Accommodate pos-only arguments when checking self name (#2626)

crates/ruff/resources/test/fixtures/pep8_naming/N805.py

@@ -51,3 +51,11 @@ class MetaClass(ABCMeta):
 
 def func(x):
     return x
+
+
+class PosOnlyClass:
+    def good_method_pos_only(self, blah, /, something: str):
+        pass
+
+    def bad_method_pos_only(this, blah, /, self, something: str):
+        pass

crates/ruff/src/rules/pep8_naming/rules.rs

@@ -316,7 +316,7 @@ pub fn invalid_first_argument_name_for_method(
     ) {
         return None;
     }
-    let arg = args.args.first()?;
+    let arg = args.posonlyargs.first().or_else(|| args.args.first())?;
     if arg.node.arg == "self" {
         return None;
     }

crates/ruff/src/rules/pep8_naming/snapshots/ruff__rules__pep8_naming__tests__N805_N805.py.snap

@@ -1,5 +1,5 @@
 ---
-source: src/rules/pep8_naming/mod.rs
+source: crates/ruff/src/rules/pep8_naming/mod.rs
 expression: diagnostics
 ---
 - kind:
@@ -42,4 +42,14 @@ expression: diagnostics
     column: 17
   fix: ~
   parent: ~
+- kind:
+    InvalidFirstArgumentNameForMethod: ~
+  location:
+    row: 60
+    column: 28
+  end_location:
+    row: 60
+    column: 32
+  fix: ~
+  parent: ~