Add Pyflakes docs (#4588)

crates/ruff/src/rules/pyflakes/rules/imports.rs

@@ -15,6 +15,44 @@ pub(crate) enum UnusedImportContext {
     Init,
 }
 
+/// ## What it does
+/// Checks for unused imports.
+///
+/// ## Why is this bad?
+/// Unused imports add a performance overhead at runtime, and risk creating
+/// import cycles. They also increase the cognitive load of reading the code.
+///
+/// If an import statement is used to check for the availability or existence
+/// of a module, consider using `importlib.util.find_spec` instead.
+///
+/// ## Example
+/// ```python
+/// import numpy as np  # unused import
+///
+///
+/// def area(radius):
+///     return 3.14 * radius**2
+/// ```
+///
+/// Use instead:
+/// ```python
+/// def area(radius):
+///     return 3.14 * radius**2
+/// ```
+///
+/// To check the availability of a module, use `importlib.util.find_spec`:
+/// ```python
+/// from importlib.util import find_spec
+///
+/// if find_spec("numpy") is not None:
+///     print("numpy is installed")
+/// else:
+///     print("numpy is not installed")
+/// ```
+///
+/// ## References
+/// - [Python documentation](https://docs.python.org/3/reference/simple_stmts.html#the-import-statement)
+/// - [Python documentation](https://docs.python.org/3/library/importlib.html#importlib.util.find_spec)
 #[violation]
 pub struct UnusedImport {
     pub(crate) name: String,
@@ -54,6 +92,34 @@ impl Violation for UnusedImport {
         })
     }
 }
+
+/// ## What it does
+/// Checks for import bindings that are shadowed by loop variables.
+///
+/// ## Why is this bad?
+/// Shadowing an import with loop variables makes the code harder to read and
+/// reason about, as the identify of the imported binding is no longer clear.
+/// It's also often indicative of a mistake, as it's unlikely that the loop
+/// variable is intended to be used as the imported binding.
+///
+/// Consider using a different name for the loop variable.
+///
+/// ## Example
+/// ```python
+/// from os import path
+///
+/// for path in files:
+///     print(path)
+/// ```
+///
+/// Use instead:
+/// ```python
+/// from os import path
+///
+///
+/// for filename in files:
+///     print(filename)
+/// ```
 #[violation]
 pub struct ImportShadowedByLoopVar {
     pub(crate) name: String,
@@ -68,6 +134,34 @@ impl Violation for ImportShadowedByLoopVar {
     }
 }
 
+/// ## What it does
+/// Checks for the use of wildcard imports.
+///
+/// ## Why is this bad?
+/// Wildcard imports (e.g., `from module import *`) make it hard to determine
+/// which symbols are available in the current namespace, and from which module
+/// they were imported.
+///
+/// ## Example
+/// ```python
+/// from math import *
+///
+///
+/// def area(radius):
+///     return pi * radius**2
+/// ```
+///
+/// Use instead:
+/// ```python
+/// from math import pi
+///
+///
+/// def area(radius):
+///     return pi * radius**2
+/// ```
+///
+/// ## References
+/// - [PEP 8](https://peps.python.org/pep-0008/#imports)
 #[violation]
 pub struct UndefinedLocalWithImportStar {
     pub(crate) name: String,
@@ -81,6 +175,31 @@ impl Violation for UndefinedLocalWithImportStar {
     }
 }
 
+/// ## What it does
+/// Checks for `__future__` imports that are not located at the beginning of a
+/// file.
+///
+/// ## Why is this bad?
+/// Imports from `__future__` must be placed the beginning of the file, before any
+/// other statements (apart from docstrings). The use of `__future__` imports
+/// elsewhere is invalid and will result in a `SyntaxError`.
+///
+/// ## Example
+/// ```python
+/// from pathlib import Path
+///
+/// from __future__ import annotations
+/// ```
+///
+/// Use instead:
+/// ```python
+/// from __future__ import annotations
+///
+/// from pathlib import Path
+/// ```
+///
+/// ## References
+/// - [PEP 8](https://peps.python.org/pep-0008/#module-level-dunder-names)
 #[violation]
 pub struct LateFutureImport;
 
@@ -91,6 +210,43 @@ impl Violation for LateFutureImport {
     }
 }
 
+/// ## What it does
+/// Checks for names that might be undefined, but may also be defined in a
+/// wildcard import.
+///
+/// ## Why is this bad?
+/// Wildcard imports (e.g., `from module import *`) make it hard to determine
+/// which symbols are available in the current namespace. If a module contains
+/// a wildcard import, and a name in the current namespace has not been
+/// explicitly defined or imported, then it's unclear whether the name is
+/// undefined or was imported by the wildcard import.
+///
+/// If the name _is_ defined in via a wildcard import, that member should be
+/// imported explicitly to avoid confusion.
+///
+/// If the name is _not_ defined in a wildcard import, it should be defined or
+/// imported.
+///
+/// ## Example
+/// ```python
+/// from math import *
+///
+///
+/// def area(radius):
+///     return pi * radius**2
+/// ```
+///
+/// Use instead:
+/// ```python
+/// from math import pi
+///
+///
+/// def area(radius):
+///     return pi * radius**2
+/// ```
+///
+/// ## References
+/// - [PEP 8](https://peps.python.org/pep-0008/#imports)
 #[violation]
 pub struct UndefinedLocalWithImportStarUsage {
     pub(crate) name: String,
@@ -109,6 +265,33 @@ impl Violation for UndefinedLocalWithImportStarUsage {
     }
 }
 
+/// ## What it does
+/// Check for the use of wildcard imports outside of the module namespace.
+///
+/// ## Why is this bad?
+/// The use of wildcard imports outside of the module namespace (e.g., within
+/// functions) can lead to confusion, as the import can shadow local variables.
+///
+/// Though wildcard imports are discouraged, when necessary, they should be placed
+/// in the module namespace (i.e., at the top-level of a module).
+///
+/// ## Example
+/// ```python
+/// def foo():
+///     from math import *
+/// ```
+///
+/// Use instead:
+/// ```python
+/// from math import *
+///
+///
+/// def foo():
+///     ...
+/// ```
+///
+/// ## References
+/// - [PEP 8](https://peps.python.org/pep-0008/#imports)
 #[violation]
 pub struct UndefinedLocalWithNestedImportStarUsage {
     pub(crate) name: String,
@@ -122,6 +305,16 @@ impl Violation for UndefinedLocalWithNestedImportStarUsage {
     }
 }
 
+/// ## What it does
+/// Checks for `__future__` imports that are not defined in the current Python
+/// version.
+///
+/// ## Why is this bad?
+/// Importing undefined or unsupported members from the `__future__` module is
+/// a `SyntaxError`.
+///
+/// ## References
+/// - [Python documentation](https://docs.python.org/3/library/__future__.html)
 #[violation]
 pub struct FutureFeatureNotDefined {
     name: String,