Creating a file with a single colon in its name strips the part after the colon from the file name and produces an empty file (on Windows)

[FeldrinH]

I tried this code:

let mut f = File::create("before:after").unwrap();
f.write("test".as_bytes()).unwrap();
f.sync_all().unwrap();

I expected to see this happen:
The file creation produces an error, because file paths cannot contain colons on Windows.

Instead, this happened:
There were no errors. Instead, an empty file named before was created.

Note in particular:

• The file was named before, despite the requested name being before:after.
• The file was empty, despite the fact that the code succesfully wrote some text to it and synchronized the writes to disk.

Other observations:
When the file name contains multiple colons then the file creation does produce an error, as expected.

Meta

rustc --version --verbose:

rustc 1.83.0 (90b35a623 2024-11-26)
binary: rustc
commit-hash: 90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf
commit-date: 2024-11-26
host: x86_64-pc-windows-msvc
release: 1.83.0
LLVM version: 19.1.1

Tested on Windows 10 Home version 10.0.19045 Build 19045.

[FeldrinH]

It appears that Rust is not the only language with this bizarre behavior. Python on Windows does the same thing.
The following code produces the exact same result as the Rust code above (tested on Python 3.12.6).

f = open("before:after", mode="w")
f.write("test")
f.close()

[theemathas]

Python deemed this behavior as "not a bug". https://bugs.python.org/issue23463

[ChrisDenton]

This is just Windows filesystem behaviour. The colon indicates a "file stream" https://learn.microsoft.com/en-us/windows/win32/fileio/file-streams#naming-conventions-for-streams

Specifically in this case you're creating a data stream called "after" in a file called "before".

You can see streams in powershell using the -Stream * option of Get-Item https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.management/get-item?view=powershell-7.4#-stream

[FeldrinH]

Python deemed this behavior as "not a bug". https://bugs.python.org/issue23463

I mostly agree with what Tom Edwards said in that thread. The fact that it is possible to write to alternate file streams without any explicit opt-in is a footgun for almost anyone writing to files and especially anyone that generates file names from user input.
(Case in point: I first encountered this issue while writing files that included a video timestamp in their names and was extremely confused.)

Practically no one ever wants to write to alternate file streams and there is currently no way to avoid accidentally doing this other than explicitly checking for colons any time you open a file on Windows.

[the8472]

see also: #107276