Add C-string literals.

src/SUMMARY.md

@@ -33,3 +33,4 @@
   - [Reserving syntax](rust-2021/reserving-syntax.md)
   - [Warnings promoted to errors](rust-2021/warnings-promoted-to-error.md)
   - [Or patterns in macro-rules](rust-2021/or-patterns-macro-rules.md)
+  - [C-string literals](rust-2021/c-string-literals.md)

src/rust-2021/c-string-literals.md

@@ -0,0 +1,72 @@
+# C-string literals
+
+## Summary
+
+- Literals of the form `c"foo"` or `cr"foo"` represent a string of type [`&core::ffi::CStr`][CStr].
+
+[CStr]: ../../core/ffi/struct.CStr.html
+
+## Details
+
+Starting with Rust 1.76, C-strings can be written using C-string literal syntax with the `c` or `cr` prefix.
+
+Previously, it was challenging to properly produce a valid string literal that could interoperate with C APIs which terminate with a NUL byte.
+The [`cstr`] crate was a popular solution, but that required compiling a proc-macro which was quite expensive.
+Now, C-strings can be written directly using literal syntax notation, which will generate a value of type [`&core::ffi::CStr`][CStr] which is automatically terminated with a NUL byte.
+
+```rust,edition2021
+# use core::ffi::CStr;
+
+assert_eq!(c"hello", CStr::from_bytes_with_nul(b"hello\0").unwrap());
+assert_eq!(
+    c"byte escapes \xff work",
+    CStr::from_bytes_with_nul(b"byte escapes \xff work\0").unwrap()
+);
+assert_eq!(
+    c"unicode escapes \u{00E6} work",
+    CStr::from_bytes_with_nul(b"unicode escapes \xc3\xa6 work\0").unwrap()
+);
+assert_eq!(
+    c"unicode characters αβγ encoded as UTF-8",
+    CStr::from_bytes_with_nul(
+        b"unicode characters \xce\xb1\xce\xb2\xce\xb3 encoded as UTF-8\0"
+    )
+    .unwrap()
+);
+assert_eq!(
+    c"strings can continue \
+        on multiple lines",
+    CStr::from_bytes_with_nul(b"strings can continue on multiple lines\0").unwrap()
+);
+```
+
+C-strings do not allow interior NUL bytes (such as with a `\0` escape).
+
+Similar to regular strings, C-strings also support "raw" syntax with the `cr` prefix.
+These raw C-strings do not process backslash escapes which can make it easier to write strings that contain backslashes.
+Double-quotes can be included by surrounding the quotes with the `#` character.
+Multiple `#` characters can be used to avoid ambiguity with internal `"#` sequences.
+
+```rust,edition2021
+assert_eq!(cr"foo", c"foo");
+// Number signs can be used to embed interior double quotes.
+assert_eq!(cr#""foo""#, c"\"foo\"");
+// This requires two #.
+assert_eq!(cr##""foo"#"##, c"\"foo\"#");
+// Escapes are not processed.
+assert_eq!(cr"C:\foo", c"C:\\foo");
+```
+
+See [The Reference] for more details.
+
+[`cstr`]: https://crates.io/crates/cstr
+[The Reference]: ../../reference/tokens.html#c-string-and-raw-c-string-literals
+
+## Migration
+
+Migration is only necessary for macros which may have been assuming a sequence of tokens that looks similar to `c"…"` or `cr"…"`, which previous to the 2021 edition would tokenize as two separate tokens, but in 2021 appears as a single token.
+
+As part of the [syntax reservation] for the 2021 edition, any macro input which may run into this issue should issue a warning from the `rust_2021_prefixes_incompatible_syntax` migration lint.
+See that chapter for more detail.
+
+[syntax reservation]: reserving-syntax.md