Make target dir self-ignoring

[senekor]

Fixes #15061

How to test:

• run cargo new, observer that no .gitignore is generated
• run cargo build, observe that git still ignores the target directory
• run cargo build with different ways to override the default target directory location:
  • CLI arg (--target-dir)
  • environment variable (CARGO_TARGET_DIR)
  • .cargo/config.toml: build.target-dir

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @epage (or someone else) some time within the next two weeks.

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[senekor]

This is currently a proof of concept, meant as a starting point for discussing how to implement it properly.

The easy approach is to just make sure that the gitignore file exists whenever the target directory path is determined.
This leads to some (maybe unacceptable) quirks.
For example, many cargo commands would touch the file system when the user doesn't expect it (even cargo clean --dry-run creates the gitignore file).

[epage]

Note that #11548 is not labeled with S-accepted which is a prerequisite for being reviewed, see https://doc.crates.io/contrib/process/working-on-cargo.html

[senekor]

makes sense!

[senekor]

I removed the part dealing with cargo new. The PR now points to #15061, which reflects the smaller scope. (approval still pending)