Implement B021 (#719)

src/flake8_bugbear/plugins/f_string_docstring.rs

@@ -0,0 +1,19 @@
+use rustpython_ast::{ExprKind, Stmt, StmtKind};
+
+use crate::ast::types::Range;
+use crate::check_ast::Checker;
+use crate::checks::{Check, CheckKind};
+
+/// B021
+pub fn f_string_docstring(checker: &mut Checker, body: &[Stmt]) {
+    if let Some(stmt) = body.first() {
+        if let StmtKind::Expr { value } = &stmt.node {
+            if let ExprKind::JoinedStr { .. } = value.node {
+                checker.add_check(Check::new(
+                    CheckKind::FStringDocstring,
+                    Range::from_located(stmt),
+                ));
+            }
+        }
+    }
+}