The “Conditional compilation” section of the Reference has a list of configurations that must be defined (as of Rust 1.14):
target_archwith values like:x86x86_64mipspowerpcpowerpc64armaarch64
target_oswith values like:windowsmacosioslinuxandroidfreebsddragonflybitrigopenbsdnetbsd
target_familywith values like:unixwindows
unix(shortcut fortarget_family)windows(shortcut fortarget_family)target_envwith values like:gnumsvcmusl""(empty string)
target_endianwith values:littlebig
target_pointer_widthwith values like:3264
target_has_atomicwith values like:8163264ptr
target_vendorwith values like:applepcunknown
testdebug_assertions