## Package ID Specifications

### Package ID specifications

Subcommands of Cargo frequently need to refer to a particular package within a dependency graph for various operations like updating, cleaning, building, etc. To solve this problem, Cargo supports Package ID Specifications. A specification is a string which is used to uniquely refer to one package within a graph of packages.

#### Specification grammar

The formal grammar for a Package Id Specification is:

pkgid := pkgname
| [ proto "://" ] hostname-and-path [ "#" ( pkgname | semver ) ]
pkgname := name [ ":" semver ]

proto := "http" | "git" | ...


Here, brackets indicate that the contents are optional.

#### Example specifications

These could all be references to a package foo version 1.2.3 from the registry at crates.io

pkgidnameversionurl
foofoo**
foo:1.2.3foo1.2.3*
crates.io/foofoo**://crates.io/foo
crates.io/foo#1.2.3foo1.2.3*://crates.io/foo
crates.io/bar#foo:1.2.3foo1.2.3*://crates.io/bar
https://crates.io/foo#1.2.3foo1.2.3https://crates.io/foo

#### Brevity of specifications

The goal of this is to enable both succinct and exhaustive syntaxes for referring to packages in a dependency graph. Ambiguous references may refer to one or more packages. Most commands generate an error if more than one package could be referred to with the same specification.