and few more

maybe bug here?

warning: this argument is a mutable reference, but not used mutably
    --> compiler\rustc_borrowck\src\diagnostics\conflict_errors.rs:3857:35
     |
3857 |     pub(crate) fn emit(&self, cx: &mut MirBorrowckCtxt<'_, 'tcx>, diag: &mut Diag<'_>) -> String {
     |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&MirBorrowckCtxt<'_, 'tcx>`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

warning: this argument is a mutable reference, but not used mutably
   --> compiler\rustc_borrowck\src\type_check\liveness\trace.rs:601:17
    |
601 |         typeck: &mut TypeChecker<'_, 'tcx>,
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&TypeChecker<'_, 'tcx>`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -337,7 +337,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     fn suggest_ref_or_clone(
-        &mut self,
+        &self,
         mpi: MovePathIndex,
         move_span: Span,
         err: &mut Diag<'tcx>,
@@ -1125,7 +1125,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     pub(crate) fn report_use_while_mutably_borrowed(
-        &mut self,
+        &self,
         location: Location,
         (place, _span): (Place<'tcx>, Span),
         borrow: &BorrowData<'tcx>,
@@ -1174,7 +1174,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     pub(crate) fn report_conflicting_borrow(
-        &mut self,
+        &self,
         location: Location,
         (place, span): (Place<'tcx>, Span),
         gen_borrow_kind: BorrowKind,
@@ -2463,7 +2463,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     fn report_local_value_does_not_live_long_enough(
-        &mut self,
+        &self,
         location: Location,
         name: &str,
         borrow: &BorrowData<'tcx>,
@@ -2642,7 +2642,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     fn report_thread_local_value_does_not_live_long_enough(
-        &mut self,
+        &self,
         drop_span: Span,
         borrow_span: Span,
     ) -> Diag<'tcx> {
@@ -2663,7 +2663,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     #[instrument(level = "debug", skip(self))]
     fn report_temporary_value_does_not_live_long_enough(
-        &mut self,
+        &self,
         location: Location,
         borrow: &BorrowData<'tcx>,
         drop_span: Span,
@@ -2921,7 +2921,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     #[instrument(level = "debug", skip(self))]
     fn report_escaping_closure_capture(
-        &mut self,
+        &self,
         use_span: UseSpans<'tcx>,
         var_span: Span,
         fr_name: &RegionName,
@@ -3031,7 +3031,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     fn report_escaping_data(
-        &mut self,
+        &self,
         borrow_span: Span,
         name: &Option<String>,
         upvar_span: Span,
@@ -3065,7 +3065,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     fn get_moved_indexes(
-        &mut self,
+        &self,
         location: Location,
         mpi: MovePathIndex,
     ) -> (Vec<MoveSite>, Vec<Location>) {
@@ -3854,7 +3854,7 @@ enum AnnotatedBorrowFnSignature<'tcx> {
 impl<'tcx> AnnotatedBorrowFnSignature<'tcx> {
     /// Annotate the provided diagnostic with information about borrow from the fn signature that
     /// helps explain.
-    pub(crate) fn emit(&self, cx: &mut MirBorrowckCtxt<'_, 'tcx>, diag: &mut Diag<'_>) -> String {
+    pub(crate) fn emit(&self, cx: &MirBorrowckCtxt<'_, 'tcx>, diag: &mut Diag<'_>) -> String {
         match self {
             &AnnotatedBorrowFnSignature::Closure { argument_ty, argument_span } => {
                 diag.span_label(

compiler/rustc_borrowck/src/type_check/liveness/polonius.rs

@@ -87,6 +87,8 @@ pub(super) fn populate_access_facts<'a, 'tcx>(
     body: &Body<'tcx>,
     location_table: &LocationTable,
     move_data: &MoveData<'tcx>,
+    //FIXME: this is not mutated, but expected to be modified as
+    // out param, bug?
     dropped_at: &mut Vec<(Local, Location)>,
 ) {
     debug!("populate_access_facts()");

compiler/rustc_borrowck/src/type_check/liveness/trace.rs

@@ -200,7 +200,7 @@ impl<'me, 'typeck, 'flow, 'tcx> LivenessResults<'me, 'typeck, 'flow, 'tcx> {
         for local in boring_locals {
             let local_ty = self.cx.body.local_decls[local].ty;
             let drop_data = self.cx.drop_data.entry(local_ty).or_insert_with({
-                let typeck = &mut self.cx.typeck;
+                let typeck = &self.cx.typeck;
                 move || LivenessContext::compute_drop_data(typeck, local_ty)
             });
 
@@ -542,7 +542,7 @@ impl<'tcx> LivenessContext<'_, '_, '_, 'tcx> {
         );
 
         let drop_data = self.drop_data.entry(dropped_ty).or_insert_with({
-            let typeck = &mut self.typeck;
+            let typeck = &self.typeck;
             move || Self::compute_drop_data(typeck, dropped_ty)
         });
 
@@ -597,10 +597,7 @@ impl<'tcx> LivenessContext<'_, '_, '_, 'tcx> {
         });
     }
 
-    fn compute_drop_data(
+    fn compute_drop_data(typeck: &TypeChecker<'_, 'tcx>, dropped_ty: Ty<'tcx>) -> DropData<'tcx> {
-        typeck: &mut TypeChecker<'_, 'tcx>,
-        dropped_ty: Ty<'tcx>,
-    ) -> DropData<'tcx> {
         debug!("compute_drop_data(dropped_ty={:?})", dropped_ty,);
 
         match typeck