[pylint] Implement singledispatchmethod-function (PLE5120) (#10428)

## Summary

Implement `singledispatchmethod-function` from pylint, part of #970.

This is essentially a copy paste of #8934 for `@singledispatchmethod`
decorator.

## Test Plan

Text fixture added.

Author: augustelalande

crates/ruff_linter/resources/test/fixtures/pylint/singledispatchmethod_function.py

@@ -0,0 +1,23 @@
+from functools import singledispatchmethod
+
+
+@singledispatchmethod  # [singledispatchmethod-function]
+def convert_position(position):
+    pass
+
+
+class Board:
+
+    @singledispatchmethod  # Ok
+    @classmethod
+    def convert_position(cls, position):
+        pass
+
+    @singledispatchmethod  # Ok
+    def move(self, position):
+        pass
+
+    @singledispatchmethod  # [singledispatchmethod-function]
+    @staticmethod
+    def do(position):
+        pass

crates/ruff_linter/src/checkers/ast/analyze/deferred_scopes.rs

@@ -43,6 +43,7 @@ pub(crate) fn deferred_scopes(checker: &mut Checker) {
         Rule::UnusedStaticMethodArgument,
         Rule::UnusedVariable,
         Rule::SingledispatchMethod,
+        Rule::SingledispatchmethodFunction,
     ]) {
         return;
     }
@@ -419,6 +420,10 @@ pub(crate) fn deferred_scopes(checker: &mut Checker) {
                 pylint::rules::singledispatch_method(checker, scope, &mut diagnostics);
             }
 
+            if checker.enabled(Rule::SingledispatchmethodFunction) {
+                pylint::rules::singledispatchmethod_function(checker, scope, &mut diagnostics);
+            }
+
             if checker.any_enabled(&[
                 Rule::InvalidFirstArgumentNameForClassMethod,
                 Rule::InvalidFirstArgumentNameForMethod,

crates/ruff_linter/src/codes.rs

@@ -256,6 +256,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pylint, "E1310") => (RuleGroup::Stable, rules::pylint::rules::BadStrStripCall),
         (Pylint, "E1507") => (RuleGroup::Stable, rules::pylint::rules::InvalidEnvvarValue),
         (Pylint, "E1519") => (RuleGroup::Preview, rules::pylint::rules::SingledispatchMethod),
+        (Pylint, "E1520") => (RuleGroup::Preview, rules::pylint::rules::SingledispatchmethodFunction),
         (Pylint, "E1700") => (RuleGroup::Stable, rules::pylint::rules::YieldFromInAsyncFunction),
         (Pylint, "E2502") => (RuleGroup::Stable, rules::pylint::rules::BidirectionalUnicode),
         (Pylint, "E2510") => (RuleGroup::Stable, rules::pylint::rules::InvalidCharacterBackspace),

crates/ruff_linter/src/rules/pylint/mod.rs

@@ -21,6 +21,10 @@ mod tests {
     use crate::test::test_path;
 
     #[test_case(Rule::SingledispatchMethod, Path::new("singledispatch_method.py"))]
+    #[test_case(
+        Rule::SingledispatchmethodFunction,
+        Path::new("singledispatchmethod_function.py")
+    )]
     #[test_case(Rule::AssertOnStringLiteral, Path::new("assert_on_string_literal.py"))]
     #[test_case(Rule::AwaitOutsideAsync, Path::new("await_outside_async.py"))]
     #[test_case(Rule::BadOpenMode, Path::new("bad_open_mode.py"))]

crates/ruff_linter/src/rules/pylint/rules/mod.rs

@@ -57,6 +57,7 @@ pub(crate) use return_in_init::*;
 pub(crate) use self_assigning_variable::*;
 pub(crate) use single_string_slots::*;
 pub(crate) use singledispatch_method::*;
+pub(crate) use singledispatchmethod_function::*;
 pub(crate) use subprocess_popen_preexec_fn::*;
 pub(crate) use subprocess_run_without_check::*;
 pub(crate) use super_without_brackets::*;
@@ -147,6 +148,7 @@ mod return_in_init;
 mod self_assigning_variable;
 mod single_string_slots;
 mod singledispatch_method;
+mod singledispatchmethod_function;
 mod subprocess_popen_preexec_fn;
 mod subprocess_run_without_check;
 mod super_without_brackets;

