Address review comments.

compiler/rustc_borrowck/src/diagnostics/region_name.rs

@@ -626,8 +626,8 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
                     | GenericArgKind::Const(_),
                     _,
                 ) => {
-                    // HIR lowering sometimes doesn't catch this in erroneous
-                    // programs, so we need to use span_delayed_bug here. See #82126.
+                    // This was previously a `span_delayed_bug` and could be
+                    // reached by the test for #82126, but no longer.
                     self.dcx().span_bug(
                         hir_arg.span(),
                         format!("unmatched arg and hir arg: found {kind:?} vs {hir_arg:?}"),

compiler/rustc_borrowck/src/type_check/free_region_relations.rs

@@ -316,7 +316,11 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
                     .and(type_op::normalize::Normalize::new(ty))
                     .fully_perform(self.infcx, span)
                 else {
-                    tcx.dcx().span_bug(span, format!("failed to normalize {ty:?}"));
+                    // Note: this path is currently not reached in any test, so
+                    // any example that triggers this would be worth minimizing
+                    // and converting into a test.
+                    tcx.dcx().span_delayed_bug(span, format!("failed to normalize {ty:?}"));
+                    continue;
                 };
                 constraints.extend(c);
 

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -622,7 +622,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                 );
                 // If this was a hard error, don't bother continuing evaluation.
                 if is_error {
-                    let guard: rustc_errors::ErrorGuaranteed = ecx
+                    let guard = ecx
                         .tcx
                         .dcx()
                         .span_delayed_bug(span, "The deny lint should have already errored");

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -734,6 +734,10 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
                 remapped_types.insert(def_id, ty::EarlyBinder::bind(ty));
             }
             Err(err) => {
+                // This code path is not reached in any tests, but may be
+                // reachable. If this is triggered, it should be converted to
+                // `span_delayed_bug` and the triggering case turned into a
+                // test.
                 tcx.dcx()
                     .span_bug(return_span, format!("could not fully resolve: {ty} => {err:?}"));
             }
@@ -914,7 +918,13 @@ impl<'tcx> ty::FallibleTypeFolder<TyCtxt<'tcx>> for RemapHiddenTyRegions<'tcx> {
                         .with_note(format!("hidden type inferred to be `{}`", self.ty))
                         .emit()
                 }
-                _ => self.tcx.dcx().bug("should've been able to remap region"),
+                _ => {
+                    // This code path is not reached in any tests, but may be
+                    // reachable. If this is triggered, it should be converted
+                    // to `delayed_bug` and the triggering case turned into a
+                    // test.
+                    self.tcx.dcx().bug("should've been able to remap region");
+                }
             };
             return Err(guar);
         };
@@ -1273,6 +1283,8 @@ fn compare_number_of_generics<'tcx>(
     // inheriting the generics from will also have mismatched arguments, and
     // we'll report an error for that instead. Delay a bug for safety, though.
     if trait_.is_impl_trait_in_trait() {
+        // FIXME: no tests trigger this. If you find example code that does
+        // trigger this, please add it to the test suite.
         tcx.dcx()
             .bug("errors comparing numbers of generics of trait/impl functions were not emitted");
     }

compiler/rustc_hir_analysis/src/check/compare_impl_item/refine.rs

@@ -154,6 +154,9 @@ pub(super) fn check_refining_return_position_impl_trait_in_trait<'tcx>(
         trait_m_sig.inputs_and_output,
     ));
     if !ocx.select_all_or_error().is_empty() {
+        // This code path is not reached in any tests, but may be reachable. If
+        // this is triggered, it should be converted to `delayed_bug` and the
+        // triggering case turned into a test.
         tcx.dcx().bug("encountered errors when checking RPITIT refinement (selection)");
     }
     let outlives_env = OutlivesEnvironment::with_bounds(
@@ -162,10 +165,16 @@ pub(super) fn check_refining_return_position_impl_trait_in_trait<'tcx>(
     );
     let errors = infcx.resolve_regions(&outlives_env);
     if !errors.is_empty() {
+        // This code path is not reached in any tests, but may be reachable. If
+        // this is triggered, it should be converted to `delayed_bug` and the
+        // triggering case turned into a test.
         tcx.dcx().bug("encountered errors when checking RPITIT refinement (regions)");
     }
     // Resolve any lifetime variables that may have been introduced during normalization.
     let Ok((trait_bounds, impl_bounds)) = infcx.fully_resolve((trait_bounds, impl_bounds)) else {
+        // This code path is not reached in any tests, but may be reachable. If
+        // this is triggered, it should be converted to `delayed_bug` and the
+        // triggering case turned into a test.
         tcx.dcx().bug("encountered errors when checking RPITIT refinement (resolution)");
     };
 

