`cargo install` should read `$HOME/.cargo/config` only

[traviscross]

The cargo install command should read configuration from $HOME/.cargo/config but not from other configuration files.

Normally cargo looks for configuration in $PWD/.cargo/config, $PWD/../.cargo/config, etc., and in $HOME/.cargo/config. For cargo install, however, the tree walking behavior is surprising. cargo install foo should work the same regardless of the directory from which we run it.

This surprising behavior resulted in #5850 and PR #5874. That PR causes cargo install to ignore any configured build.target value from a configuration file.

However, that change does both too little and too much. It does too much because we don't want to ignore configured values in $HOME/.cargo/config. Values set there make sense for cargo install. E.g., we might use that to set musl as our default build target. On the other hand, the change does too little because we want to ignore all configuration values from $PWD/(../)*/.cargo/config, not just build.target.

The correct behavior is that cargo install should load only $HOME/.cargo/config and should apply all values found there.

cc @alexcrichton @japaric

[dwijnand]

The walking up the filesystem and looking in ~/.cargo happens in:

cargo/src/cargo/util/config.rs

Lines 1538 to 1567 in a5d8294

	fn walk_tree<F>(pwd: &Path, mut walk: F) -> CargoResult<()>
	where
	F: FnMut(&Path) -> CargoResult<()>,
	{
	let mut stash: HashSet<PathBuf> = HashSet::new();
	for current in paths::ancestors(pwd) {
	let possible = current.join(".cargo").join("config");
	if fs::metadata(&possible).is_ok() {
	walk(&possible)?;
	stash.insert(possible);
	}
	}
	// Once we're done, also be sure to walk the home directory even if it's not
	// in our history to be sure we pick up that standard location for
	// information.
	let home = homedir(pwd).ok_or_else(|| {
	format_err!(
	"Cargo couldn't find your home directory. \
	This probably means that $HOME was not set."
	)
	})?;
	let config = home.join("config");
	if !stash.contains(&config) && fs::metadata(&config).is_ok() {
	walk(&config)?;
	}
	Ok(())
	}

[marcbowes]

How can you run cargo install with a source-replacement for crates.io after this change?

[alexcrichton]

@marcbowes I believe you should be able to configure via env vars or place configuration in $HOME

[marcbowes]

@alexcrichton the env vars don't allow you to do source replacement. https://doc.rust-lang.org/cargo/reference/config.html#environment-variables

Environment variables will take precedent over TOML configuration, and currently only integer, boolean, and string keys are supported to be defined by environment variables. This means that source replacement, which is expressed by tables, cannot be configured through environment variables.

Moving configuration in $HOME is a global thing, i.e. it can create a mess that needs to be cleaned up, can be difficult to actually do (merge, undo) and prevents parallel installs with different config (e.g. registries).

Moving $HOME to a different folder temporarily breaks rustup. That can be worked around too, of course.

A much simpler option would be to have the config load from $CARGO_HOME as well as $HOME. Then I can just change $CARGO_HOME (and already do).

Alternatively, can we have an option or env var to switch out the root config?

[ehuss]

Would the change recommended in #6026 (comment) work? It sounds like a good idea to me.

[marcbowes]

Not for the use-case I have in mind.

At my company, we want to control our use of 3P packages (e.g. for licensing reasons). We mirror crates.io internally and use .cargo/config to source-replace registries:

[source.crates-io]
replace-with = 'local-registry'

[source.local-registry]
local-registry = 'PATH-TO/cargo-registry'

In addition to removing some third-party packages, this registry also includes first-party packages.

Now, we want to be able to use cargo install against that registry. Before the changes in this issue, we could place the .cargo/config file in the cwd and run cargo install. Now, we can't.

The comment you refer to would help in that we could fetch the package, tar xf it, then put in the config package and run install with --path. Which is what we sort of do, except it short-circuits most of what install buys you.