Use fewer delayed bugs.

For some cases where it's clear that an error has already occurred,
e.g.:
- there's a comment stating exactly that, or
- things like HIR lowering, where we are lowering an error kind

The commit also tweaks some comments around delayed bug sites.

Author: nnethercote

Diff for: compiler/rustc_ast_lowering/src/expr.rs

@@ -323,9 +323,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     )
                 }
                 ExprKind::Yield(opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
-                ExprKind::Err => {
-                    hir::ExprKind::Err(self.dcx().span_delayed_bug(e.span, "lowered ExprKind::Err"))
-                }
+                ExprKind::Err => hir::ExprKind::Err(self.dcx().has_errors().unwrap()),
                 ExprKind::Try(sub_expr) => self.lower_expr_try(e.span, sub_expr),
 
                 ExprKind::Paren(_) | ExprKind::ForLoop { .. } => {

Diff for: compiler/rustc_ast_lowering/src/item.rs

@@ -1068,7 +1068,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
         match block {
             Some(block) => self.lower_block_expr(block),
-            None => self.expr_err(span, self.dcx().span_delayed_bug(span, "no block")),
+            None => self.expr_err(span, self.dcx().has_errors().unwrap()),
         }
     }
 

Diff for: compiler/rustc_ast_lowering/src/lib.rs

@@ -1285,9 +1285,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_ty_direct(&mut self, t: &Ty, itctx: ImplTraitContext) -> hir::Ty<'hir> {
         let kind = match &t.kind {
             TyKind::Infer => hir::TyKind::Infer,
-            TyKind::Err => {
-                hir::TyKind::Err(self.dcx().span_delayed_bug(t.span, "TyKind::Err lowered"))
-            }
+            TyKind::Err => hir::TyKind::Err(self.dcx().has_errors().unwrap()),
             // Lower the anonymous structs or unions in a nested lowering context.
             //
             // ```

Diff for: compiler/rustc_const_eval/src/transform/validate.rs

@@ -117,18 +117,14 @@ struct CfgChecker<'a, 'tcx> {
 impl<'a, 'tcx> CfgChecker<'a, 'tcx> {
     #[track_caller]
     fn fail(&self, location: Location, msg: impl AsRef<str>) {
-        let span = self.body.source_info(location).span;
-        // We use `span_delayed_bug` as we might see broken MIR when other errors have already
-        // occurred.
-        self.tcx.dcx().span_delayed_bug(
-            span,
-            format!(
-                "broken MIR in {:?} ({}) at {:?}:\n{}",
-                self.body.source.instance,
-                self.when,
-                location,
-                msg.as_ref()
-            ),
+        // We might see broken MIR when other errors have already occurred.
+        assert!(
+            self.tcx.dcx().has_errors().is_some(),
+            "broken MIR in {:?} ({}) at {:?}:\n{}",
+            self.body.source.instance,
+            self.when,
+            location,
+            msg.as_ref(),
         );
     }
 

Diff for: compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -34,10 +34,10 @@ pub(super) fn failed_to_match_macro<'cx>(
     if try_success_result.is_ok() {
         // Nonterminal parser recovery might turn failed matches into successful ones,
         // but for that it must have emitted an error already
-        tracker
-            .cx
-            .dcx()
-            .span_delayed_bug(sp, "Macro matching returned a success on the second try");
+        assert!(
+            tracker.cx.dcx().has_errors().is_some(),
+            "Macro matching returned a success on the second try"
+        );
     }
 
     if let Some(result) = tracker.result {

Diff for: compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -758,8 +758,8 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             // since we should have emitten an error for them earlier, and they will
             // not be well-formed!
             if polarity == ty::ImplPolarity::Negative {
-                self.tcx().dcx().span_delayed_bug(
-                    binding.span,
+                assert!(
+                    self.tcx().dcx().has_errors().is_some(),
                     "negative trait bounds should not have bindings",
                 );
                 continue;

