The Cargo Book

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.

The specification may be fully qualified, such as https://github.com/rust-lang/crates.io-index#regex@1.4.3 or it may be abbreviated, such as regex. The abbreviated form may be used as long as it uniquely identifies a single package in the dependency graph. If there is ambiguity, additional qualifiers can be added to make it unique. For example, if there are two versions of the regex package in the graph, then it can be qualified with a version to make it unique, such as regex@1.4.3.

Specification grammar

The formal grammar for a Package Id Specification is:

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

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

Here, brackets indicate that the contents are optional.

The URL form can be used for git dependencies, or to differentiate packages that come from different sources such as different registries.

Example specifications

The following are references to the regex package on crates.io:

The following are some examples of specs for several different git dependencies:

Local packages on the filesystem can use file:// URLs to reference them:

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.