RFC: Add cargo install

text/0000-cargo-install.md

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+- Feature Name: N/A
+- Start Date: 2015-07-10
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Add a new subcommand to Cargo, `install`, which will install `[[bin]]`-based
+packages onto the local system in a Cargo-specific directory.
+
+# Motivation
+
+There has [almost always been a desire][cargo-37] to be able to install Cargo
+packages locally, but it's been somewhat unclear over time what the precise
+meaning of this is. Now that we have crates.io and lots of experience with
+Cargo, however, the niche that `cargo install` would fill is much clearer.
+
+[cargo-37]: https://github.com/rust-lang/cargo/issues/37
+
+Fundamentally, however, Cargo is a ubiquitous tool among the Rust community and
+implementing `cargo install` would facilitate sharing Rust code among its
+developers.  Simple tasks like installing a new cargo subcommand, installing an
+editor plugin, etc, would be just a `cargo install` away. Cargo can manage
+dependencies and versions itself to make the process as seamless as possible.
+
+Put another way, enabling easily sharing code is one of Cargo's fundamental
+design goals, and expanding into binaries is simply an extension of Cargo's core
+functionality.
+
+# Detailed design
+
+The following new subcommand will be added to Cargo:
+
+```
+Install a crate onto the local system
+
+Installing new crates:
+    cargo install [options]
+    cargo install [options] [-p CRATE | --package CRATE] [--vers VERS]
+    cargo install [options] --git URL [--branch BRANCH | --tag TAG | --rev SHA]
+    cargo install [options] --path PATH
+
+Managing installed crates:
+    cargo install [options] --list
+
+Options:
+    -h, --help              Print this message
+    -j N, --jobs N          The number of jobs to run in parallel
+    --features FEATURES     Space-separated list of features to activate
+    --no-default-features   Do not build the `default` feature
+    --debug                 Build in debug mode instead of release mode
+    --bin NAME              Only install the binary NAME
+    --example EXAMPLE       Install the example EXAMPLE instead of binaries
+    -p, --package CRATE     Install this crate from crates.io or select the
+                            package in a repository/path to install.
+    -v, --verbose           Use verbose output
+    --root                  Directory to install packages into
+
+This command manages Cargo's local set of install binary crates. Only packages
+which have [[bin]] targets can be installed, and all binaries are installed into
+`$HOME/.cargo/bin` by default (or `$CARGO_HOME/bin` if you change the home
+directory).
+
+There are multiple methods of installing a new crate onto the system. The
+`cargo install` command with no arguments will install the current crate (as
+specifed by the current directory). Otherwise the `-p`, `--package`, `--git`,
+and `--path` options all specify the source from which a crate is being
+installed. The `-p` and `--package` options will download crates from crates.io.
+
+Crates from crates.io can optionally specify the version they wish to install
+via the `--vers` flags, and similarly packages from git repositories can
+optionally specify the branch, tag, or revision that should be installed. If a
+crate has multiple binaries, the `--bin` argument can selectively install only
+one of them, and if you'd rather install examples the `--example` argument can
+be used as well.
+
+The `--list` option will list all installed packages (and their versions).
+```
+
+## Installing Crates
+
+Cargo attempts to be as flexible as possible in terms of installing crates from
+various locations and specifying what should be installed. All binaries will be
+stored in a **cargo-local** directory, and more details on where exactly this is
+located can be found below.
+
+Cargo will not attempt to install binaries or crates into system directories
+(e.g. `/usr`) as that responsibility is intended for system package managers.
+
+To use installed crates one just needs to add the binary path to their `PATH`
+environment variable. This will be recommended when `cargo install` is run if
+`PATH` does not already look like it's configured.
+
+#### Crate Sources
+
+The `cargo install` command will be able to install crates from any source that
+Cargo already understands. For example it will start off being able to install
+from crates.io, git repositories, and local paths. Like with normal
+dependencies, downloads from crates.io can specify a version, git repositories
+can specify branches, tags, or revisions.
+
+#### Sources with multiple crates
+
+Sources like git repositories and paths can have multiple crates inside them,
+and Cargo needs a way to figure out which one is being installed. If there is
+more than one crate in a repo (or path), then Cargo will apply the following
+heuristics to select a crate, in order:
+
+1. If the `-p` argument is specified, use that crate.
+2. If only one crate has binaries, use that crate.
+3. If only one crate has examples, use that crate.
+4. Print an error suggesting the `-p` flag.
+
+#### Multiple binaries in a crate
+
+Once a crate has been selected, Cargo will by default build all binaries and
+install them. This behavior can be modified with the `--bin` or `--example`
+flags to configure what's installed on the local system.
+
+#### Building a Binary
+
+The `cargo install` command has some standard build options found on `cargo
+build` and friends, but a key difference is that `--release` is the default for
+installed binaries so a `--debug` flag is present to switch this back to
+debug-mode. Otherwise the `--features` flag can be specified to activate various
+features of the crate being installed.
+
+The `--target` option is omitted as `cargo install` is not intended for creating
+cross-compiled binaries to ship to other platforms.
