You are probably here because you want to add a new lint to Clippy. If this is the first time you're contributing to Clippy, this document guides you through creating an example lint from scratch.
To get started, we will create a lint that detects functions called foo
,
because that's clearly a non-descriptive name.
- Adding a new lint
- Setup
- Getting Started
- Testing
- Rustfix tests
- Testing manually
- Lint declaration
- Lint registration
- Lint passes
- Emitting a lint
- Adding the lint logic
- Specifying the lint's minimum supported Rust version (MSRV)
- Author lint
- Print HIR lint
- Documentation
- Running rustfmt
- Debugging
- Conflicting lints
- PR Checklist
- Adding configuration to a lint
- Cheat Sheet
See the Basics documentation.
There is a bit of boilerplate code that needs to be set up when creating a new
lint. Fortunately, you can use the Clippy dev tools to handle this for you. We
are naming our new lint foo_functions
(lints are generally written in snake
case), and we don't need type information, so it will have an early pass type
(more on this later). If you're unsure if the name you chose fits the lint,
take a look at our lint naming guidelines.
To get started, there are two ways to define our lint.
Command: cargo dev new_lint --name=foo_functions --pass=early --category=pedantic
(category will default to nursery if not provided)
This command will create a new file: clippy_lints/src/foo_functions.rs
, as well
as register the lint.
Command: cargo dev new_lint --name=foo_functions --type=functions --category=pedantic
This command will create a new file: clippy_lints/src/{type}/foo_functions.rs
.
Notice how this command has a --type
flag instead of --pass
. Unlike a standalone
definition, this lint won't be registered in the traditional sense. Instead, you will
call your lint from within the type's lint pass, found in clippy_lints/src/{type}/mod.rs
.
A "type" is just the name of a directory in clippy_lints/src
, like functions
in
the example command. These are groupings of lints with common behaviors, so if your
lint falls into one, it would be best to add it to that type.
Both commands will create a file: tests/ui/foo_functions.rs
. For cargo lints,
two project hierarchies (fail/pass) will be created by default under tests/ui-cargo
.
Next, we'll open up these files and add our lint!
Let's write some tests first that we can execute while we iterate on our lint.
Clippy uses UI tests for testing. UI tests check that the output of Clippy is
exactly as expected. Each test is just a plain Rust file that contains the code
we want to check. The output of Clippy is compared against a .stderr
file.
Note that you don't have to create this file yourself, we'll get to generating
the .stderr
files further down.
We start by opening the test file created at tests/ui/foo_functions.rs
.
Update the file with some examples to get started:
#![allow(unused)]
#![warn(clippy::foo_functions)]
// Impl methods
struct A;
impl A {
pub fn fo(&self) {}
pub fn foo(&self) {}
pub fn food(&self) {}
}
// Default trait methods
trait B {
fn fo(&self) {}
fn foo(&self) {}
fn food(&self) {}
}
// Plain functions
fn fo() {}
fn foo() {}
fn food() {}
fn main() {
// We also don't want to lint method calls
foo();
let a = A;
a.foo();
}
Now we can run the test with TESTNAME=foo_functions cargo uibless
, currently
this test is meaningless though.
While we are working on implementing our lint, we can keep running the UI test.
That allows us to check if the output is turning into what we want by checking the
.stderr
file that gets updated on every test run.
Running TESTNAME=foo_functions cargo uitest
should pass on its own. When we
commit our lint, we need to commit the generated .stderr
files, too. In
general, you should only commit files changed by cargo bless
for the
specific lint you are creating/editing.
Note: you can run multiple test files by specifying a comma separated list:
TESTNAME=foo_functions,test2,test3
.
For cargo lints, the process of testing differs in that we are interested in the
Cargo.toml
manifest file. We also need a minimal crate associated with that
manifest.