Diff for: compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1865,13 +1865,13 @@ fn check_variances_for_type_defn<'tcx>(
         let hir_param = &hir_generics.params[index];
 
         if ty_param.def_id != hir_param.def_id.into() {
-            // valid programs always have lifetimes before types in the generic parameter list
+            // Valid programs always have lifetimes before types in the generic parameter list.
             // ty_generics are normalized to be in this required order, and variances are built
             // from ty generics, not from hir generics. but we need hir generics to get
-            // a span out
+            // a span out.
             //
-            // if they aren't in the same order, then the user has written invalid code, and already
-            // got an error about it (or I'm wrong about this)
+            // If they aren't in the same order, then the user has written invalid code, and already
+            // got an error about it (or I'm wrong about this).
             tcx.dcx().span_delayed_bug(
                 hir_param.span,
                 "hir generics and ty generics in different order",

Diff for: compiler/rustc_hir_analysis/src/coherence/unsafety.rs

@@ -85,7 +85,7 @@ pub(super) fn check_item(
 
         (_, _, Unsafety::Unsafe, Negative) => {
             // Reported in AST validation
-            tcx.dcx().span_delayed_bug(tcx.def_span(def_id), "unsafe negative impl");
+            assert!(tcx.dcx().has_errors().is_some(), "unsafe negative impl");
             Ok(())
         }
         (_, _, Unsafety::Normal, Negative)

Diff for: compiler/rustc_hir_typeck/src/cast.rs

@@ -139,10 +139,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             | ty::Never
             | ty::Dynamic(_, _, ty::DynStar)
             | ty::Error(_) => {
-                let reported = self
+                let guar = self
                     .dcx()
                     .span_delayed_bug(span, format!("`{t:?}` should be sized but is not?"));
-                return Err(reported);
+                return Err(guar);
             }
         })
     }

Diff for: compiler/rustc_hir_typeck/src/writeback.rs

@@ -221,8 +221,8 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
             if base_ty.is_none() {
                 // When encountering `return [0][0]` outside of a `fn` body we can encounter a base
                 // that isn't in the type table. We assume more relevant errors have already been
-                // emitted, so we delay an ICE if none have. (#64638)
-                self.tcx().dcx().span_delayed_bug(e.span, format!("bad base: `{base:?}`"));
+                // emitted. (#64638)
+                assert!(self.tcx().dcx().has_errors().is_some(), "bad base: `{base:?}`");
             }
             if let Some(base_ty) = base_ty
                 && let ty::Ref(_, base_ty_inner, _) = *base_ty.kind()

Diff for: compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs

@@ -802,14 +802,12 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
         }
 
         // Errors in earlier passes can yield error variables without
-        // resolution errors here; delay ICE in favor of those errors.
-        self.tcx().dcx().span_delayed_bug(
-            self.var_infos[node_idx].origin.span(),
-            format!(
-                "collect_error_for_expanding_node() could not find \
-                 error for var {node_idx:?} in universe {node_universe:?}, lower_bounds={lower_bounds:#?}, \
-                 upper_bounds={upper_bounds:#?}"
-            ),
+        // resolution errors here; ICE if no errors have been emitted yet.
+        assert!(
+            self.tcx().dcx().has_errors().is_some(),
+            "collect_error_for_expanding_node() could not find error for var {node_idx:?} in \
+            universe {node_universe:?}, lower_bounds={lower_bounds:#?}, \
+            upper_bounds={upper_bounds:#?}",
         );
     }
 

Diff for: compiler/rustc_infer/src/infer/outlives/obligations.rs

@@ -300,9 +300,9 @@ where
                     self.components_must_outlive(origin, subcomponents, region, category);
                 }
                 Component::UnresolvedInferenceVariable(v) => {
-                    // 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
+                    // 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_delayed_bug(
                         origin.span(),
                         format!("unresolved inference variable in outlives: {v:?}"),

Diff for: compiler/rustc_infer/src/infer/outlives/verify.rs

@@ -172,13 +172,13 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
                 self.bound_from_components(components, visited)
             }
             Component::UnresolvedInferenceVariable(v) => {
-                // 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
+                // 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()
                     .delayed_bug(format!("unresolved inference variable in outlives: {v:?}"));
-                // add a bound that never holds
+                // Add a bound that never holds.
                 VerifyBound::AnyBound(vec![])
             }
         }

