Document that nightly features should be opt-in using a --cfg flag

src/checklist.md

@@ -68,6 +68,7 @@
   - [ ] Structs have private fields ([C-STRUCT-PRIVATE])
   - [ ] Newtypes encapsulate implementation details ([C-NEWTYPE-HIDE])
   - [ ] Data structures do not duplicate derived trait bounds ([C-STRUCT-BOUNDS])
+  - [ ] Nightly features use an explicit opt-in ([C-NIGHTLY-OPTIN])
 - **Necessities** *(to whom they matter, they really matter)*
   - [ ] Public dependencies of a stable crate are stable ([C-STABLE])
   - [ ] Crate and its dependencies have a permissive license ([C-PERMISSIVE])
@@ -135,6 +136,7 @@
 [C-STRUCT-PRIVATE]: future-proofing.html#c-struct-private
 [C-NEWTYPE-HIDE]: future-proofing.html#c-newtype-hide
 [C-STRUCT-BOUNDS]: future-proofing.html#c-struct-bounds
+[C-NIGHTLY-OPTIN]: future-proofing.html#c-nightly-optin
 
 [C-STABLE]: necessities.html#c-stable
 [C-PERMISSIVE]: necessities.html#c-permissive

src/future-proofing.md

@@ -202,3 +202,21 @@ on the data structure.
 [`std::borrow::Cow`]: https://doc.rust-lang.org/std/borrow/enum.Cow.html
 [`std::boxed::Box`]: https://doc.rust-lang.org/std/boxed/struct.Box.html
 [`std::io::BufWriter`]: https://doc.rust-lang.org/std/io/struct.BufWriter.html
+
+<a id="c-nightly-optin"></a>
+## Nightly features use an explicit opt-in (C-NIGHTLY-OPTIN)
+
+Some libraries need to use, or want to experiment with, the [nightly channel].
+To avoid accidental breakage, libraries should either:
+- Use nightly features unconditionally, so that people depending on the library must always use a nightly toolchain to build
+- Add a `--cfg` flag which opts-in to the nightly features (optionally, with a test compilation to give a better error message if the feature is not present in the current compiler version). This allows people to avoid opting-in if they do not want to be exposed to possible breakage.
+
+Each nightly feature should be under a separate flag so that breakage to one feature does not cause breakage for others.
+Each flag should be prefixed with the crate name to avoid accidentally triggering features in other crates.
+For example, if your crate is named `awesome-lib` and depends on two different nightly features, `std::intrinsics::black_box` and `std::intrinsics::catch_unwind`, create two cfgs named `awesome_lib_black_box` and `awesome_lib_catch_unwind`.
+
+When doing feature detection, we recommend *against* simply checking whether the compiler is a nightly channel, as nightly features frequently change between compiler versions.
+Feature detection should compile a sample rust program and verify that it works.
+Note that flags like `-Z allow-features` may affect whether features are allowed.
+
+[nightly channel]: https://rust-lang.github.io/rustup/concepts/channels.html