Add regression test for issue #41261

tests/ui/inference/ambiguous-cast-suggestion-issue-41261.rs

@@ -0,0 +1,19 @@
+//@ edition: 2021
+
+// Regression test for issue #41261.
+// The diagnostic should point at the ambiguous cast, not at the later method call.
+
+struct S {
+    v: Vec<(u32, Vec<u32>)>,
+}
+
+impl S {
+    pub fn remove(&mut self, i: u32) -> Option<std::vec::Drain<'_, u32>> {
+        self.v.get_mut(i as _).map(|&mut (_, ref mut v2)| {
+            //~^ ERROR type annotations needed
+            v2.drain(..)
+        })
+    }
+}
+
+fn main() {}

tests/ui/inference/ambiguous-cast-suggestion-issue-41261.stderr

@@ -0,0 +1,17 @@
+error[E0282]: type annotations needed for `&mut (_, _)`
+  --> $DIR/ambiguous-cast-suggestion-issue-41261.rs:12:37
+   |
+LL |         self.v.get_mut(i as _).map(|&mut (_, ref mut v2)| {
+   |                                     ^^^^^^^^^^^^^^^^^^^^
+LL |
+LL |             v2.drain(..)
+   |             -- type must be known at this point
+   |
+help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
+   |
+LL |         self.v.get_mut(i as _).map(|&mut (_, ref mut v2): &mut (_, _)| {
+   |                                                         +++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0282`.