Currently, the exception message makes sense only if the problem is with the 'to' file path.

1) BAD CASE:

import std.file;

void main() {
    rename("some_non_existing_file.txt", "bar.txt");
}

Unfortunately, the exception includes the target file name:

std.file.FileException@std/file.d(681): bar.txt: No such file or directory
[...]

DISCUSSION: The fix seems to be trivial by replacing the last argument with fromz below:

phobos/std/file.d:681:
        cenforce(core.stdc.stdio.rename(fromz, toz) == 0, t, toz);

However, it's not that simple because in some cases it's toz that needs to be reported.

2) GOOD CASE:

import std.file;

void main() {
    // Assuming that foo.txt exists:
    rename("foo.txt", "some/non/existing/path/bar.txt");
}

std.file.FileException@std/file.d(681): some/non/existing/path/bar.txt: No such file or directory
[...]

So, it makes sense to include both paths.