default .gitignore from `cargo new` should explicitly exclude vendor/

[jyn514]

Problem

cargo vendor creates a bunch of files with paths from other people's crates. people sometimes write poorly specified .gitignore files that are too broad and don't anchor specific directories; this is silly but people do silly things, see for example rust-lang/rust#138978. in this case git add vendor/ doesn't do what they expect, some of the paths are ignored.

Proposed Solution

add !/vendor to the default gitignore.

Notes

i took a look at the code for this. currently the logic lives here:

cargo/src/cargo/ops/cargo_new.rs

Lines 747 to 750 in 6344cad

	// Using the push method with multiple arguments ensures that the entries
	// for all mutually-incompatible VCS in terms of syntax are in sync.
	let mut ignore = IgnoreList::new();
	ignore.push("/target", "^target$", "target");

it's not as simple as adding another .push line, because Fossil requires these to be configured in a separate ignore-glob file and Mercurial doesn't support this kind of thing at all. Not sure if this has interactions with pijul.

the current design makes it hard to handle two different files for include and exclude, so that would need to be refactored somehow.

[epage]

If I understand correctly, the reproduction steps are roughly

1. Having a Rust project with a .gitignore of target instead of /target
2. cargo add cc
3. cargo vendor

To get to Step 1, the user either needed to edit an auto-generated .gitignore, or they hand wrote it.

Changing cargo new wouldn't help with someone hand writing .gitignore. It wouldn't help people already in this situation. It will help someone who runs a newer cargo new and then later needed to hand edit .gitignore to ignore more target directories. That is a limited case but maybe its sufficient.

Its been a while since I've dug into the details of gitignore too much. How might this interact with .gitignores inside of vendor/?

Are there any other downsides to doing this?

people sometimes write poorly specified .gitignore files that are too broad and don't anchor specific directories; this is silly but people do silly things,

Please keep in mind that there may be motivations you do not directly see at this time. For example, I've set my template .gitignore to just target because I've been in enough situations where I've had multiple target directories and it was the simplest way. We even had it this way in the cargo repo until 433635b, likely because we previously didn't have a workspace.