compiler/rustc_hir_analysis/src/check/dropck.rs

@@ -67,10 +67,11 @@ pub fn check_drop_impl(tcx: TyCtxt<'_>, drop_impl_did: DefId) -> Result<(), Erro
             // already checked by coherence, but compilation may
             // not have been terminated.
             let span = tcx.def_span(drop_impl_did);
-            tcx.dcx().span_bug(
+            let reported = tcx.dcx().span_delayed_bug(
                 span,
                 format!("should have been rejected by coherence check: {dtor_self_type}"),
             );
+            Err(reported)
         }
     }
 }

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1087,12 +1087,8 @@ fn check_type_defn<'tcx>(
                 packed && {
                     let ty = tcx.type_of(variant.tail().did).instantiate_identity();
                     let ty = tcx.erase_regions(ty);
-                    if ty.has_infer() {
-                        // Unresolved type expression.
-                        tcx.dcx().span_bug(item.span, format!("inference variables in {ty:?}"));
-                    } else {
-                        ty.needs_drop(tcx, tcx.param_env(item.owner_id))
-                    }
+                    assert!(!ty.has_infer());
+                    ty.needs_drop(tcx, tcx.param_env(item.owner_id))
                 }
             };
             // All fields (except for possibly the last) should be sized.

compiler/rustc_hir_typeck/src/expr.rs

@@ -1315,9 +1315,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // the LUB of the breaks (possibly ! if none); else, it
         // is nil. This makes sense because infinite loops
         // (which would have type !) are only possible iff we
-        // permit break with a value [1].
+        // permit break with a value.
         if ctxt.coerce.is_none() && !ctxt.may_break {
-            // [1]
             self.dcx().span_bug(body.span, "no coercion, but loop may not break");
         }
         ctxt.coerce.map(|c| c.complete(self)).unwrap_or_else(|| Ty::new_unit(self.tcx))

compiler/rustc_hir_typeck/src/intrinsicck.rs

@@ -48,7 +48,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let to = normalize(to);
         trace!(?from, ?to);
         if from.has_non_region_infer() || to.has_non_region_infer() {
-            tcx.dcx().span_bug(span, "argument to transmute has inference variables");
+            // Note: this path is currently not reached in any test, so any
+            // example that triggers this would be worth minimizing and
+            // converting into a test.
+            tcx.dcx().span_delayed_bug(span, "argument to transmute has inference variables");
         }
         // Transmutes that are only changing lifetimes are always ok.
         if from == to {

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -924,7 +924,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         span: item_span,
                         ..
                     })) => {
-                        tcx.dcx().span_bug(
+                        tcx.dcx().span_delayed_bug(
                             *item_span,
                             "auto trait is invoked with no method error, but no error reported?",
                         );

compiler/rustc_infer/src/infer/outlives/obligations.rs

@@ -303,7 +303,7 @@ where
                     // Ignore this, we presume it will yield an error later,
                     // since if a type variable is not resolved by this point
                     // it never will be.
-                    self.tcx.dcx().span_bug(
+                    self.tcx.dcx().span_delayed_bug(
                         origin.span(),
                         format!("unresolved inference variable in outlives: {v:?}"),
                     );

compiler/rustc_infer/src/infer/outlives/verify.rs

@@ -176,7 +176,11 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
                 // Ignore this, we presume it will yield an error later, since
                 // if a type variable is not resolved by this point it never
                 // will be.
-                self.tcx.dcx().bug(format!("unresolved inference variable in outlives: {v:?}"));
+                self.tcx
+                    .dcx()
+                    .delayed_bug(format!("unresolved inference variable in outlives: {v:?}"));
+                // Add a bound that never holds.
+                VerifyBound::AnyBound(vec![])
             }
         }
     }

compiler/rustc_infer/src/infer/relate/nll.rs

@@ -557,9 +557,7 @@ where
         match b.kind() {
             ty::ConstKind::Infer(InferConst::Var(_)) if D::forbid_inference_vars() => {
                 // Forbid inference variables in the RHS.
-                self.infcx
-                    .dcx()
-                    .span_bug(self.delegate.span(), format!("unexpected inference var {b:?}",));
+                self.infcx.dcx().bug(format!("unexpected inference var {b:?}"));
             }
             // FIXME(invariance): see the related FIXME above.
             _ => self.infcx.super_combine_consts(self, a, b),

