Strange format!() behavior

[engstad]

The program:

fn main() { println!("{}", "\"\\\\\"") }

prints out:

"\\"

But the program:

fn main() { println!("\"\\\\\"") }

prints out:

"\"

They ought to be the same.

[engstad]

I could not use mark-down for this issue, because it took away backslashes as well. To clarify:
If back-slash+back-slash is found in the format-string, then it is escaped one too many times.

[alexcrichton]

Closing as working as intended. In the second example the rust string is "\", and then that goes through the format string parser which interprets backslashes again, and \" is not a valid escape.
The error message indicates that it was an error in the format string parser, not the rust string parser:

foo.rs:3:14: 3:22 error: invalid format string: invalid escape character `"`
foo.rs:3     println!("\"\\\"");
                      ^~~~~~~~
error: aborting due to previous error

cc #9970

[engstad]

That is strange, both programs compiles fine.

[engstad]

Alex: You didn't paste the program correctly. Please reopen.

[alexcrichton]

This is the same thing going on, when using format strings you need to escape twice, both the rust string parser and the format string parser. If you're doing lots of escapes, I recommend:

println!(r#""\\""#);

[engstad]

Thanks Alex,

I am just not sure this is good for C/C++ programmers. For instance, in C:

void main() { printf("\n"); } // Three back-slashes and an n

Simply outputs a back-slash and a new-line.

I noticed in src/test/run-pass/ifmt.rs things like:

t!(format!("hello {"), "hello {"); // Note: Formatting screwed up, see source.

I wouldn't expect the double-backslash here. So, the larger question is -- why this inconsistency? I really think this should be fixed before 1.0 is out.

[alexcrichton]

This is a dupe of #9970, I would recommend putting your concerns on that bug instead.