Add autocomplete for nested examples

[MrMinimal]

What does this PR try to resolve?

Not all examples show up in auto-completion.
If the examples directory has nested examples, only the top level ones will be shown with cargo build --example<TAB>.cargo build --example<TAB>.
Bevy examples only show hello_world instead of all available examples.

Old:

New:

How should we test and review this PR?

The completion logic for me is in ~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/etc/bash_completion.d/cargo.
So to test, this has to be edited as it doesn't get updated when you simply build cargo (I think rustup installs it).

Then go into a Rust project with nested examples (e. g. Bevy) and type cargo build --example and press <TAB>.

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[epage]

Should we do something similar for zsh?

cargo/src/etc/_cargo

Lines 472 to 477 in c2c636a

	_cargo_example_names() {
	if [[ -d examples ]]; then
	local -a files=(${(@f)$(echo examples/*.rs(:t:r))})
	_values 'example' "${files[@]}"
	fi
	}

[epage]

This could pick up both examples and submodules. Unsure whether that is worth it or not

[ehuss]

I would be a little worried that the false-positive rate would be too high. In bevy for example, this would auto-complete a ton of names that aren't examples.

I think the only real solution here is to parse Cargo.toml, and I think native-completions is the only way to realistically do that. This bashcomp could do some json parsing of cargo metadata, but I wouldn't want to go there.

[epage]

Are you aware of our experimental completion system (docs)? It uses cargo to do things like look up all examples.

[MrMinimal]

@epage I wasn't aware of the experimental system, just wanted to contribute back a quick fix I had.
If the new system replaces this one, I'll close the PR.