ignore boring locals when explaining borrow due to drop

Polonius liveness has to contain boring locals, and we ignore them in
diagnostics to match NLL diagnostics, since they doesn't contain boring locals.

We ignored these when explaining why a loan contained a point due to
a use of a live var, but not when it contained a point due to a drop of
a live var.

Author: lqd

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -687,7 +687,7 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
                 }
             }
 
-            Some(Cause::DropVar(local, location)) => {
+            Some(Cause::DropVar(local, location)) if !is_local_boring(local) => {
                 let mut should_note_order = false;
                 if self.local_name(local).is_some()
                     && let Some((WriteKind::StorageDeadOrDrop, place)) = kind_place
@@ -705,7 +705,7 @@ impl<'tcx> MirBorrowckCtxt<'_, '_, 'tcx> {
                 }
             }
 
-            Some(Cause::LiveVar(..)) | None => {
+            Some(Cause::LiveVar(..) | Cause::DropVar(..)) | None => {
                 // Here, under NLL: no cause was found. Under polonius: no cause was found, or a
                 // boring local was found, which we ignore like NLLs do to match its diagnostics.
                 if let Some(region) = self.to_error_region_vid(borrow_region_vid) {

tests/ui/nll/polonius/lending-iterator-sanity-checks.polonius.stderr

@@ -1,13 +1,15 @@
 error[E0499]: cannot borrow `*t` as mutable more than once at a time
   --> $DIR/lending-iterator-sanity-checks.rs:19:19
    |
+LL | fn use_live<T: LendingIterator>(t: &mut T) -> Option<(T::Item<'_>, T::Item<'_>)> {
+   |                                    - let's call the lifetime of this reference `'1`
 LL |     let Some(i) = t.next() else { return None };
    |                   - first mutable borrow occurs here
 LL |     let Some(j) = t.next() else { return None };
    |                   ^ second mutable borrow occurs here
 ...
-LL | }
-   | - first borrow might be used here, when `i` is dropped and runs the destructor for type `<T as LendingIterator>::Item<'_>`
+LL |     Some((i, j))
+   |     ------------ returning this value requires that `*t` is borrowed for `'1`
 
 error[E0499]: cannot borrow `*t` as mutable more than once at a time
   --> $DIR/lending-iterator-sanity-checks.rs:31:13