format!() should support the same set of \-escapes that regular string literals do

[lilyball]

As long as format!() treats \ as an escape character, it should handle all the same escapes that normal string literals do. This way a raw string literal can be passed to format!() and still use the expected escapes. Right now, attempting to do so:

println!(r#"this is a "string": {}\n"#, "example code");

throws a compiler error:

 error: invalid escape character `n`
println!(r#"this is a "string": {}\n"#, "example code");
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[alexcrichton]

I personally don't believe that the format-string format should not be conforming to what rust as a language uses as a string format. I believe that if we start recognizing \n as an escape sequence, then it will start to just lead to confusion.

What I do think, however, is that the error message could be improved in this case. It's no clear that the error is coming from the format string parser and not the rust string parser, and I believe that distinction would alleviate the problem.

[engstad]

Coming from a C/C++ and Ocaml background, rust's behavior is quite confusing. In C:

printf("\\\n"); // Outputs: \<new-line>

And in Ocaml, format-strings are special strings that enforce types of the arguments:

Printf.printf "\\\n"; (* Outputs: \<new-line> *)

In Python:

print("\\\n")

In Rust, we have to write:

print!("\\\\\n"); // Outputs: \<new-line>

Clearly, for format-strings, the escape character ought to be back-slash, but then

format string	meaning
{	literal '{'
}	literal '}'
\	literal ''
\n	new-line
\t	tab

Given the goals of Rust 1.0, I think this issue should be resolved before 1.0 is released.

[rprichard]

I think this issue is already fixed -- format! no longer uses \ as an escape character. Instead, {{ and }} are escape sequences.

[alexcrichton]

I believe so as well, thanks @rprichard!