`.gitignore`, with `:` prepended to indicate a pattern.

## Git submodules

**NOTE**: submodules are a nice thing to know about, but it *isn't* an absolute
prerequisite to contribute to `rustc`. If you are using Git for the first time,
you might want to get used to the main concepts of Git before reading this section.

The `rust-lang/rust` repository uses [Git submodules] as a way to use other
Rust projects from within the `rust` repo. Examples include Rust's fork of
`llvm-project`, `cargo`  and libraries like `stdarch` and `backtrace`.

Those projects are developed and maintained in an separate Git (and GitHub)
repository, and they have their own Git history/commits, issue tracker and PRs.
Submodules allow us to create some sort of embedded sub-repository inside the
`rust` repository and use them like they were directories in the `rust` repository.

Take `llvm-project` for example. `llvm-project` is maintained in the [`rust-lang/llvm-project`]
repository, but it is used in `rust-lang/rust` by the compiler for code generation and
optimization. We bring it in `rust` as a submodule, in the `src/llvm-project` folder.

The contents of submodules are ignored by Git: submodules are in some sense isolated
from the rest of the repository. However, if you try to `cd src/llvm-project` and then
run `git status`:

```console
HEAD detached at 9567f08afc943
nothing to commit, working tree clean
```

As far as git is concerned, you are no longer in the `rust` repo, but in the `llvm-project` repo.
You will notice that we are in "detached HEAD" state, i.e. not on a branch but on a
particular commit.

This is because, like any dependency, we want to be able to control which version to use.
Submodules allow us to do just that: every submodule is "pinned" to a certain
commit, which doesn't change unless modified manually. If you use `git checkout <commit>`
in the `llvm-project` directory and go back to the `rust` directory, you can stage this
change like any other, e.g. by running `git add src/llvm-project`. (Note that if
you *don't* stage the change to commit, then you run the risk that running
`x` will just undo your change by switching back to the previous commit when
it automatically "updates" the submodules.)

This version selection is usually done by the maintainers of the project, and
looks like [this][llvm-update].

Git submodules take some time to get used to, so don't worry if it isn't perfectly
clear yet. You will rarely have to use them directly and, again, you don't need
to know everything about submodules to contribute to Rust. Just know that they
exist and that they correspond to some sort of embedded subrepository dependency