A well-intentioned crate author wants to ensure their code builds without warnings. So they annotate their crate root with the following:
#![deny(warnings)] // All is well.
It is short and will stop the build if anything is amiss.
By disallowing the compiler to build with warnings, a crate author opts out of
Rust's famed stability. Sometimes new features or old misfeatures need a change
in how things are done, thus lints are written that
warn for a certain grace
period before being turned to
For example, it was discovered that a type could have two
impls with the same
method. This was deemed a bad idea, but in order to make the transition smooth,
overlapping-inherent-impls lint was introduced to give a warning to those
stumbling on this fact, before it becomes a hard error in a future release.
Also sometimes APIs get deprecated, so their use will emit a warning where before there was none.
All this conspires to potentially break the build whenever something changes.
Furthermore, crates that supply additional lints (e.g. rust-clippy) can no longer be used unless the annotation is removed.
There are two ways of tackling this problem: First, we can decouple the build setting from the code, and second, we can name the lints we want to deny explicitly.
The following command line will build with all warnings set to
RUSTFLAGS="-D warnings" cargo build
This can be done by any individual developer (or be set in a CI tool like Travis, but remember that this may break the build when something changes) without requiring a change to the code.
Alternatively, we can specify the lints that we want to
deny in the code.
Here is a list of warning lints that is (hopefully) safe to deny:
#[deny(bad-style, const-err, dead-code, extra-requirement-in-impl, improper-ctypes, legacy-directory-ownership, non-shorthand-field-patterns, no-mangle-generic-items, overflowing-literals, path-statements , patterns-in-fns-without-body, plugin-as-library, private-in-public, private-no-mangle-fns, private-no-mangle-statics, raw-pointer-derive, safe-extern-statics, unconditional-recursion, unions-with-drop-fields, unused, unused-allocation, unused-comparisons, unused-parens, while-true)]
In addition, the following
allowed lints may be a good idea to
#[deny(missing-debug-implementations, missing-docs, trivial-casts, trivial-numeric-casts, unused-extern-crates, unused-import-braces, unused-qualifications, unused-results)]
Some may also want to add
missing-copy-implementations to their list.
Note that we explicitly did not add the
deprecated lint, as it is fairly
certain that there will be more deprecated APIs in the future.