`#![deny(warnings)]`

Description

A well-intentioned crate author wants to ensure their code builds without warnings. So they annotate their crate root with the following:

Example

#![deny(warnings)]

// All is well.

Advantages

It is short and will stop the build if anything is amiss.

Drawbacks

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 deny.

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, the 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. This is mitigated with --cap-lints. The --cap-lints=warn command line argument, turns all deny lint errors into warnings. But be aware that forbid lints are stronger than deny hence the 'forbid' level cannot be overridden to be anything lower than an error. As a result forbid lints will still stop compilation.

Alternatives

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 deny:

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 (as of Rustc 1.48.0):

#[deny(bad-style,
       const-err,
       dead-code,
       improper-ctypes,
       non-shorthand-field-patterns,
       no-mangle-generic-items,
       overflowing-literals,
       path-statements ,
       patterns-in-fns-without-body,
       private-in-public,
       unconditional-recursion,
       unused,
       unused-allocation,
       unused-comparisons,
       unused-parens,
       while-true)]

In addition, the following allowed lints may be a good idea to deny:

#[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.

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#![deny(warnings)]