RFC: Support char::encode_utf8 in const scenarios.

text/3696-const-char-encode-utf8.md

@@ -0,0 +1,90 @@
+- Feature Name: `const_char_encode_utf8`
+- Start Date: 2024-09-17
+- RFC PR: [rust-lang/rfcs#3696](https://github.com/rust-lang/rfcs/pull/3696)
+- Rust Issue: [rust-lang/rust#130512](https://github.com/rust-lang/rust/issues/130512)
+
+# Summary
+[summary]: #summary
+
+`char::encode_utf8` should be marked const to allow for compile-time conversions.
+Considering mutable references now being stable in const environments, this implementation would be trivial even without compiler magic.
+
+# Motivation
+[motivation]: #motivation
+
+The `encode_utf8` method (in `char`) is currently **not** marked as "const" and is therefore rendered unusable in scenarios that require const-compatibility.
+
+With the recent stabilisation of [`const_mut_refs`](https://github.com/rust-lang/rust/issues/57349/), implementing `encode_utf8` with the current signature is trivial and would (in practice) yield no incompatibilities with existing code.
+
+I expect that implementing this RFC – despite its limited scope – will however prove useful in supporting compile-time string handling in the future.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Currently, the `encode_utf8` method has the following prototype:
+
+```rust
+pub fn encode_utf8(self, dst: &mut [u8]) -> &mut str;
+```
+
+This is to simply be marked as const:
+
+```rust
+pub const fn encode_utf8(self, dst: &mut [u8]) -> &mut str;
+```
+
+This is not a breaking change.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Other than just adding the `const` qualifier to the function prototype, the function body would have to be changed due to some constructs currently not being supported in constant expressions.
+
+A working implementation can be found at [`bjoernager/rust:const-char-encode-utf8`](https://github.com/bjoernager/rust/tree/const-char-encode-utf8).
+Required changes are in [`/library/core/src/char/methods.rs`](https://github.com/bjoernager/rust/blob/const-char-encode-utf8/library/core/src/char/methods.rs/).
+
+Note that this implementation assumes [`const_slice_from_raw_parts_mut`](https://github.com/rust-lang/rust/issues/67456/).
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Implementing this RFC at the current moment could degenerate diagnostics as the `assert` call in the `encode_utf8_raw` function relies on formatters that are non-const.
+
+The reference implementation resolves this by instead using a generic message, although this may not be desired:
+
+```
+encode_utf8: buffer does not have enough bytes to encode code point
+```
+
+This *could* be changed to have the number of bytes required hard-coded, but doing so may instead sacrifice code readability.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+If the initial diagnostics are deemed to be worth more than const-compatibility then an `encode_utf8_unchecked` method could be considered instead:
+
+```rust
+pub const unsafe fn encode_utf8_unchecked(self, dst: &mut [u8]) -> &mut str;
+
+// ... or...
+
+pub const unsafe fn encode_utf8_unchecked(self, dst: *mut u8) -> *mut str;
+```
+
+This function would perform the same operation but without testing the length of `dst`, allowing for const conversions at least in the short-term (until formatters are stabilised).
+
+# Prior art
+[prior-art]: #prior-art
+
+Currently none that I know of.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+The problem with diagnostic degeneration could be solved by allowing the used formatters in const environments.
+I do not know if there already exists such a feature for use by the standard library.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+I suspect that having a similar `decode_utf8` method may be desired.