If our new lint is named e.g. foo_categories
, after running cargo dev new_lint --name=foo_categories --type=cargo --category=cargo
we will find by
default two new crates, each with its manifest file:
tests/ui-cargo/foo_categories/fail/Cargo.toml
: this file should cause the new lint to raise an error.tests/ui-cargo/foo_categories/pass/Cargo.toml
: this file should not trigger the lint.
If you need more cases, you can copy one of those crates (under
foo_categories
) and rename it.
The process of generating the .stderr
file is the same, and prepending the
TESTNAME
variable to cargo uitest
works too.
If the lint you are working on is making use of structured suggestions, the test
will create a .fixed
file by running rustfix for that test.
Rustfix will apply the suggestions
from the lint to the code of the test file and compare that to the contents of a
.fixed
file.
Use cargo bless
to automatically generate the .fixed
file while running
the tests.
Manually testing against an example file can be useful if you have added some
println!
s and the test suite output becomes unreadable. To try Clippy with
your local modifications, run the following from the Clippy directory:
cargo dev lint input.rs
To run Clippy on an existing project rather than a single file you can use
cargo dev lint /path/to/project
Or set up a rustup toolchain that points to the local Clippy binaries
cargo dev setup toolchain
# Then in `/path/to/project` you can run
cargo +clippy clippy
Let's start by opening the new file created in the clippy_lints
crate at
clippy_lints/src/foo_functions.rs
. That's the crate where all the lint code
is. This file has already imported some initial things we will need:
use rustc_lint::{EarlyLintPass, EarlyContext};
use rustc_session::declare_lint_pass;
use rustc_ast::ast::*;
The next step is to update the lint declaration. Lints are declared using the
declare_clippy_lint!
macro, and we just need to update
the auto-generated lint declaration to have a real description, something like
this:
declare_clippy_lint! {
/// ### What it does
///
/// ### Why is this bad?
///
/// ### Example
/// ```rust
/// // example code
/// ```
#[clippy::version = "1.29.0"]
pub FOO_FUNCTIONS,
pedantic,
"function named `foo`, which is not a descriptive name"
}
- The section of lines prefixed with
///
constitutes the lint documentation section. This is the default documentation style and will be displayed like this. To render and open this documentation locally in a browser, runcargo dev serve
. - The
#[clippy::version]
attribute will be rendered as part of the lint documentation. The value should be set to the current Rust version that the lint is developed in, it can be retrieved by runningrustc -vV
in the rust-clippy directory. The version is listed under release. (Use the version without the-nightly
) suffix. FOO_FUNCTIONS
is the name of our lint. Be sure to follow the lint naming guidelines here when naming your lint. In short, the name should state the thing that is being checked for and read well when used withallow
/warn
/deny
.pedantic
sets the lint level toAllow
. The exact mapping can be found here- The last part should be a text that explains what exactly is wrong with the code
The rest of this file contains an empty implementation for our lint pass, which
in this case is EarlyLintPass
and should look like this:
// clippy_lints/src/foo_functions.rs
// .. imports and lint declaration ..
declare_lint_pass!(FooFunctions => [FOO_FUNCTIONS]);
impl EarlyLintPass for FooFunctions {}
When using cargo dev new_lint
, the lint is automatically registered and
nothing more has to be done.
When declaring a new lint by hand and cargo dev update_lints
is used, the lint
pass may have to be registered manually in the register_lints
function in
clippy_lints/src/lib.rs
:
store.register_early_pass(|| Box::new(foo_functions::FooFunctions));
As one may expect, there is a corresponding register_late_pass
method
available as well. Without a call to one of register_early_pass
or
register_late_pass
, the lint pass in question will not be run.
One reason that cargo dev update_lints
does not automate this step is that
multiple lints can use the same lint pass, so registering the lint pass may
already be done when adding a new lint. Another reason that this step is not
automated is that the order that the passes are registered determines the order
the passes actually run, which in turn affects the order that any emitted lints
are output in.
Writing a lint that only checks for the name of a function means that we only have to deal with the AST and don't have to deal with the type system at all. This is good, because it makes writing this particular lint less complicated.
