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Lint passes
Before working on the logic of a new lint, there is an important decision
that every Clippy developer must make: to use
EarlyLintPass or LateLintPass.
In short, the LateLintPass has access to type and symbol information while the
EarlyLintPass doesn't. If you don't need access to type information, use the
EarlyLintPass.
Let us expand on these two traits more below.
EarlyLintPass
If you examine the documentation on EarlyLintPass closely,
you'll see that every method defined for this trait utilizes a
EarlyContext. In EarlyContext's documentation, it states:
Context for lint checking of the AST, after expansion, before lowering to HIR.
Voilà. EarlyLintPass works only on the Abstract Syntax Tree (AST) level.
And AST is generated during the lexing and parsing phase
of code compilation. Therefore, it doesn't know what a symbol means or information about types, and it should
be our trait choice for a new lint if the lint only deals with syntax-related issues.
While linting speed has not been a concern for Clippy,
the EarlyLintPass is faster, and it should be your choice
if you know for sure a lint does not need type information.
As a reminder, run the following command to generate boilerplate for lints
that use EarlyLintPass:
$ cargo dev new_lint --name=<your_new_lint> --pass=early --category=<your_category_choice>
Example for EarlyLintPass
Take a look at the following code:
let x = OurUndefinedType;
x.non_existing_method();
From the AST perspective, both lines are "grammatically" correct.
The assignment uses a let and ends with a semicolon. The invocation
of a method looks fine, too. As programmers, we might raise a few
questions already, but the parser is okay with it. This is what we
mean when we say EarlyLintPass deals with only syntax on the AST level.
Alternatively, think of the foo_functions lint we mentioned in
define new lints chapter.
We want the foo_functions lint to detect functions with foo as their name.
Writing a lint that only checks for the name of a function means that we only
work with the AST and don't have to access the type system at all (the type system is where
LateLintPass comes into the picture).
LateLintPass
In contrast to EarlyLintPass, LateLintPass contains type information.
If you examine the documentation on LateLintPass closely,
you see that every method defined in this trait utilizes a
LateContext.
In LateContext's documentation we will find methods that
deal with type-checking, which do not exist in EarlyContext, such as:
Example for LateLintPass
Let us take a look with the following example:
let x = OurUndefinedType;
x.non_existing_method();
These two lines of code are syntactically correct code from the perspective of the AST. We have an assignment and invoke a method on the variable that is of a type. Grammatically, everything is in order for the parser.
However, going down a level and looking at the type information,
the compiler will notice that both OurUndefinedType and non_existing_method()
are undefined.
As Clippy developers, to access such type information, we must implement
LateLintPass on our lint.
When you browse through Clippy's lints, you will notice that almost every lint
is implemented in a LateLintPass, specifically because we often need to check
not only for syntactic issues but also type information.
Another limitation of the EarlyLintPass is that the nodes are only identified
by their position in the AST. This means that you can't just get an id and
request a certain node. For most lints that is fine, but we have some lints
that require the inspection of other nodes, which is easier at the HIR level.
In these cases, LateLintPass is the better choice.
As a reminder, run the following command to generate boilerplate for lints
that use LateLintPass:
$ cargo dev new_lint --name=<your_new_lint> --pass=late --category=<your_category_choice>