Rust Compiler Lints

Jan. 16, 2019 · Matt

What

The Rust compiler allows to enable or disable various lints during code compilation. This is a short overview of these lints.

TLDR: The quickest way to discover these lints is to have the compiler list them.

$ rustc -W help

Why

Lints are useful for various use cases:

  • preventing bugs by catching common gotchas such as unused variables.
  • enforcing coding standards such as having documentation.
  • relaxing compiler defaults that might not fit your use case e.g. allowing use of CamelCase

The above list is not exhaustive but gives an ideas of where linting is applicable.

How

The Rust compiler lints are categorised into four levels:

  • allow
  • warn
  • deny
  • forbid

These are pretty self-explanatory. The compiler with either stay silent, issue an warning or throw an error. The extra forbid level is same as deny but cannot be overridden.

How do you configure these lint levels? There are a couple of ways:

  • via compiler flags

    Running rustc -W help on your terminal will, among other things, show you how to set lint levels:

    Available lint options:
-W <foo>           Warn about <foo>
-A <foo>           Allow <foo>
-D <foo>           Deny <foo>
-F <foo>           Forbid <foo> (deny <foo> and all attempts to override)

  • via attributes

    You can also have attributes, such as #![warn(missing_docs)], in your lib.rs file.

The compiler can also set the maximum level of all lints using the --cap-lints flag. e.g.

$ rustc --cap-lints warn

There is a list of lints that the compiler allows, warns and denies by default. The help command rustc -W help lists out all lints with their corresponding default level.

Reference