We have to make this decision with every new Clippy lint. It boils down to using
either EarlyLintPass
or LateLintPass
.
In short, the EarlyLintPass
runs before type checking and
HIR lowering and the LateLintPass
has access to type information. Consider using the LateLintPass
unless you need
something specific from the EarlyLintPass
.
Since we don't need type information for checking the function name, we used
--pass=early
when running the new lint automation and all the imports were
added accordingly.
With UI tests and the lint declaration in place, we can start working on the implementation of the lint logic.
Let's start by implementing the EarlyLintPass
for our FooFunctions
:
impl EarlyLintPass for FooFunctions {
fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
// TODO: Emit lint here
}
}
We implement the check_fn
method from the
EarlyLintPass
trait. This gives us access to various
information about the function that is currently being checked. More on that in
the next section. Let's worry about the details later and emit our lint for
every function definition first.
Depending on how complex we want our lint message to be, we can choose from a
variety of lint emission functions. They can all be found in
clippy_utils/src/diagnostics.rs
.
span_lint_and_help
seems most appropriate in this case. It allows us to
provide an extra help message, and we can't really suggest a better name
automatically. This is how it looks:
impl EarlyLintPass for FooFunctions {
fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
span_lint_and_help(
cx,
FOO_FUNCTIONS,
span,
"function named `foo`",
None,
"consider using a more meaningful name"
);
}
}
Running our UI test should now produce output that contains the lint message.
According to the rustc-dev-guide, the text should be matter of fact and avoid capitalization and periods, unless multiple sentences are needed. When code or an identifier must appear in a message or label, it should be surrounded with single grave accents `.
Writing the logic for your lint will most likely be different from our example, so this section is kept rather short.
Using the check_fn
method gives us access to FnKind
that has the FnKind::Fn
variant. It provides access to the name of the
function/method via an Ident
.
With that we can expand our check_fn
method to:
impl EarlyLintPass for FooFunctions {
fn check_fn(&mut self, cx: &EarlyContext<'_>, fn_kind: FnKind<'_>, span: Span, _: NodeId) {
if is_foo_fn(fn_kind) {
span_lint_and_help(
cx,
FOO_FUNCTIONS,
span,
"function named `foo`",
None,
"consider using a more meaningful name"
);
}
}
}
We separate the lint conditional from the lint emissions because it makes the code a bit easier to read. In some cases this separation would also allow to write some unit tests (as opposed to only UI tests) for the separate function.
In our example, is_foo_fn
looks like:
// use statements, impl EarlyLintPass, check_fn, ..
fn is_foo_fn(fn_kind: FnKind<'_>) -> bool {
match fn_kind {
FnKind::Fn(_, ident, ..) => {
// check if `fn` name is `foo`
ident.name.as_str() == "foo"
}
// ignore closures
FnKind::Closure(..) => false
}
}
Now we should also run the full test suite with cargo test
. At this point
running cargo test
should produce the expected output. Remember to run cargo bless
to update the .stderr
file.
cargo test
(as opposed to cargo uitest
) will also ensure that our lint
implementation is not violating any Clippy lints itself.
That should be it for the lint implementation. Running cargo test
should now
pass.
Sometimes a lint makes suggestions that require a certain version of Rust. For
example, the manual_strip
lint suggests using str::strip_prefix
and
str::strip_suffix
which is only available after Rust 1.45. In such cases, you
need to ensure that the MSRV configured for the project is >= the MSRV of the
required Rust feature. If multiple features are required, just use the one with
a lower MSRV.
First, add an MSRV alias for the required feature in clippy_utils::msrvs
.
This can be accessed later as msrvs::STR_STRIP_PREFIX
, for example.
msrv_aliases! {
..
