Add mixed utf8 literals rfc.

Author: m-ou-se

text/0000-mixed-utf8-literals.md

@@ -0,0 +1,109 @@
+- Feature Name: `mixed_utf8_literals`
+- Start Date: 2022-11-15
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Allow the exact same characters and escape codes in `"…"` and `b"…"` literals.
+
+That is:
+
+- Allow unicode characters, including `\u{…}` escape codes, in byte string literals. E.g. `b"hello\xff我叫\u{1F980}"`
+- Allow `\x…` escape codes in regular string literals, as long as they are valid UTF-8. E.g. `"\xf0\x9f\xa6\x80"`
+
+# Motivation
+[motivation]: #motivation
+
+Byte strings (`[u8]`) are a strict superset of regular (utf-8) strings (`str`),
+but Rust's byte string literals are currently not a superset of regular string literals:
+they reject non-ascii characters and `\u{…}` escape codes.
+
+```
+error: non-ASCII character in byte constant
+ --> src/main.rs:2:16
+  |
+2 |     b"hello\xff你\u{597d}"
+  |                ^^ byte constant must be ASCII
+  |
+
+error: unicode escape in byte string
+ --> src/main.rs:2:17
+  |
+2 |     b"hello\xff你\u{597d}"
+  |                  ^^^^^^^^ unicode escape in byte string
+  |
+```
+
+This can be annoying when working with "conventionally UTF-8" strings, such as with the popular [`bstr` crate](https://docs.rs/bstr/latest/bstr/).
+For example, right now, there is no convenient way to write a literal like `b"hello\xff你好"`.
+
+Allowing all characters and escape codes in both types of string literals reduces the complexity of the language.
+We'd no longer have [different escape codes](https://doc.rust-lang.org/reference/tokens.html#characters-and-strings)
+for different literal types. We'd only require regular string literals to be valid UTF-8.
+
+If we can postpone the UTF-8 validation until the point where tokens are turned into literals, then this not only simplifies the job of the tokenizer,
+but allows macros to take string literals with invalid UTF-8 (through `$_:tt` or `TokenTree`).
+That can be useful for macros like `cstr!("…")` and `wide!("…")`, etc., which currently unnecessarily result in errors for non-UTF-8 data:
+
+```
+error: out of range hex escape
+ --> src/main.rs:3:13
+  |
+3 |     cstr!("¿\xff");
+  |             ^^^^ must be a character in the range [\x00-\x7f]
+```
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Regular string literals (`""`) must be valid UTF-8. For example, valid strings are `"abc"`, `"🦀"`, `"\u{1F980}"` and `"\xf0\x9f\xa6\x80"`.
+`"\x80"` is not valid, however, as that is not valid UTF-8.
+
+Byte string literals (`b""`) may include non-ascii characters and unicode escape codes (`\u{…}`), which will be encoded as UTF-8.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The tokenizer should accept all escape codes in both `""` and `b""` literals.
+Only a regular string literal is checked for invalid UTF-8, but only at the point where the token is converted to a string literal AST node.
+
+Just like how `$_:tt` accepts a thousand-digit integer literal but `$_:literal` does not,
+a `$_:tt` should accept `"\x80"`, but `$_:literal` should not.
+Similar, proc macros should be able to consume invalid UTF-8 string literals as `TokenTree`.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+One might unintentionally write `\xf0` instead of `\u{f0}`.
+However, for regular string literals that will result in an error in nearly all cases, since that's not valid UTF-8 by itself.
+
+# Alternatives
+[alternatives]: #alternatives
+
+- Only extend `b""`, but still don't accept `\x` in regular string literals (`""`).
+
+- Stabilize `concat_bytes!()` and require writing `"hello\xff你好"` as `concat_bytes!(b"hello\xff", "你好")`.
+  (Assuming we extend the macro to accept a mix of byte string literals and regular string literals.)
+
+# Prior art
+[prior-art]: #prior-art
+
+- C and C++ do the same. (Assuming UTF-8 character set.)
+- [The `bstr` crate](https://docs.rs/bstr/latest/bstr/)
+- Python and Javascript do it differently: `\xff` mean `\u{ff}`, because their strings behave like UTF-32 or UTF-16 rather than UTF-8.
+  (Also, Python's byte strings "accept" `\u` escape codes as just `'\\', 'u'`, without any warning or error.)
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- Should `concat!("\xf0\x9f", "\xa6\x80")` work? (The string literals are not valid UTF-8 individually, but are valid UTF-8 after being concatenated.)
+
+  (I don't care. I guess we should do whatever is easiest to implement.)
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+- Update the `concat!()` macro to accept `b""` strings and also not implicitly convert integers to strings, such that `concat!(b"", $x, b"\0")` becomes usable.
+  (This would need to happen over an edition.)