We stopped using allow_internal_unstable a while ago (#1142)

Co-authored-by: Ralf Jung <post@ralfj.de>
Co-authored-by: Yuki Okushi <jtitor@2k36.org>
Co-authored-by: Noah Lev <camelidcamel@gmail.com>

1 files changed, 21 insertions, 13 deletions

diff --git a/src/doc/rustc-dev-guide/src/stability.md b/src/doc/rustc-dev-guide/src/stability.md
index d3221217ad4..0f1aec17ee9 100644
--- a/src/doc/rustc-dev-guide/src/stability.md
+++ b/src/doc/rustc-dev-guide/src/stability.md
@@ -92,24 +92,32 @@ and the associated
 
 ## allow_internal_unstable
 
-Macros, compiler desugarings and `const fn`s expose their bodies to the call
+Macros and compiler desugarings expose their bodies to the call
 site. To work around not being able to use unstable things in the standard
 library's macros, there's the `#[allow_internal_unstable(feature1, feature2)]`
-attribute that allows the given features to be used in stable macros or
-`const fn`s.
-
-Note that `const fn`s are even more special in this regard. You can't just
-allow any feature, the features need an implementation in
-`qualify_min_const_fn.rs`. For example the `const_fn_union` feature gate allows
-accessing fields of unions inside stable `const fn`s. The rules for when it's
-ok to use such a feature gate are that behavior matches the runtime behavior of
-the same code (see also [this blog post][blog]). This means that you may not
-create a `const fn` that e.g. transmutes a memory address to an integer,
+attribute that allows the given features to be used in stable macros.
+
+## rustc_allow_const_fn_unstable
+
+`const fn`, while not directly exposing their body to the world, are going to get
+evaluated at compile time in stable crates. If their body does something const-unstable,
+that could lock us into certain features indefinitely by accident. Thus no unstable const
+features are allowed inside stable `const fn`.
+
+However, sometimes we do know that a feature will get
+stabilized, just not when, or there is a stable (but e.g. runtime-slow) workaround, so we
+could always fall back to some stable version if we scrapped the unstable feature.
+In those cases, the rustc_allow_const_fn_unstable attribute can be used to allow some
+unstable features in the body of a stable `const fn`.
+
+You also need to take care to uphold the `const fn` invariant that calling it at runtime and
+compile-time needs to behave the same (see also [this blog post][blog]). This means that you
+may not create a `const fn` that e.g. transmutes a memory address to an integer,
 because the addresses of things are nondeterministic and often unknown at
 compile-time.
 
-Always ping @oli-obk, @RalfJung, and @Centril if you are adding more
-`allow_internal_unstable` attributes to any `const fn`
+Always ping @rust-lang/wg-const-eval if you are adding more
+`rustc_allow_const_fn_unstable` attributes to any `const fn`.
 
 ## staged_api