std::fs::copy unconditionally sets permission bits and fails

I'm not writing any code in particular, but I encountered this issue in the software I use.

The software tries to copy a file using std::fs::copy
https://github.com/IntQuant/noita_entangled_worlds/blob/master/noita_proxy/src/player_cosmetics.rs#L586-L592
both source and destination are in a directory with ACLs which make all created files have me (lassebq) the owner of those files, even if they're created by another user.

  File: files/system/player/unmodified.png
  Size: 18680     	Blocks: 40         IO Block: 4096   regular file
Device: 8,17	Inode: 6689021     Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/ lassebq)   Gid: (  100/   users)
Access: 2026-07-22 11:54:33.889921747 +0300
Modify: 2026-07-22 11:54:33.889953747 +0300
Change: 2026-07-22 13:47:40.571769277 +0300
 Birth: 2026-07-22 11:54:33.889921747 +0300
  File: data/generated/sprite_uv_maps/mods.quant.ew.files.system.player.tmp.0000000000000001.png
  Size: 18680     	Blocks: 40         IO Block: 4096   regular file
Device: 8,17	Inode: 6689280     Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/ lassebq)   Gid: (  100/   users)
Access: 2026-07-22 11:56:41.143151945 +0300
Modify: 2026-07-22 14:32:49.975744501 +0300
Change: 2026-07-22 14:32:49.975744501 +0300
 Birth: 2026-07-22 11:56:41.143151945 +0300

ACLs:

# file: files/system/player/unmodified.png
# owner: lassebq
# group: users
user::rw-
group::rwx			#effective:rw-
group:users:rw-
mask::rw-
other::r--

# file: data/generated/sprite_uv_maps/mods.quant.ew.files.system.player.tmp.0000000000000001.png
# owner: lassebq
# group: users
user::rw-
group::rwx			#effective:rw-
group:users:rw-
mask::rw-
other::r--

So, when I run the software as user guest, it attempts to copy file and I get a permission denied error on that file

2026-07-22T11:47:01.492768Z ERROR noita_proxy: Panic occured at src/player_cosmetics.rs:592:6 : called `Result::unwrap()` on an `Err` value: Os { code: 1, kind: PermissionDenied, message: "Operation not permitted" }

But when I perform cp files/system/player/unmodified.png data/generated/sprite_uv_maps/mods.quant.ew.files.system.player.tmp.0000000000000001.png as guest there is no issue, and the files remain owned by lassebq

Start by reproducing the ACL setup described around noita_proxy/src/player_cosmetics.rs:586-592, then inspect std::fs::copy and its filesystem permission handling. Compare its behavior with the successful cp command. Done means copying under these ACLs no longer fails with PermissionDenied, with a regression test covering the case.