crates/ruff_linter/src/rules/pylint/rules/singledispatchmethod_function.rs

@@ -0,0 +1,121 @@
+use ruff_diagnostics::{Diagnostic, Edit, Fix, FixAvailability, Violation};
+use ruff_macros::{derive_message_formats, violation};
+use ruff_python_ast as ast;
+use ruff_python_semantic::analyze::function_type;
+use ruff_python_semantic::Scope;
+use ruff_text_size::Ranged;
+
+use crate::checkers::ast::Checker;
+use crate::importer::ImportRequest;
+
+/// ## What it does
+/// Checks for `@singledispatchmethod` decorators on functions or static
+/// methods.
+///
+/// ## Why is this bad?
+/// The `@singledispatchmethod` decorator is intended for use with class and
+/// instance methods, not functions.
+///
+/// Instead, use the `@singledispatch` decorator.
+///
+/// ## Example
+/// ```python
+/// from functools import singledispatchmethod
+///
+///
+/// @singledispatchmethod
+/// def func(arg):
+///     ...
+/// ```
+///
+/// Use instead:
+/// ```python
+/// from functools import singledispatchmethod
+///
+///
+/// @singledispatch
+/// def func(arg):
+///     ...
+/// ```
+///
+/// ## Fix safety
+/// This rule's fix is marked as unsafe, as migrating from `@singledispatchmethod` to
+/// `@singledispatch` may change the behavior of the code.
+#[violation]
+pub struct SingledispatchmethodFunction;
+
+impl Violation for SingledispatchmethodFunction {
+    const FIX_AVAILABILITY: FixAvailability = FixAvailability::Sometimes;
+
+    #[derive_message_formats]
+    fn message(&self) -> String {
+        format!("`@singledispatchmethod` decorator should not be used on non-method functions")
+    }
+
+    fn fix_title(&self) -> Option<String> {
+        Some("Replace with `@singledispatch`".to_string())
+    }
+}
+
+/// E1520
+pub(crate) fn singledispatchmethod_function(
+    checker: &Checker,
+    scope: &Scope,
+    diagnostics: &mut Vec<Diagnostic>,
+) {
+    let Some(func) = scope.kind.as_function() else {
+        return;
+    };
+
+    let ast::StmtFunctionDef {
+        name,
+        decorator_list,
+        ..
+    } = func;
+
+    let Some(parent) = &checker.semantic().first_non_type_parent_scope(scope) else {
+        return;
+    };
+
+    let type_ = function_type::classify(
+        name,
+        decorator_list,
+        parent,
+        checker.semantic(),
+        &checker.settings.pep8_naming.classmethod_decorators,
+        &checker.settings.pep8_naming.staticmethod_decorators,
+    );
+    if !matches!(
+        type_,
+        function_type::FunctionType::Function | function_type::FunctionType::StaticMethod
+    ) {
+        return;
+    }
+
+    for decorator in decorator_list {
+        if checker
+            .semantic()
+            .resolve_qualified_name(&decorator.expression)
+            .is_some_and(|qualified_name| {
+                matches!(
+                    qualified_name.segments(),
+                    ["functools", "singledispatchmethod"]
+                )
+            })
+        {
+            let mut diagnostic = Diagnostic::new(SingledispatchmethodFunction, decorator.range());
+            diagnostic.try_set_fix(|| {
+                let (import_edit, binding) = checker.importer().get_or_import_symbol(
+                    &ImportRequest::import("functools", "singledispatch"),
+                    decorator.start(),
+                    checker.semantic(),
+                )?;
+                Ok(Fix::unsafe_edits(
+                    Edit::range_replacement(binding, decorator.expression.range()),
+                    [import_edit],
+                ))
+            });
+            diagnostics.push(diagnostic);
+        }
+    }
+}

crates/ruff_linter/src/rules/pylint/snapshots/ruff_linter__rules__pylint__tests__PLE1520_singledispatchmethod_function.py.snap

@@ -0,0 +1,49 @@
+---
+source: crates/ruff_linter/src/rules/pylint/mod.rs
+---
+singledispatchmethod_function.py:4:1: PLE1520 [*] `@singledispatchmethod` decorator should not be used on non-method functions
+  |
+4 | @singledispatchmethod  # [singledispatchmethod-function]
+  | ^^^^^^^^^^^^^^^^^^^^^ PLE1520
+5 | def convert_position(position):
+6 |     pass
+  |
+  = help: Replace with `@singledispatch`
+
+ℹ Unsafe fix
+1   |-from functools import singledispatchmethod
+  1 |+from functools import singledispatchmethod, singledispatch
+2 2 | 
+3 3 | 
+4   |-@singledispatchmethod  # [singledispatchmethod-function]
+  4 |+@singledispatch  # [singledispatchmethod-function]
+5 5 | def convert_position(position):
+6 6 |     pass
+7 7 | 
+
+singledispatchmethod_function.py:20:5: PLE1520 [*] `@singledispatchmethod` decorator should not be used on non-method functions
+   |
+18 |         pass
+19 | 
+20 |     @singledispatchmethod  # [singledispatchmethod-function]
+   |     ^^^^^^^^^^^^^^^^^^^^^ PLE1520
+21 |     @staticmethod
+22 |     def do(position):
+   |
+   = help: Replace with `@singledispatch`
+
+ℹ Unsafe fix
+1     |-from functools import singledispatchmethod
+   1  |+from functools import singledispatchmethod, singledispatch
+2  2  | 
+3  3  | 
+4  4  | @singledispatchmethod  # [singledispatchmethod-function]
+--------------------------------------------------------------------------------
+17 17 |     def move(self, position):
+18 18 |         pass
+19 19 | 
+20    |-    @singledispatchmethod  # [singledispatchmethod-function]
+   20 |+    @singledispatch  # [singledispatchmethod-function]
+21 21 |     @staticmethod
+22 22 |     def do(position):
+23 23 |         pass