write double backslashes in paths in #line directives

[mariusbancila]

When the #line directives are emitted, the path of the compiled file is written in the output cpp file. However, double backslashes are not used which may lead to interpreting characters as escape sequences when a relative path is supplied as argument.

Consider this source code located in a file called demo.cpp2 in a folder called learn:

main: () -> int = {
   std::cout << "Hello, world!\n";
}

Compile it with a relative path:

cppfront.exe ..\learn\demo.cpp2

It produces this output:

// ----- Cpp2 support -----
#include "cpp2util.h"


#line 1 "..\learn\demo.cpp2"
[[nodiscard]] auto main() -> int;

//=== Cpp2 definitions ==========================================================

#line 1 "..\learn\demo.cpp2"
[[nodiscard]] auto main() -> int{
   std::cout << "Hello, world!\n";
}

When you run this with cl.exe you get the following warnings:

demo.cpp
..\learn\demo.cpp(5): warning C4129: 'l': unrecognized character escape sequence
..\learn\demo.cpp(5): warning C4129: 'd': unrecognized character escape sequence
..learndemo.cpp2(5): warning C4129: 'l': unrecognized character escape sequence
..learndemo.cpp2(5): warning C4129: 'd': unrecognized character escape sequence

This PR fixes this so that the output is as follows:

// ----- Cpp2 support -----
#include "cpp2util.h"


#line 1 "..\\learn\\demo.cpp2"
[[nodiscard]] auto main() -> int;

//=== Cpp2 definitions ==========================================================

#line 1 "..\\learn\\demo.cpp2"
[[nodiscard]] auto main() -> int{
   std::cout << "Hello, world!\n";
}

[MichaelCook]

Could also just stop using backslashes entirely.
At the earliest opportunity, convert any backslashes in pathnames to forward slashes.
In Windows, they're equivalent (though some Windows applications treat forward slashes on the command line specially).

[JohelEGP]

That'd mean using std::filesystem::path in the generic format instead of std::string, right?

[hsutter]

Thanks, good point -- I'll add a std::quoted there in an upcoming checkin separately that will close this one.