1,45,0 { STR_STRIP_PREFIX }
}
In order to access the project-configured MSRV, you need to have an msrv
field
in the LintPass struct, and a constructor to initialize the field. The msrv
value is passed to the constructor in clippy_lints/lib.rs
.
pub struct ManualStrip {
msrv: Msrv,
}
impl ManualStrip {
pub fn new(conf: &'static Conf) -> Self {
Self { msrv: conf.msrv.clone() }
}
}
The project's MSRV can then be matched against the feature MSRV in the LintPass
using the Msrv::meets
method.
if !self.msrv.meets(msrvs::STR_STRIP_PREFIX) {
return;
}
The project's MSRV can also be specified as an attribute, which overrides
the value from clippy.toml
. This can be accounted for using the
extract_msrv_attr!(LintContext)
macro and passing
LateContext
/EarlyContext
.
impl<'tcx> LateLintPass<'tcx> for ManualStrip {
fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
...
}
extract_msrv_attr!(LateContext);
}
Once the msrv
is added to the lint, a relevant test case should be added to
the lint's test file, tests/ui/manual_strip.rs
in this example. It should
have a case for the version below the MSRV and one with the same contents but
for the MSRV version itself.
...
#[clippy::msrv = "1.44"]
fn msrv_1_44() {
/* something that would trigger the lint */
}
#[clippy::msrv = "1.45"]
fn msrv_1_45() {
/* something that would trigger the lint */
}
As a last step, the lint should be added to the lint documentation. This is done
in clippy_config/src/conf.rs
:
define_Conf! {
/// Lint: LIST, OF, LINTS, <THE_NEWLY_ADDED_LINT>. The minimum rust version that the project supports
(msrv: Option<String> = None),
...
}
Afterwards update the documentation for the book as described in Adding configuration to a lint.
If you have trouble implementing your lint, there is also the internal author
lint to generate Clippy code that detects the offending pattern. It does not
work for all the Rust syntax, but can give a good starting point.
The quickest way to use it, is the Rust playground:
play.rust-lang.org. Put the code you want to lint into the
editor and add the #[clippy::author]
attribute above the item. Then run Clippy
via Tools -> Clippy
and you should see the generated code in the output below.
Here is an example on the playground.
If the command was executed successfully, you can copy the code over to where you are implementing your lint.
To implement a lint, it's helpful to first understand the internal
representation that rustc uses. Clippy has the #[clippy::dump]
attribute that
prints the High-Level Intermediate Representation (HIR) of the item,
statement, or expression that the attribute is attached to. To attach the
attribute to expressions you often need to enable
#![feature(stmt_expr_attributes)]
.
Here you can find an example, just select Tools and run Clippy.
The final thing before submitting our PR is to add some documentation to our lint declaration.
Please document your lint with a doc comment akin to the following:
declare_clippy_lint! {
/// ### What it does
/// Checks for ... (describe what the lint matches).
///
/// ### Why is this bad?
/// Supply the reason for linting the code.
///
/// ### Example
///
/// ```rust,ignore
/// // A short example of code that triggers the lint
/// ```
///
/// Use instead:
/// ```rust,ignore
/// // A short example of improved code that doesn't trigger the lint
/// ```
#[clippy::version = "1.29.0"]
pub FOO_FUNCTIONS,
pedantic,
"function named `foo`, which is not a descriptive name"
}
If the lint is in the restriction
group because it lints things that are not
necessarily “bad” but are more of a style choice, then replace the
“Why is this bad?” section heading with “Why restrict this?”, to avoid writing
“Why is this bad? It isn't, but ...”.
Once your lint is merged, this documentation will show up in the lint list.
Rustfmt is a tool for formatting Rust code according to style guidelines. Your
code has to be formatted by rustfmt
before a PR can be merged. Clippy uses
nightly rustfmt
in the CI.
It can be installed via rustup
:
rustup component add rustfmt --toolchain=nightly
Use cargo dev fmt
to format the whole codebase. Make sure that rustfmt
is
installed for the nightly toolchain.
If you want to debug parts of your lint implementation, you can use the dbg!
macro anywhere in your code. Running the tests should then include the debug
output in the stdout
part.
