[beta] backport fix for #94502

compiler/rustc_borrowck/src/diagnostics/outlives_suggestion.rs

@@ -256,6 +256,6 @@ impl OutlivesSuggestionBuilder {
         diag.sort_span = mir_span.shrink_to_hi();
 
         // Buffer the diagnostic
-        mbcx.buffer_error(diag);
+        mbcx.buffer_non_error_diag(diag);
     }
 }

compiler/rustc_borrowck/src/lib.rs

@@ -398,7 +398,7 @@ fn do_mir_borrowck<'a, 'tcx>(
                 diag.message = initial_diag.styled_message().clone();
                 diag.span = initial_diag.span.clone();
 
-                mbcx.buffer_error(diag);
+                mbcx.buffer_non_error_diag(diag);
             },
         );
         initial_diag.cancel();
@@ -2317,6 +2317,11 @@ mod error {
             t.buffer(&mut self.buffered);
         }
 
+        // For diagnostics we must not set `tainted_by_errors`.
+        pub fn buffer_non_error_diag(&mut self, t: DiagnosticBuilder<'_>) {
+            t.buffer(&mut self.buffered);
+        }
+
         pub fn set_tainted_by_errors(&mut self) {
             self.tainted_by_errors = Some(ErrorReported {});
         }
@@ -2327,6 +2332,10 @@ mod error {
             self.errors.buffer_error(t);
         }
 
+        pub fn buffer_non_error_diag(&mut self, t: DiagnosticBuilder<'_>) {
+            self.errors.buffer_non_error_diag(t);
+        }
+
         pub fn buffer_move_error(
             &mut self,
             move_out_indices: Vec<MoveOutIndex>,

compiler/rustc_borrowck/src/nll.rs

@@ -417,7 +417,7 @@ pub(super) fn dump_annotation<'a, 'tcx>(
         err.note(&format!("Inferred opaque type values:\n{:#?}", opaque_type_values));
     }
 
-    errors.buffer_error(err);
+    errors.buffer_non_error_diag(err);
 }
 
 fn for_each_region_constraint(

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

@@ -0,0 +1,28 @@
+// check-pass
+
+// mir borrowck previously incorrectly set `tainted_by_errors`
+// when buffering lints, which resulted in ICE later on,
+// see #94502.
+
+// Errors with `nll` which is already tested in enough other tests,
+// so we ignore it here.
+//
+// ignore-compare-mode-nll
+
+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:25:5
+   |
+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
+