Provide a write_file() method for ChildPath

[vincentdephily]

What this would bring over using the existing copy_from() for a single file is the ability to copy the file to a different name. Basically I'd like the semantics of write_binary() and write_str() but using an existing file as source.

use assert_fs::prelude::*;

let temp = assert_fs::TempDir::new().unwrap();
temp
   .child("input.txt")
   .write_file("unittest.42.txt")
   .unwrap();
temp.close().unwrap();

The rationale is that my program uses fixed names for various input files (and/or I don't want to bother configuring my program to use other filenames), but I have many different input files for as many different unittests. I want to say "for this test, the content of input.txt will be taken from that example file".

[vincentdephily]

The workaround seems to be fs::copy(Path::new(".").join(src), tempdir.path().join(dst)), which is arguably simple enough. But it feels weird to not be able to use assert_fs directly for this.

[epage]

Thanks for using assert_fs and letting me know about this! I definitely see the value in this. Besides the ability to rename, this just in general is simpler than having to use copy_from which I think would be worthwhile.