C-string literals

Summary

• Literals of the form c"foo" or cr"foo" represent a string of type &core::ffi::CStr.

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 which is automatically terminated with a NUL byte.

#![allow(unused)]
fn main() {
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.

#![allow(unused)]
fn main() {
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.

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.