Ignore generators in flake8-return rules (#2126)

We could do a better job of handling them, but they cause too many false-positives right now.

Closes #2119.

resources/test/fixtures/flake8_return/RET503.py

@@ -114,3 +114,12 @@ def bar3(x, y, z):
         else:
             return z
         return None
+
+
+def prompts(self, foo):
+    if not foo:
+        return []
+
+    for x in foo:
+        yield x
+        yield x + 1

src/rules/flake8_return/rules.rs

@@ -325,6 +325,11 @@ pub fn function(checker: &mut Checker, body: &[Stmt]) {
         visitor.stack
     };
 
+    // Avoid false positives for generators.
+    if !stack.yields.is_empty() {
+        return;
+    }
+
     if checker.settings.rules.enabled(&Rule::SuperfluousElseReturn)
         || checker.settings.rules.enabled(&Rule::SuperfluousElseRaise)
         || checker

src/rules/flake8_return/visitor.rs

@@ -7,6 +7,7 @@ use crate::ast::visitor::Visitor;
 #[derive(Default)]
 pub struct Stack<'a> {
     pub returns: Vec<(&'a Stmt, Option<&'a Expr>)>,
+    pub yields: Vec<&'a Expr>,
     pub ifs: Vec<&'a Stmt>,
     pub elifs: Vec<&'a Stmt>,
     pub refs: FxHashMap<&'a str, Vec<Location>>,
@@ -116,7 +117,6 @@ impl<'a> Visitor<'a> for ReturnVisitor<'a> {
                     .push((stmt.location, stmt.end_location.unwrap()));
                 visitor::walk_stmt(self, stmt);
             }
-
             _ => {
                 visitor::walk_stmt(self, stmt);
             }
@@ -143,8 +143,8 @@ impl<'a> Visitor<'a> for ReturnVisitor<'a> {
                     .or_insert_with(Vec::new)
                     .push(expr.location);
             }
-            ExprKind::JoinedStr { .. } => {
-                visitor::walk_expr(self, expr);
+            ExprKind::YieldFrom { .. } | ExprKind::Yield { .. } => {
+                self.stack.yields.push(expr);
             }
             _ => visitor::walk_expr(self, expr),
         }