RFC: Add OsUnknown as a target operating system

active/0000-unknown-os-target.md

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+# Summary
+
+Add support for triple of the form `*-unknown-*` that relies on minimal
+dependencies and doesn't support dynamic linking. Targeting newlib for the
+dependencies - it comes with libc, libm and libstdc++, can be ported to new
+platforms very easily, and should be compatible with most other 'larger'
+glibc alternatives.
+
+# Motivation
+
+Rust is a very attractive language for embedded systems; it provides
+limited unsafety while still giving full control of memory allocation.
+Adding an OsUnknown to rustc permits `#[cfg(target_os = "unknown")]`
+attributes which should be enough to get libstd building for embedded
+targets.
+
+# Detailed design
+
+Add a new OS to librustc for the unknown target. Mostly duplicating all the
+code of OsLinux and naming it OsUnknown. Any code for dynamic linking should
+error as OsUnknown can't support dynamic linking.
+
+Modify librustc to ignore `#[crate-type = "dylib"];` attributes for
+OsUnknown. If `--crate-type=dylib` is specified on the command line, then an
+error should be produced. Ignoring the crate attribute is the easiest
+solution, it doesn't look like
+`#[cfg(target_os = "unknown")] [crate-type = "dylib"];` is supported
+(and would look messy).
+
+Subsequent work would be in libstd to get it to build a rlib and then be
+able to link against newlib. Majority of the changes are in rtdeps and libc
+(adding lots of target_os = unknown). Should this RFC be for the whole
+process or just the OsUnknown in librustc?
+
+Newlib supports POSIX like threads and is/can-be fully reentrant. It also
+supports C++ exceptions, although it can be built in a -nano spec that drops
+this support to minimise size.
+
+# Alternatives
+
+There was a previous pull request for a "none" OS to support the
+arm-none-eabi toolchain for ARM-Cortex-M0 procesors. There was an issue
+raised with this that .so was used as the dynamic lib extension (but dynamic
+linking isn't supported).
+
+Could probably get libstd to build using
+`--target *-linux-* --cfg no-dynamic --cfg with-newlib` (because OsUnknown
+is mostly just OsLinux).
+
+# Unresolved questions
+
+What is the best way to deal with linking Rust applications without
+exception support. Can utilise weak linkage in the fail macro, or build a
+custom libstd with `--cfg no-unwind`.
+
+I'm not sure how much work is required to get 'rt' to build against newlib,
+nor how compiler-rt will work (for one, it isn't able to work out the target
+endianness when bult without `__linux__` and similar defines).