Stop using DiagnosticBuilder::buffer in Checker.

This requires cancelling the "secondary" errors when they're not
emitted, to prevent panics due to unconsumed `DiagnosticBuilder`s.

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -1,6 +1,6 @@
 //! The `Visitor` responsible for actually checking a `mir::Body` for invalid operations.
 
-use rustc_errors::{Diagnostic, ErrorGuaranteed};
+use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_index::bit_set::BitSet;
@@ -214,7 +214,7 @@ pub struct Checker<'mir, 'tcx> {
     local_has_storage_dead: Option<BitSet<Local>>,
 
     error_emitted: Option<ErrorGuaranteed>,
-    secondary_errors: Vec<Diagnostic>,
+    secondary_errors: Vec<DiagnosticBuilder<'tcx>>,
 }
 
 impl<'mir, 'tcx> Deref for Checker<'mir, 'tcx> {
@@ -272,14 +272,17 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
         }
 
         // If we got through const-checking without emitting any "primary" errors, emit any
-        // "secondary" errors if they occurred.
+        // "secondary" errors if they occurred. Otherwise, cancel the "secondary" errors.
         let secondary_errors = mem::take(&mut self.secondary_errors);
         if self.error_emitted.is_none() {
             for error in secondary_errors {
-                self.tcx.dcx().emit_diagnostic(error);
+                error.emit();
             }
         } else {
             assert!(self.tcx.dcx().has_errors().is_some());
+            for error in secondary_errors {
+                error.cancel();
+            }
         }
     }
 
@@ -347,7 +350,7 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
                 self.error_emitted = Some(reported);
             }
 
-            ops::DiagnosticImportance::Secondary => err.buffer(&mut self.secondary_errors),
+            ops::DiagnosticImportance::Secondary => self.secondary_errors.push(err),
         }
     }