cargo-install: build directory should be in $TEMPDIR

[japaric]

and each instance of cargo-install should have its own build directory.

Right now cargo-install uses a target-install directory in $PWD. So you run into issues like these:

You don't have permission to write to $PWD:

$ cd /
$ cargo install cargo-graph
    Updating registry `https://github.com/rust-lang/crates.io-index`
 Downloading cargo-graph v0.2.2
error: failed to compile `cargo-graph v0.2.2`, intermediate artifacts can be found at `/target-install`

Caused by:
  failed to open: /target-install/release/.cargo-lock

To learn more, run the command again with --verbose.

Calling cargo install multiple times from the same directory will build each crate sequentially instead of in parallel (because the build directory gets locked -- without the lock things would be worse, so removing the lock is not a solution)

$ cargo install cargo-graph & cargo install cargo-count
[1] 28438
    Updating registry `https://github.com/rust-lang/crates.io-index`
    Blocking waiting for file lock on the registry index
    Updating registry `https://github.com/rust-lang/crates.io-index`
    Blocking waiting for file lock on the registry index
    Blocking waiting for file lock on crate metadata
    Blocking waiting for file lock on build directory
   Compiling rustc-serialize v0.3.19
(...)

If the build directory was $(mktemp -d) then you wouldn't have problem calling cargo install from a directory you don't permission to write to, and you could spawn several cargo install (from the same directory) that actually run in parallel.

Thoughts?

[alexcrichton]

The problem with tempdirs is that if you ctrl-c they end up lying around and they're hard to find to remove. I wonder though if Cargo could just try a few locations? For example if $PWD isn't workable because it's not writable, it could switch to a deterministic directory in /tmp. Otherwise if it fails it could switch to cargo-install2 or something like that.

Although I guess it's just amounting to a tempdir that's slightly more readable...

Meh, I guess just using a tempdir seems fine to me!