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+- Feature Name: relax_const_restrictions
+- Start Date: 2022-12-02
+- RFC PR: [rust-lang/rfcs#3351](https://github.com/rust-lang/rfcs/pull/3351)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Allow `const` functions to behave differently during constant-evaluation and runtime and remove all restrictions from `const` functions if they are called at runtime.
+
+# Motivation
+[motivation]: #motivation
+
+The past restriction of `const` functions having to behave the same way no matter where they were called has been a limitation and it has been unclear whether such a difference in behaviour could cause unsoundness. While the Rust language does, at the time of writing, not expose a way to determine whether a function has been called during constant evaluation or runtime (and this RFC does not propose adding such a feature), such an intrinsic (`const_eval_select`) does currently exist internally in the standard library.
+
+The precondition of this intrinsic has always been that the const-eval and runtime code have to exhibit the exact same behavior. Verifying this property about the two different implementations is often not trivial, which makes sound use of this intrinsic for non-trivial functions tricky. But it can often be desirable to use such an intrinsic to do various optimizations in runtime code that are not possible in constant evaluation.
+
+Exposing such an intrinsic or a language feature that allows the same can be useful, allowing for more efficient code in `const fn` during runtime (like using SIMD-intrinsics). With the current rules, such a feature would have to be unsafe.
+
+Also, floats are currently not supported in `const fn`. This is because many different hardware implementations exhibit subtly different floating point behaviors and trying to emulate all of them correctly at compile time is close to impossible. Allowing const-eval and runtime behavior to differ will enable unrestricted floats in a const context in the future.
+
+Rust code often contains debug assertions or preconditions that must be upheld but are too expensive to check in release mode. It is desirable to also check these preconditions during constant evaluation (for example with a `debug_or_const_assert!` macro). This is unsound under the old rules, as this would be different behavior during const evaluation in release mode. This RFC allows such debug assertions to also run during constant evaluation (but does not propose this itself).
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+This RFC allows `const fn` to exhibit different behavior during constant evaluation and runtime. While such a difference is often undesirable, it is not considered to be undefined behavior.
+
+If a `const fn` is able to detect whether it has been called during constant evaluation or at runtime (either through an intrinsic or a future language feature), then it is allowed to exhibit different behavior. Also, a `const fn` called at runtime can do anything a normal function can do, with no additional restrictions applied to it. It could open a file, call a system randomness API or gracefully exit the program. 
+
+At the time of writing, there is no way for a function to detect whether it was called at runtime or during constant evaluation in stable Rust and this RFC is not concerned with adding any, but it unblocks future RFCs for adding this capability.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Each execution of a function stands in a particular "constness context". A `const fn` is executed in a const context if it was called inside another `const fn` that was executed in a const context or if it was called in one of the following places:
+
+- `const` initializers (`const X: _ = CONST;`)
+- `static` initializers (`static X: _ = CONST;`)
+- array lengths (`[T; CONST]`)
+- enum discriminants (`enum A { B = CONST }`)
+- inline-const block (`const { CONST }`)
+- const generic arguments (`function::<CONST>()`)
+
+This list may be extended by future language features.
+
+All other calls to a `const fn` (for example in `main`) are in a runtime context.
+
+We can therefore say that statically, code can either be:
+- Always in a const context (code inside one of the places listed above)
+- Maybe in a const context (`const fn`), where the context can differ between calls depending on the call-site
+- Always in a runtime const context (non-`const` functions)
+
+A `const fn` is now allowed to exhibit different behavior depending on it being called in a const or runtime context. Language features and standard library functions may also differ in behavior depending on the context they have been used or called in, though the Rust language and standard library will explicitly document such behavioral differences.
+
+This makes the context of a function observable behavior.
+
+If a `const fn` is called in a runtime context, no additional restrictions are applied to it, and it may do anything a non-`const fn` can (for example, calling into FFI).
+
+A `const fn` being called in a const context will still be required to be deterministic, as this is required for type system soundness. This invariant is required by the compiler and cannot be broken, even with unsafe code.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Pure `const fn` under the old rules can be seen as a simple optimization opportunity for naive optimizers, as they could just reuse constant evaluation for `const fn` if the argument is known at compile time, even if the function is in a runtime context. This RFC makes such an optimization impossible. This is not seen as a problem by the author, as a more advanced optimizer (like LLVM) is able to remove these calls at compile time through means other than Rust's constant evaluation (inlining and constant folding). Also, a constant evaluation system can still evaluate executions in a runtime context, as long as it behaves exactly like runtime. The optimizer could also manually annotate functions as being truly pure by looking at the body.
+
+Secondly, with the current rules around `const fn` purity, unsafe code could choose to rely on purity, e.g. by caching function return values and assuming this is not observable to clients. The author does not see this as a significant drawback, as this functionality is better served by language features that target this use case directly (like a `pure` attribute) and is therefore out of scope for the language feature of "functions evaluatable at compile time". This could break code that already relies on this, but since Rust doesn't have proper support for this, the impact should be minimal at most.
+
+The old rules, which say that `const fn` always has to behave the same way are already well-known in the community. Changing this will require teaching people about the new change. Since this is a simple change, this should not be too hard (for example with a mention in the release notes).
+
+This is technically a breaking change. Code could rely on this behavior right now, as the [internal documentation](https://doc.rust-lang.org/1.65.0/std/intrinsics/fn.const_eval_select.html#safety) for `std::intrinsics::const_eval_select` explains. Relying on this was never endorsed or officially documented and there are no known cases of code relying on it. This is deemed to be highly unlikely and even if some code did rely on this, it will continue to work as long as no new behavioral differences are introduced by the code. The internal docs will have to be adjusted after this RFC is accepted.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+An alternative is to keep the current rules. This is bad because the current rules are highly restrictive and don't have a significant benefit to them. With the current rules, floats cannot be used in `const fn` without significant restrictions.
+
+It would also be possible to allow them to behave differently, but keep the restrictions around purity and determinism at runtime. This would still allow unsafe code to treat `const fn` specially, but this is not seen as a desirable feature of `const fn`.
+
+# Prior art
+[prior-art]: #prior-art
+
+C++ with `constexpr`, a compile-time evaluation system similar to Rusts `const fn`, has a [`std::is_constant_evaluated`](std-is-constant-evaluated) function which can be used to determine whether the function is being executed during constant evaluation or at runtime. It does not impose restrictions that code has to behave the same during constant evaluation or runtime.
+
+Rust has rejected having pure functions before. Back in early pre-1.0 Rust, functions could be annotated as `pure`. This was later removed because it was not deemed useful enough.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None for now.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+An intrinsic like `const_eval_select` (in the form of an intrinsic or a more complete language feature) could now be added safely, enabling more parts of the ecosystem to make functions `const` without losing runtime optimizations.
+
+Allowing all floating point operations in a const context without any restrictions.
+
+[std-is-constant-evaluated]: https://en.cppreference.com/w/cpp/types/is_constant_evaluated
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