BorrowckDiags tweaks.

- Store a mut ref to a `BorrowckDiags` in `MirBorrowckCtxt` instead of
  owning it, to save having to pass ownership in and out of
  `promoted_mbcx`.
- Use `buffer_error` in a couple of suitable places.

Author: nnethercote

compiler/rustc_borrowck/src/lib.rs

@@ -279,11 +279,10 @@ fn do_mir_borrowck<'tcx>(
             &[],                                                   // upvars
             IndexVec::from_elem(None, &promoted_body.local_decls), // local_names
             None,                                                  // polonius_output
-            diags,
+            &mut diags,
         );
         MoveVisitor { ctxt: &mut promoted_mbcx }.visit_body(promoted_body);
         promoted_mbcx.report_move_errors();
-        diags = promoted_mbcx.diags;
 
         struct MoveVisitor<'a, 'b, 'infcx, 'tcx> {
             ctxt: &'a mut MirBorrowckCtxt<'b, 'infcx, 'tcx>,
@@ -311,7 +310,7 @@ fn do_mir_borrowck<'tcx>(
         tcx.closure_captures(def), // upvars
         local_names,
         polonius_output,
-        diags,
+        &mut diags,
     );
 
     // Compute and report region errors, if any.
@@ -562,7 +561,7 @@ struct MirBorrowckCtxt<'a, 'infcx, 'tcx> {
     /// Results of Polonius analysis.
     polonius_output: Option<Box<PoloniusOutput>>,
 
-    diags: diags::BorrowckDiags<'infcx, 'tcx>,
+    diags: &'a mut diags::BorrowckDiags<'infcx, 'tcx>,
     move_errors: Vec<MoveError<'tcx>>,
 }
 
@@ -580,7 +579,7 @@ impl<'a, 'infcx, 'tcx> MirBorrowckCtxt<'a, 'infcx, 'tcx> {
         upvars: &'tcx [&'tcx ty::CapturedPlace<'tcx>],
         local_names: IndexVec<Local, Option<Symbol>>,
         polonius_output: Option<Box<PoloniusOutput>>,
-        diags: diags::BorrowckDiags<'infcx, 'tcx>,
+        diags: &'a mut diags::BorrowckDiags<'infcx, 'tcx>,
     ) -> Self {
         MirBorrowckCtxt {
             infcx,
@@ -2524,15 +2523,15 @@ mod diags {
             // Buffer any move errors that we collected and de-duplicated.
             for (_, (_, diag)) in std::mem::take(&mut self.diags.buffered_move_errors) {
                 // We have already set tainted for this error, so just buffer it.
-                self.diags.buffered_diags.push(BufferedDiag::Error(diag));
+                self.diags.buffer_error(diag);
             }
             for (_, (mut diag, count)) in std::mem::take(&mut self.diags.buffered_mut_errors) {
                 if count > 10 {
                     #[allow(rustc::diagnostic_outside_of_impl)]
                     #[allow(rustc::untranslatable_diagnostic)]
                     diag.note(format!("...and {} other attempted mutable borrows", count - 10));
                 }
-                self.diags.buffered_diags.push(BufferedDiag::Error(diag));
+                self.diags.buffer_error(diag);
             }
 
             if !self.diags.buffered_diags.is_empty() {