library developers should use `core::net` to implement abstractions in order
+for them to work in `no_std` contexts as well.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Since https://github.com/rust-lang/rust/pull/78802 has been merged, IP and
+socket address types are implemented in ideal Rust layout instead of wrapping
+their corresponding `libc` representation.
+
+Formatting for these types has also been adjusted in
+https://github.com/rust-lang/rust/pull/100625 and
+https://github.com/rust-lang/rust/pull/100640 in order to remove the dependency
+on `std::io::Write`.
+
+This means the types are now platform-agnostic, allowing them to be moved from
+`std::net` into `core::net`.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Moving the `std::net` types to `core::net` makes the core library less *minimal*.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- Eliminates the need to use different abstractions for `no_std` and `std`.
+
+- Alternatively, move these types into a library other than `core`, so they
+  can be used without `std`, and re-export them in `std`.
+
+# Prior art
+[prior-art]: #prior-art
+
+There was a prior discussion at
+
+https://internals.rust-lang.org/t/std-ipv4addr-in-core/11247/15
+
+and an experimental branch from [@Nemo157](https://github.com/Nemo157) at
+
+https://github.com/Nemo157/rust/tree/core-ip
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+Move the `ToSocketAddrs` trait to `core::net` as well. This depends on having `core::io::Result`.