Suppress unused_mut lint if mutation fails due to borrowck error

Remedying the borrowck error will likely result in the mut becoming
used.

Author: eggyal

compiler/rustc_borrowck/src/lib.rs

@@ -1207,6 +1207,17 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                 "access_place: suppressing error place_span=`{:?}` kind=`{:?}`",
                 place_span, kind
             );
+
+            // If the place is being mutated, then mark it as such anyway in order to suppress the
+            // `unused_mut` lint, which is likely incorrect once the access place error has been
+            // resolved.
+            if rw == ReadOrWrite::Write(WriteKind::Mutate)
+                && let Ok(root_place) =
+                    self.is_mutable(place_span.0.as_ref(), is_local_mutation_allowed)
+            {
+                self.add_used_mut(root_place, state);
+            }
+
             return;
         }
 

tests/ui/lint/unused/mut-used-despite-borrowck-error.rs

@@ -0,0 +1,19 @@
+//! Do not fire unused_mut lint when mutation of the bound variable fails due to a borrow-checking
+//! error.
+//!
+//! Regression test for https://github.com/rust-lang/rust/issues/152024
+//@ compile-flags: -W unused_mut
+
+struct Thing;
+impl Drop for Thing {
+    fn drop(&mut self) {}
+}
+
+fn main() {
+    let mut t;
+    let mut b = None;
+    loop {
+        t = Thing; //~ ERROR cannot assign to `t` because it is borrowed
+        b.insert(&t);
+    }
+}

tests/ui/lint/unused/mut-used-despite-borrowck-error.stderr

@@ -0,0 +1,13 @@
+error[E0506]: cannot assign to `t` because it is borrowed
+  --> $DIR/mut-used-despite-borrowck-error.rs:16:9
+   |
+LL |         t = Thing;
+   |         ^ `t` is assigned to here but it was already borrowed
+LL |         b.insert(&t);
+   |         -        -- `t` is borrowed here
+   |         |
+   |         borrow later used here
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0506`.