Add regression test

Author: joculatrix

tests/ui/borrowck/do-not-suggest-duplicate-semicolons-issue-133941.rs

@@ -0,0 +1,31 @@
+// Regression test for #133941: Don't suggest adding a semicolon during borrowck
+// errors when one already exists.
+
+use std::marker::PhantomData;
+
+struct Bar<'a>(PhantomData<&'a mut i32>);
+
+impl<'a> Drop for Bar<'a> {
+    fn drop(&mut self) {}
+}
+
+struct Foo();
+
+impl Foo {
+    fn f(&mut self) -> Option<Bar<'_>> {
+        None
+    }
+
+    fn g(&mut self) {}
+}
+
+fn main() {
+    let mut foo = Foo();
+    while let Some(_) = foo.f() {
+        //~^ NOTE first mutable borrow occurs here
+        //~| a temporary with access to the first borrow is created here ...
+        foo.g(); //~ ERROR cannot borrow `foo` as mutable more than once at a time
+        //~^ second mutable borrow occurs here
+    };
+    //~^ ... and the first borrow might be used here, when that temporary is dropped and runs the destructor for type `Option<Bar<'_>>`
+}

tests/ui/borrowck/do-not-suggest-duplicate-semicolons-issue-133941.stderr

@@ -0,0 +1,18 @@
+error[E0499]: cannot borrow `foo` as mutable more than once at a time
+  --> $DIR/do-not-suggest-duplicate-semicolons-issue-133941.rs:27:9
+   |
+LL |     while let Some(_) = foo.f() {
+   |                         -------
+   |                         |
+   |                         first mutable borrow occurs here
+   |                         a temporary with access to the first borrow is created here ...
+...
+LL |         foo.g();
+   |         ^^^ second mutable borrow occurs here
+LL |
+LL |     };
+   |     - ... and the first borrow might be used here, when that temporary is dropped and runs the destructor for type `Option<Bar<'_>>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0499`.