compiler/rustc_middle/src/ty/util.rs

@@ -356,7 +356,9 @@ impl<'tcx> TyCtxt<'tcx> {
             }
 
             let Some(item_id) = self.associated_item_def_ids(impl_did).first() else {
-                self.dcx().span_bug(self.def_span(impl_did), "Drop impl without drop function");
+                self.dcx()
+                    .span_delayed_bug(self.def_span(impl_did), "Drop impl without drop function");
+                return;
             };
 
             if let Some((old_item_id, _)) = dtor_candidate {

compiler/rustc_mir_build/src/build/expr/as_constant.rs

@@ -112,9 +112,7 @@ fn lit_to_mir_constant<'tcx>(
         let param_ty = ty::ParamEnv::reveal_all().and(ty);
         let width = tcx
             .layout_of(param_ty)
-            .map_err(|_| {
-                tcx.dcx().bug(format!("couldn't compute width of literal: {:?}", lit_input.lit))
-            })?
+            .expect(&format!("couldn't compute width of literal: {:?}", lit_input.lit))
             .size;
         trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
         let result = width.truncate(n);

compiler/rustc_mir_build/src/thir/constant.rs

@@ -14,9 +14,7 @@ pub(crate) fn lit_to_const<'tcx>(
         let param_ty = ParamEnv::reveal_all().and(ty);
         let width = tcx
             .layout_of(param_ty)
-            .map_err(|_| {
-                tcx.dcx().bug(format!("couldn't compute width of literal: {:?}", lit_input.lit))
-            })?
+            .expect(&format!("couldn't compute width of literal: {:?}", lit_input.lit))
             .size;
         trace!("trunc {} with size {} and shift {}", n, width.bits(), 128 - width.bits());
         let result = width.truncate(n);

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -216,6 +216,8 @@ fn do_normalize_predicates<'tcx>(
     // the normalized predicates.
     let errors = infcx.resolve_regions(&outlives_env);
     if !errors.is_empty() {
+        // @lcnr: Let's still ICE here for now. I want a test case
+        // for that.
         tcx.dcx().span_bug(
             span,
             format!("failed region resolution while normalizing {elaborated_env:?}: {errors:?}"),

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1464,9 +1464,7 @@ pub fn compute_inherent_assoc_ty_args<'a, 'b, 'tcx>(
         Err(_) => {
             tcx.dcx().span_bug(
                 cause.span,
-                format!(
-                    "{self_ty:?} was a subtype of {impl_ty:?} during selection but now it is not"
-                ),
+                format!("{self_ty:?} was equal to {impl_ty:?} during selection but now it is not"),
             );
         }
     }
@@ -2007,10 +2005,11 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
             ImplSource::Builtin(BuiltinImplSource::TraitUpcasting { .. }, _)
             | ImplSource::Builtin(BuiltinImplSource::TupleUnsizing, _) => {
                 // These traits have no associated types.
-                selcx.tcx().dcx().span_bug(
+                selcx.tcx().dcx().span_delayed_bug(
                     obligation.cause.span,
                     format!("Cannot project an associated type from `{impl_source:?}`"),
                 );
+                return Err(())
             }
         };
 

compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs

@@ -82,7 +82,7 @@ where
     let value = infcx.commit_if_ok(|_| {
         let ocx = ObligationCtxt::new(infcx);
         let value = op(&ocx).map_err(|_| {
-            infcx.dcx().span_bug(span, format!("error performing operation: {name}"))
+            infcx.dcx().span_delayed_bug(span, format!("error performing operation: {name}"))
         })?;
         let errors = ocx.select_all_or_error();
         if errors.is_empty() {

tests/ui/drop/missing-drop-method.rs

@@ -0,0 +1,4 @@
+struct DropNoMethod;
+impl Drop for DropNoMethod {} //~ ERROR not all trait items implemented, missing: `drop`
+
+fn main() {}

tests/ui/drop/missing-drop-method.stderr

@@ -0,0 +1,11 @@
+error[E0046]: not all trait items implemented, missing: `drop`
+  --> $DIR/missing-drop-method.rs:2:1
+   |
+LL | impl Drop for DropNoMethod {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `drop` in implementation
+   |
+   = help: implement the missing item: `fn drop(&mut self) { todo!() }`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0046`.