+
+#### Conflicting Crates
+
+Cargo will not namespace the installation directory for crates, so conflicts may
+arise in terms of binary names. For example if crates A and B both provide a
+binary called `foo` they cannot be both installed at once. Cargo will reject
+these situations and recommend that a binary is selected via `--bin` or the
+conflicting crate is uninstalled.
+
+#### Placing output artifacts
+
+The `cargo install` command can be customized where it puts its output artifacts
+to install packages in a custom location. The root directory of the installation
+will be determined in a hierarchical fashion, choosing the first of the
+following that is specified:
+
+1. The `--root` argument on the command line.
+2. The environment variable `CARGO_INSTALL_ROOT`.
+3. The `install.root` configuration option.
+4. The value of `$CARGO_HOME` (also determined in an independent and
+   hierarchical fashion).
+
+Once the root directory is found, Cargo will place all binaries in the
+`$INSTALL_ROOT/bin` folder. Cargo will also reserve the right to retain some
+metadata in this folder in order to keep track of what's installed and what
+binaries belong to which package.
+
+## Managing Installations
+
+If Cargo gives access to installing packages, it should surely provide the
+ability to manage what's installed! The first part of this is just discovering
+what's installed, and this is provided via `cargo install --list`.
+
+## Removing Crates
+
+To remove an installed crate, another subcommand will be added to Cargo:
+
+```
+Remove a locally installed crate
+
+Usage:
+    cargo uninstall [options] SPEC
+
+Options:
+    -h, --help              Print this message
+    --bin NAME              Only uninstall the binary NAME
+    --example EXAMPLE       Only uninstall the example EXAMPLE
+    -v, --verbose           Use verbose output
+
+The argument SPEC is a package id specification (see `cargo help pkgid`) to
+specify which crate should be uninstalled. By default all binaries are
+uninstalled for a crate but the `--bin` and `--example` flags can be used to
+only uninstall particular binaries.
+```
+
+Cargo won't remove the source for uninstalled crates, just the binaries that
+were installed by Cargo itself.
+
+## Non-binary artifacts
+
+Cargo will not currently attempt to manage anything other than a binary artifact
+of `cargo build`. For example the following items will not be available to
+installed crates:
+
+* Dynamic native libraries built as part of `cargo build`.
+* Native assets such as images not included in the binary itself.
+* The source code is not guaranteed to exist, and the binary doesn't know where
+  the source code is.
+
+Additionally, Cargo will not immediately provide the ability to configure the
+installation stage of a package. There is often a desire for a "pre-install
+script" which runs various house-cleaning tasks. This is left as a future
+extension to Cargo.
+
+# Drawbacks
+
+Beyond the standard "this is more surface area" and "this may want to
+aggressively include more features initially" concerns there are no known
+drawbacks at this time.
+
+# Alternatives
+
+### System Package Managers
+
+The primary alternative to putting effort behind `cargo install` is to instead
+put effort behind system-specific package managers. For example the line between
+a system package manager and `cargo install` is a little blurry, and the
+"official" way to distribute a package should in theory be through a system
+package manager. This also has the upside of benefiting those outside the Rust
+community as you don't have to have Cargo installed to manage a program. This
+approach is not without its downsides, however:
+
+* There are *many* system package managers, and it's unclear how much effort it
+  would be for Cargo to support building packages for all of them.
+* Actually preparing a package for being packaged in a system package manager
+  can be quite onerous and is often associated with a high amount of overhead.
+* Even once a system package is created, it must be added to an online
+  repository in one form or another which is often different for each
+  distribution.
+
+All in all, even if Cargo invested effort in facilitating creation of system
+packages, **the threshold for distribution a Rust program is still too high**.
+If everything went according to plan it's just unfortunately inherently complex
+to only distribute packages through a system package manager because of the
+various requirements and how diverse they are. The `cargo install` command
+provides a cross-platform, easy-to-use, if Rust-specific interface to installing
+binaries.
+
+It is expected that all major Rust projects will still invest effort into
+distribution through standard package managers, and Cargo will certainly have
+room to help out with this, but it doesn't obsolete the need for
+`cargo install`.
+
+### Installing Libraries
+
+Another possibility for `cargo install` is to not only be able to install
+binaries, but also libraries. The meaning of this however, is pretty nebulous
+and it's not clear that it's worthwhile. For example all Cargo builds will not
+have access to these libraries (as Cargo retains control over dependencies). It
+may mean that normal invocations of `rustc` have access to these libraries (e.g.
+for small one-off scripts), but it's not clear that this is worthwhile enough to
+support installing libraries yet.
+
+Another possible interpretation of installing libraries is that a developer is
+informing Cargo that the library should be available in a pre-compiled form. If
+any compile ends up using the library, then it can use the precompiled form
+instead of recompiling it. This job, however, seems best left to `cargo build`
+as it will automatically handle when the compiler version changes, for example.
+It may also be more appropriate to add the caching layer at the `cargo build`
+layer instead of `cargo install`.
+
+# Unresolved questions
+
+None yet