Avoid nested-if violations when outer-if has else clause (#2095)

It looks like we need `do`-`while`-like semantics here with an additional outer check.

Closes #2094.

resources/test/fixtures/flake8_simplify/SIM102.py

@@ -92,3 +92,40 @@ if node.module:
         "multiprocessing."
     ):
         print("Bad module!")
+
+
+# OK
+if a:
+    if b:
+        print("foo")
+else:
+    print("bar")
+
+# OK
+if a:
+    if b:
+        if c:
+            print("foo")
+        else:
+            print("bar")
+else:
+    print("bar")
+
+# OK
+if a:
+    # SIM 102
+    if b:
+        if c:
+            print("foo")
+else:
+    print("bar")
+
+
+# OK
+if a:
+    if b:
+        if c:
+            print("foo")
+        print("baz")
+else:
+    print("bar")

src/checkers/ast.rs

@@ -1386,6 +1386,7 @@ where
                         stmt,
                         test,
                         body,
+                        orelse,
                         self.current_stmt_parent().map(Into::into),
                     );
                 }

src/rules/flake8_simplify/rules/ast_if.rs

@@ -50,7 +50,7 @@ fn is_main_check(expr: &Expr) -> bool {
 /// ```
 fn find_last_nested_if(body: &[Stmt]) -> Option<(&Expr, &Stmt)> {
     let [Stmt { node: StmtKind::If { test, body: inner_body, orelse }, ..}] = body else { return None };
-    if !(orelse.is_empty() && body.len() == 1) {
+    if !orelse.is_empty() {
         return None;
     }
     find_last_nested_if(inner_body).or_else(|| {
@@ -67,12 +67,10 @@ pub fn nested_if_statements(
     stmt: &Stmt,
     test: &Expr,
     body: &[Stmt],
+    orelse: &[Stmt],
     parent: Option<&Stmt>,
 ) {
-    if is_main_check(test) {
-        return;
-    }
-
+    // If the parent could contain a nested if-statement, abort.
     if let Some(parent) = parent {
         if let StmtKind::If { body, orelse, .. } = &parent.node {
             if orelse.is_empty() && body.len() == 1 {
@@ -81,6 +79,16 @@ pub fn nested_if_statements(
         }
     }
 
+    // If this if-statement has an else clause, or more than one child, abort.
+    if !(orelse.is_empty() && body.len() == 1) {
+        return;
+    }
+
+    if is_main_check(test) {
+        return;
+    }
+
+    // Find the deepest nested if-statement, to inform the range.
     let Some((test, first_stmt)) = find_last_nested_if(body) else {
         return;
     };

src/rules/flake8_simplify/snapshots/ruff__rules__flake8_simplify__tests__SIM102_SIM102.py.snap

@@ -148,4 +148,21 @@ expression: diagnostics
       row: 95
       column: 0
   parent: ~
+- kind:
+    NestedIfStatements: ~
+  location:
+    row: 117
+    column: 4
+  end_location:
+    row: 118
+    column: 13
+  fix:
+    content: "    if b and c:\n        print(\"foo\")\n"
+    location:
+      row: 117
+      column: 0
+    end_location:
+      row: 120
+      column: 0
+  parent: ~