Forbid wildcard dependencies on crates.io

text/0000-no-wildcard-deps.md

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+- Feature Name: N/A
+- Start Date: 2015-07-23
+- RFC PR:
+- Rust Issue:
+
+# Summary
+
+A Cargo crate's dependencies are associated with constraints that specify the
+set of versions of the dependency with which the crate is compatible. These
+constraints range from accepting exactly one version (`=1.2.3`), to
+accepting a range of versions (`^1.2.3`, `~1.2.3`, `>= 1.2.3, < 3.0.0`), to
+accepting any version at all (`*`). This RFC proposes to update crates.io to
+reject publishes of crates that have compile or build dependencies with
+version constraints that have no upper bound.
+
+# Motivation
+
+Version constraints are a delicate balancing act between stability and
+flexibility. On one extreme, one can lock dependencies to an exact version.
+From one perspective, this is great, since the dependencies a user will consume
+will be the same that the developers tested against. However, on any nontrival
+project, one will inevitably run into conflicts where library A depends on
+version `1.2.3` of library B, but library C depends on version `1.2.4`, at
+which point, the only option is to force the version of library B to one of
+them and hope everything works.
+
+On the other hand, a wildcard (`*`) constraint will never conflict with
+anything! There are other things to worry about here, though. A version
+constraint is fundamentally an assertion from a library's author to its users
+that the library will work with any version of a dependency that matches its
+constraint. A wildcard constraint is claiming that the library will work with
+any version of the dependency that has ever been released *or will ever be
+released, forever*. This is a somewhat absurd guarantee to make - forever is a
+long time!
+
+Absurd guarantees on their own are not necessarily sufficient motivation to
+make a change like this. The real motivation is the effect that these
+guarantees have on consumers of libraries.
+
+As an example, consider the [openssl](https://crates.io/crates/openssl) crate.
+It is one of the most popular libraries on crates.io, with several hundred
+downloads every day. 50% of the [libraries that depend on it](https://crates.io/crates/openssl/reverse_dependencies)
+have a wildcard constraint on the version. Almost all of them them will fail
+to compile against version 0.7 of openssl when it is released. When that
+happens, users of those libraries will be forced to manually override Cargo's
+version selection every time it is recalculated. This is not a fun time.
+
+Bad version restrictions are also "viral". Even if a developer is careful to
+pick dependencies that have reasonable version restrictions, there could be a
+wildcard constraint hiding five transitive levels down.  Manually searching the
+entire dependency graph is an exercise in frustration that shouldn't be
+necessary.
+
+On the other hand, consider a library that has a version constraint of `^0.6`.
+When openssl 0.7 releases, the library will either continue to work against
+version 0.7, or it won't. In the first case, the author can simply extend the
+constraint to `>= 0.6, < 0.8` and consumers can use it with version 0.6 or 0.7
+without any trouble. If it does not work against version 0.7, consumers of the
+library are fine! Their code will continue to work without any manual
+intervention. The author can update the library to work with version 0.7 and
+release a new version with a constraint of `^0.7` to support consumers that
+want to use that newer release.
+
+Making crates.io more picky than Cargo itself is not a new concept; it
+currently [requires several items](https://github.com/rust-lang/crates.io/blob/8c85874b6b967e1f46ae2113719708dce0c16d32/src/krate.rs#L746-L759) in published crates that Cargo will not:
+
+ * A valid license
+ * A description
+ * A list of authors
+
+All of these requirements are in place to make it easier for developers to use
+the libraries uploaded to crates.io - that's why crates are published, after
+all! A restriction on wildcards is another step down that path.
+
+Note that this restriction would only apply to normal compile dependencies and
+build dependencies, but not to dev dependencies. Dev dependencies are only used
+when testing a crate, so it doesn't matter to downstream consumers if they
+break.
+
+# Detailed design
+
+Alter crates.io's pre-publish behavior to check the version constraints of all
+compile and build dependencies, and reject those that have no upper bound. For
+example, these would be rejected:
+
+ * `*`
+ * `> 0.3`
+ * `>= 0.3`
+
+While these would not:
+
+ * `>= 0.3, < 0.5`
+ * `^0.3`
+ * `~0.3`
+ * `=0.3.1`
+
+# Drawbacks
+
+The barrier to entry when publishing a crate will be mildly higher.
+
+In theory, there could be contexts where an unbounded version constraint is
+actually appropriate?
+
+# Alternatives
+
+We could continue allowing these kinds of constraints, but complain in a
+"sufficiently annoying" manner during publishes to discourage their use.
+
+# Unresolved questions
+
+Should crates.io also forbid constraints that reference versions of
+dependencies that don't yet exist? For example, a constraint of `>= 0.3, < 0.5`
+where the dependency has no published versions in the `0.4` range.