Diff for: compiler/rustc_lint/src/early.rs

@@ -431,14 +431,13 @@ pub fn check_ast_node_inner<'a, T: EarlyLintPass>(
     // If not, that means that we somehow buffered a lint for a node id
     // that was not lint-checked (perhaps it doesn't exist?). This is a bug.
     for (id, lints) in cx.context.buffered.map {
-        for early_lint in lints {
-            sess.dcx().span_delayed_bug(
-                early_lint.span,
-                format!(
-                    "failed to process buffered lint here (dummy = {})",
-                    id == ast::DUMMY_NODE_ID
-                ),
+        if !lints.is_empty() {
+            assert!(
+                sess.dcx().has_errors().is_some(),
+                "failed to process buffered lint here (dummy = {})",
+                id == ast::DUMMY_NODE_ID
             );
+            break;
         }
     }
 }

Diff for: compiler/rustc_mir_build/src/check_unsafety.rs

@@ -33,7 +33,7 @@ struct UnsafetyVisitor<'a, 'tcx> {
     body_target_features: &'tcx [Symbol],
     /// When inside the LHS of an assignment to a field, this is the type
     /// of the LHS and the span of the assignment expression.
-    assignment_info: Option<(Ty<'tcx>, Span)>,
+    assignment_info: Option<Ty<'tcx>>,
     in_union_destructure: bool,
     param_env: ParamEnv<'tcx>,
     inside_adt: bool,
@@ -473,10 +473,15 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                 if let ty::Adt(adt_def, _) = lhs.ty.kind()
                     && adt_def.is_union()
                 {
-                    if let Some((assigned_ty, assignment_span)) = self.assignment_info {
+                    if let Some(assigned_ty) = self.assignment_info {
                         if assigned_ty.needs_drop(self.tcx, self.param_env) {
-                            // This would be unsafe, but should be outright impossible since we reject such unions.
-                            self.tcx.dcx().span_delayed_bug(assignment_span, format!("union fields that need dropping should be impossible: {assigned_ty}"));
+                            // This would be unsafe, but should be outright impossible since we
+                            // reject such unions.
+                            assert!(
+                                self.tcx.dcx().has_errors().is_some(),
+                                "union fields that need dropping should be impossible: \
+                                {assigned_ty}"
+                            );
                         }
                     } else {
                         self.requires_unsafe(expr.span, AccessToUnionField);
@@ -492,14 +497,15 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                     self.requires_unsafe(expr.span, MutationOfLayoutConstrainedField);
                 }
 
-                // Second, check for accesses to union fields
-                // don't have any special handling for AssignOp since it causes a read *and* write to lhs
+                // Second, check for accesses to union fields. Don't have any
+                // special handling for AssignOp since it causes a read *and*
+                // write to lhs.
                 if matches!(expr.kind, ExprKind::Assign { .. }) {
-                    self.assignment_info = Some((lhs.ty, expr.span));
+                    self.assignment_info = Some(lhs.ty);
                     visit::walk_expr(self, lhs);
                     self.assignment_info = None;
                     visit::walk_expr(self, &self.thir()[rhs]);
-                    return; // we have already visited everything by now
+                    return; // We have already visited everything by now.
                 }
             }
             ExprKind::Borrow { borrow_kind, arg } => {

Diff for: compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

@@ -153,8 +153,7 @@ impl<'tcx> ConstToPat<'tcx> {
             // a hard error when we don't have a valtree or when we find something in
             // the valtree that is not structural; then this can all be made a lot simpler.
 
-            let structural =
-                traits::search_for_structural_match_violation(self.span, self.tcx(), cv.ty());
+            let structural = traits::search_for_structural_match_violation(self.tcx(), cv.ty());
             debug!(
                 "search_for_structural_match_violation cv.ty: {:?} returned: {:?}",
                 cv.ty(),

Diff for: compiler/rustc_mir_transform/src/check_unsafety.rs

@@ -243,10 +243,11 @@ impl<'tcx> Visitor<'tcx> for UnsafetyChecker<'_, 'tcx> {
                     // old value is being dropped.
                     let assigned_ty = place.ty(&self.body.local_decls, self.tcx).ty;
                     if assigned_ty.needs_drop(self.tcx, self.param_env) {
-                        // This would be unsafe, but should be outright impossible since we reject such unions.
-                        self.tcx.dcx().span_delayed_bug(
-                            self.source_info.span,
-                            format!("union fields that need dropping should be impossible: {assigned_ty}")
+                        // This would be unsafe, but should be outright impossible since we reject
+                        // such unions.
+                        assert!(
+                            self.tcx.dcx().has_errors().is_some(),
+                            "union fields that need dropping should be impossible: {assigned_ty}"
                         );
                     }
                 } else {

