wrong_self_convention: Match SelfKind::No more restrictively

The `wrong_self_convention` lint uses a `SelfKind` type to decide
whether a method has the right kind of "self" for its name, or whether
the kind of "self" it has makes its name confusable for a method in
a common trait.  One possibility is `SelfKind::No`, which is supposed
to mean "No `self`".

Previously, SelfKind::No matched everything _except_ Self, including
references to Self.  This patch changes it to match Self, &Self, &mut
Self, Box<Self>, and so on.

For example, this kind of method was allowed before:

```
impl S {
    // Should trigger the lint, because
    // "methods called `is_*` usually take `self` by reference or no `self`"
    fn is_foo(&mut self) -> bool { todo!() }
}
```

But since SelfKind::No matched "&mut self", no lint was triggered
(see #8142).

With this patch, the code above now gives a lint as expected.

Fixes #8142

changelog: [`wrong_self_convention`] rejects `self` references in more cases

tests/ui/issue_4266.stderr

@@ -12,5 +12,14 @@ error: explicit lifetimes given in parameter types where they could be elided (o
 LL | async fn one_to_one<'a>(s: &'a str) -> &'a str {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: methods called `new` usually take no `self`
+  --> $DIR/issue_4266.rs:27:22
+   |
+LL |     pub async fn new(&mut self) -> Self {
+   |                      ^^^^^^^^^
+   |
+   = note: `-D clippy::wrong-self-convention` implied by `-D warnings`
+   = help: consider choosing a less ambiguous name
+
+error: aborting due to 3 previous errors
 

tests/ui/wrong_self_convention.rs

@@ -188,3 +188,24 @@ mod issue6727 {
         }
     }
 }
+
+pub mod issue8142 {
+    struct S;
+
+    impl S {
+        // Should lint: is_ methods should only take &self, or no self at all.
+        fn is_still_buggy(&mut self) -> bool {
+            false
+        }
+
+        // Should not lint: "no self at all" is allowed.
+        fn is_forty_two(x: u32) -> bool {
+            x == 42
+        }
+
+        // Should not lint: &self is allowed.
+        fn is_test_code(&self) -> bool {
+            true
+        }
+    }
+}

tests/ui/wrong_self_convention.stderr

@@ -191,5 +191,13 @@ LL |         fn to_u64(self) -> u64 {
    |
    = help: consider choosing a less ambiguous name
 
-error: aborting due to 24 previous errors
+error: methods called `is_*` usually take `self` by reference or no `self`
+  --> $DIR/wrong_self_convention.rs:197:27
+   |
+LL |         fn is_still_buggy(&mut self) -> bool {
+   |                           ^^^^^^^^^
+   |
+   = help: consider choosing a less ambiguous name
+
+error: aborting due to 25 previous errors