warning when compiling with clang-10 on macOS

[kevinushey]

Saw this while compiling Rcpp with the latest version of LLVM on macOS:

/usr/local/opt/llvm/bin/clang++ -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -fsanitize=address,undefined -fno-omit-frame-pointer -fno-sanitize=alignment,function,float-divide-by-zero -std=gnu++11 -I"/Users/kevinushey/r/r-devel-sanitizers/lib/R/include" -DNDEBUG -I../inst/include/  -I/usr/local/include   -fPIC  -g -O1 -Wall -pedantic -DSWITCH_TO_REFCNT  -c module.cpp -o module.o
api.cpp:136:12: warning: address of stack memory associated with local variable 'f' returned [-Wreturn-stack-address]
    return f.c_str();
           ^
1 warning generated.

The offending routine is here:

Rcpp/src/api.cpp

Lines 129 to 137 in c9cf6aa

	// [[Rcpp::register]]
	const char* short_file_name(const char* file) { // #nocov start
	std::string f(file);
	size_t index = f.find("/include/");
	if (index != std::string::npos) {
	f = f.substr(index + 9);
	}
	return f.c_str();
	}

We are indeed returning a pointer to the data for a std::string on the stack, which is unsafe. Note that while this is a registered routine, it's only used internally (when Rcpp debugging is enabled via RCPP_DEBUG). Either way, two potential fixes here would be:

1. Just return a std::string rather than a const char*, or
2. Use a static std::string, and return a pointer to the data for that std::string.

[eddelbuettel]

Ugg. Option 1. is probably safer.