Diff for: compiler/rustc_mir_transform/src/lib.rs

@@ -265,7 +265,7 @@ fn mir_const_qualif(tcx: TyCtxt<'_>, def: LocalDefId) -> ConstQualifs {
     let body = &tcx.mir_const(def).borrow();
 
     if body.return_ty().references_error() {
-        tcx.dcx().span_delayed_bug(body.span, "mir_const_qualif: MIR had errors");
+        assert!(tcx.dcx().has_errors().is_some(), "mir_const_qualif: MIR had errors");
         return Default::default();
     }
 

Diff for: compiler/rustc_query_system/src/query/plumbing.rs

@@ -429,16 +429,16 @@ where
             let formatter = query.format_value();
             if old_hash != new_hash {
                 // We have an inconsistency. This can happen if one of the two
-                // results is tainted by errors. In this case, delay a bug to
-                // ensure compilation is doomed.
-                qcx.dep_context().sess().dcx().delayed_bug(format!(
+                // results is tainted by errors.
+                assert!(
+                    qcx.dep_context().sess().dcx().has_errors().is_some(),
                     "Computed query value for {:?}({:?}) is inconsistent with fed value,\n\
                         computed={:#?}\nfed={:#?}",
                     query.dep_kind(),
                     key,
                     formatter(&result),
                     formatter(&cached_result),
-                ));
+                );
             }
         }
     }

Diff for: compiler/rustc_trait_selection/src/traits/const_evaluatable.rs

@@ -62,9 +62,10 @@ pub fn is_const_evaluatable<'tcx>(
 
         match unexpanded_ct.kind() {
             ty::ConstKind::Expr(_) => {
-                // FIXME(generic_const_exprs): we have a `ConstKind::Expr` which is fully concrete, but
-                // currently it is not possible to evaluate `ConstKind::Expr` so we are unable to tell if it
-                // is evaluatable or not. For now we just ICE until this is implemented.
+                // FIXME(generic_const_exprs): we have a `ConstKind::Expr` which is fully concrete,
+                // but currently it is not possible to evaluate `ConstKind::Expr` so we are unable
+                // to tell if it is evaluatable or not. For now we just ICE until this is
+                // implemented.
                 Err(NotConstEvaluatable::Error(tcx.dcx().span_delayed_bug(
                     span,
                     "evaluating `ConstKind::Expr` is not currently supported",

Diff for: compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -236,9 +236,9 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             }
         }
 
-        // It could be that we don't report an error because we have seen an `ErrorReported` from another source.
-        // We should probably be able to fix most of these, but some are delayed bugs that get a proper error
-        // after this function.
+        // It could be that we don't report an error because we have seen an `ErrorReported` from
+        // another source. We should probably be able to fix most of these, but some are delayed
+        // bugs that get a proper error after this function.
         reported.unwrap_or_else(|| self.dcx().delayed_bug("failed to report fulfillment errors"))
     }
 
@@ -519,7 +519,11 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 trait_ref,
                                 span,
                             ) {
-                                GetSafeTransmuteErrorAndReason::Silent => return self.dcx().span_delayed_bug(span, "silent safe transmute error"),
+                                GetSafeTransmuteErrorAndReason::Silent => {
+                                    return self.dcx().span_delayed_bug(
+                                        span, "silent safe transmute error"
+                                    );
+                                }
                                 GetSafeTransmuteErrorAndReason::Error {
                                     err_msg,
                                     safe_transmute_explanation,

Diff for: compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -555,7 +555,7 @@ fn virtual_call_violations_for_method<'tcx>(
 
     // NOTE: This check happens last, because it results in a lint, and not a
     // hard error.
-    if tcx.predicates_of(method.def_id).predicates.iter().any(|&(pred, span)| {
+    if tcx.predicates_of(method.def_id).predicates.iter().any(|&(pred, _span)| {
         // dyn Trait is okay:
         //
         //     trait Trait {
@@ -594,7 +594,10 @@ fn virtual_call_violations_for_method<'tcx>(
             // would already have reported an error at the definition of the
             // auto trait.
             if pred_trait_ref.args.len() != 1 {
-                tcx.dcx().span_delayed_bug(span, "auto traits cannot have generic parameters");
+                assert!(
+                    tcx.dcx().has_errors().is_some(),
+                    "auto traits cannot have generic parameters"
+                );
             }
             return false;
         }