Make broken MIR a proper lint.

compiler/rustc_lint/src/lib.rs

@@ -597,6 +597,7 @@ fn register_internals(store: &mut LintStore) {
         "rustc::internal",
         None,
         vec![
+            LintId::of(BROKEN_MIR),
             LintId::of(DEFAULT_HASH_TYPES),
             LintId::of(POTENTIAL_QUERY_INSTABILITY),
             LintId::of(USAGE_OF_TY_TYKIND),

compiler/rustc_lint_defs/src/builtin.rs

@@ -3933,6 +3933,15 @@ declare_lint! {
     "`break` expression with label and unlabeled loop as value expression"
 }
 
+crate::declare_tool_lint! {
+    /// The `broken_mir` statically detects undefined behaviour in the MIR optimization pipeline.
+    /// This is an internal lint, and not intended to be used directly.
+    pub rustc::BROKEN_MIR,
+    Deny,
+    "broken MIR",
+    report_in_external_macro: true
+}
+
 declare_lint! {
     /// The `non_exhaustive_omitted_patterns` lint aims to help consumers of a `#[non_exhaustive]`
     /// struct or enum who want to match all of its fields/variants explicitly.

compiler/rustc_mir_transform/src/lint.rs

@@ -5,13 +5,15 @@
 use std::borrow::Cow;
 
 use rustc_data_structures::fx::FxHashSet;
+use rustc_hir::{HirId, CRATE_HIR_ID};
 use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 use rustc_mir_dataflow::impls::{MaybeStorageDead, MaybeStorageLive};
 use rustc_mir_dataflow::storage::always_storage_live_locals;
 use rustc_mir_dataflow::{Analysis, ResultsCursor};
+use rustc_session::lint::builtin::BROKEN_MIR;
 
 pub fn lint_body<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, when: String) {
     let always_live_locals = &always_storage_live_locals(body);
@@ -30,6 +32,11 @@ pub fn lint_body<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, when: String) {
         tcx,
         when,
         body,
+        lint_id: body
+            .source
+            .def_id()
+            .as_local()
+            .map_or(CRATE_HIR_ID, |def_id| tcx.local_def_id_to_hir_id(def_id)),
         is_fn_like: tcx.def_kind(body.source.def_id()).is_fn_like(),
         always_live_locals,
         maybe_storage_live,
@@ -45,6 +52,7 @@ struct Lint<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     when: String,
     body: &'a Body<'tcx>,
+    lint_id: HirId,
     is_fn_like: bool,
     always_live_locals: &'a BitSet<Local>,
     maybe_storage_live: ResultsCursor<'a, 'tcx, MaybeStorageLive<'a>>,
@@ -53,19 +61,19 @@ struct Lint<'a, 'tcx> {
 }
 
 impl<'a, 'tcx> Lint<'a, 'tcx> {
-    #[track_caller]
+    #[allow(rustc::untranslatable_diagnostic)]
+    #[allow(rustc::diagnostic_outside_of_impl)]
     fn fail(&self, location: Location, msg: impl AsRef<str>) {
         let span = self.body.source_info(location).span;
-        self.tcx.sess.dcx().span_delayed_bug(
-            span,
-            format!(
+        self.tcx.node_span_lint(BROKEN_MIR, self.lint_id, span, |lint| {
+            lint.primary_message(format!(
                 "broken MIR in {:?} ({}) at {:?}:\n{}",
                 self.body.source.instance,
                 self.when,
                 location,
                 msg.as_ref()
-            ),
-        );
+            ));
+        });
     }
 }
 

tests/mir-opt/reference_prop.rs

@@ -1,8 +1,7 @@
-//@ compile-flags: -Zlint-mir=no
 //@ test-mir-pass: ReferencePropagation
 //@ needs-unwind
 
-#![feature(raw_ref_op)]
+#![feature(raw_ref_op, lint_reasons)]
 #![feature(core_intrinsics, custom_mir)]
 
 #[inline(never)]
@@ -678,6 +677,7 @@ fn read_through_raw(x: &mut usize) -> usize {
 }
 
 #[custom_mir(dialect = "runtime", phase = "post-cleanup")]
+#[expect(rustc::broken_mir)]
 fn multiple_storage() {
     // CHECK-LABEL: multiple_storage
     // CHECK: _3 = (*_2);
@@ -704,6 +704,7 @@ fn multiple_storage() {
 }
 
 #[custom_mir(dialect = "runtime", phase = "post-cleanup")]
+#[expect(rustc::broken_mir)]
 fn dominate_storage() {
     // CHECK-LABEL: dominate_storage
     // CHECK: _5 = (*_2);
@@ -734,6 +735,7 @@ fn dominate_storage() {
 }
 
 #[custom_mir(dialect = "runtime", phase = "post-cleanup")]
+#[expect(rustc::broken_mir)]
 fn maybe_dead(m: bool) {
     // CHECK-LABEL: fn maybe_dead(
     // CHECK: (*_5) = const 7_i32;

tests/ui/mir/lint/storage-live.rs

@@ -21,6 +21,7 @@ fn multiple_storage() {
         {
             StorageLive(a);
             StorageLive(a);
+            //~^ ERROR broken MIR
             Return()
         }
     }

tests/ui/mir/lint/storage-live.stderr

@@ -5,14 +5,10 @@ error: internal compiler error: broken MIR in Item(DefId(0:8 ~ storage_live[HASH
 LL |             StorageLive(a);
    |             ^^^^^^^^^^^^^^
    |
-note: delayed at compiler/rustc_mir_transform/src/lint.rs:LL:CC - disabled backtrace
-  --> $DIR/storage-live.rs:23:13
-   |
-LL |             StorageLive(a);
-   |             ^^^^^^^^^^^^^^
-
-aborting due to `-Z treat-err-as-bug=1`
+   = aborting due to `-Z treat-err-as-bug=1`
 error: the compiler unexpectedly panicked. this is a bug.
 
 query stack during panic:
+#0 [mir_built] building MIR for `multiple_storage`
+#1 [has_ffi_unwind_calls] checking if `multiple_storage` contains FFI-unwind calls
 end of query stack