add tests for #94502

Author: lcnr

src/test/ui/nll/lint-no-err.rs

@@ -0,0 +1,23 @@
+// check-pass
+
+// mir borrowck previously incorrectly set `tainted_by_errors`
+// when buffering lints, which resulted in ICE later on,
+// see #94502.
+
+struct Repro;
+impl Repro {
+    fn get(&self) -> &i32 {
+        &3
+    }
+
+    fn insert(&mut self, _: i32) {}
+}
+
+fn main() {
+    let x = &0;
+    let mut conflict = Repro;
+    let prev = conflict.get();
+    conflict.insert(*prev + *x);
+    //~^ WARN cannot borrow `conflict` as mutable because it is also borrowed as immutable
+    //~| WARN this borrowing pattern was not meant to be accepted
+}

src/test/ui/nll/lint-no-err.stderr

@@ -0,0 +1,17 @@
+warning: cannot borrow `conflict` as mutable because it is also borrowed as immutable
+  --> $DIR/lint-no-err.rs:20:5
+   |
+LL |     let prev = conflict.get();
+   |                -------------- immutable borrow occurs here
+LL |     conflict.insert(*prev + *x);
+   |     ^^^^^^^^^^^^^^^^-----^^^^^^
+   |     |               |
+   |     |               immutable borrow later used here
+   |     mutable borrow occurs here
+   |
+   = note: `#[warn(mutable_borrow_reservation_conflict)]` on by default
+   = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
+   = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>
+
+warning: 1 warning emitted
+