There are several lints that deal with the same pattern but suggest different approaches. In other words, some lints may suggest modifications that go in the opposite direction to what some other lints already propose for the same code, creating conflicting diagnostics.
When you are creating a lint that ends up in this scenario, the following tips should be encouraged to guide classification:
- The only case where they should be in the same category is if that category is
restriction
. For example,semicolon_inside_block
andsemicolon_outside_block
. - For all the other cases, they should be in different categories with different levels of allowance. For example,
implicit_return
(restriction, allow) andneedless_return
(style, warn).
For lints that are in different categories, it is also recommended that at least one of them should be in the
restriction
category. The reason for this is that the restriction
group is the only group where we don't
recommend to enable the entire set, but cherry pick lints out of.
Before submitting your PR make sure you followed all the basic requirements:
- Followed lint naming conventions
- Added passing UI tests (including committed
.stderr
file) -
cargo test
passes locally - Executed
cargo dev update_lints
- Added lint documentation
- Run
cargo dev fmt
Clippy supports the configuration of lints values using a clippy.toml
file which is searched for in:
- The directory specified by the
CLIPPY_CONF_DIR
environment variable, or - The directory specified by the CARGO_MANIFEST_DIR environment variable, or
- The current directory.
Adding a configuration to a lint can be useful for thresholds or to constrain some behavior that can be seen as a false positive for some users. Adding a configuration is done in the following steps:
-
Adding a new configuration entry to
clippy_config::conf
like this:/// Lint: LINT_NAME. /// /// <The configuration field doc comment> (configuration_ident: Type = DefaultValue),
The doc comment is automatically added to the documentation of the listed lints. The default value will be formatted using the
Debug
implementation of the type. -
Adding the configuration value to the lint impl struct:
-
This first requires the definition of a lint impl struct. Lint impl structs are usually generated with the
declare_lint_pass!
macro. This struct needs to be defined manually to add some kind of metadata to it:// Generated struct definition declare_lint_pass!(StructName => [ LINT_NAME ]); // New manual definition struct pub struct StructName {} impl_lint_pass!(StructName => [ LINT_NAME ]);
-
Next add the configuration value and a corresponding creation method like this:
pub struct StructName { configuration_ident: Type, } // ... impl StructName { pub fn new(conf: &'static Conf) -> Self { Self { configuration_ident: conf.configuration_ident, } } }
-
-
Passing the configuration value to the lint impl struct:
First find the struct construction in the
clippy_lints
lib file. The configuration value is now cloned or copied into a local value that is then passed to the impl struct like this:// Default generated registration: store.register_*_pass(|| box module::StructName); // New registration with configuration value store.register_*_pass(move || box module::StructName::new(conf));
Congratulations the work is almost done. The configuration value can now be accessed in the linting code via
self.configuration_ident
. -
Adding tests:
- The default configured value can be tested like any normal lint in
tests/ui
. - The configuration itself will be tested separately in
tests/ui-toml
. Simply add a new subfolder with a fitting name. This folder contains aclippy.toml
file with the configuration value and a rust file that should be linted by Clippy. The test can otherwise be written as usual.
- The default configured value can be tested like any normal lint in
-
Update Lint Configuration
Run
cargo bless --test config-metadata
to generate documentation changes for the book.
Here are some pointers to things you are likely going to need for every lint:
- Clippy utils - Various helper functions. Maybe the function you need
is already in here (
is_type_diagnostic_item
,implements_trait
,snippet
, etc) - Clippy diagnostics
- Let chains
from_expansion
andin_external_macro
Span
Applicability
- Common tools for writing lints helps with common operations
- The rustc-dev-guide explains a lot of internal compiler concepts
- The nightly rustc docs which has been linked to throughout this guide
For EarlyLintPass
lints:
For LateLintPass
lints:
While most of Clippy's lint utils are documented, most of rustc's internals lack documentation currently. This is unfortunate, but in most cases you can probably get away with copying things from existing similar lints. If you are stuck, don't hesitate to ask on Zulip or in the issue/PR.