Add regression test for issue 127545

Author: tesuji

tests/ui/mismatched_types/transforming-option-ref-issue-127545.rs

@@ -0,0 +1,6 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/127545>.
+#![crate_type = "lib"]
+
+pub fn foo(arg: Option<&Vec<i32>>) -> Option<&[i32]> {
+    arg //~ ERROR 5:5: 5:8: mismatched types [E0308]
+}

tests/ui/mismatched_types/transforming-option-ref-issue-127545.stderr

@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/transforming-option-ref-issue-127545.rs:5:5
+   |
+LL | pub fn foo(arg: Option<&Vec<i32>>) -> Option<&[i32]> {
+   |                                       -------------- expected `Option<&[i32]>` because of return type
+LL |     arg
+   |     ^^^ expected `Option<&[i32]>`, found `Option<&Vec<i32>>`
+   |
+   = note: expected enum `Option<&[i32]>`
+              found enum `Option<&Vec<i32>>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.