Make broken MIR a proper lint.

Author: cjgillot

compiler/rustc_lint_defs/src/builtin.rs

@@ -23,6 +23,7 @@ declare_lint_pass! {
         BARE_TRAIT_OBJECTS,
         BINDINGS_WITH_VARIANT_NAME,
         BREAK_WITH_LABEL_AND_LOOP,
+        BROKEN_MIR,
         BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE,
         CENUM_IMPL_DROP_CAST,
         COHERENCE_LEAK_CHECK,
@@ -3955,6 +3956,15 @@ 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

@@ -1,13 +1,16 @@
 //! This pass statically detects code which has undefined behaviour or is likely to be erroneous.
 //! It can be used to locate problems in MIR building or optimizations. It assumes that all code
 //! can be executed, so it has false positives.
+
+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;
 use std::borrow::Cow;
 
 pub fn lint_body<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, when: String) {
@@ -28,6 +31,11 @@ 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,
         reachable_blocks,
@@ -41,6 +49,7 @@ 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>,
     reachable_blocks: BitSet<BasicBlock>,
@@ -49,10 +58,13 @@ 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(
+        self.tcx.struct_span_lint_hir(
+            BROKEN_MIR,
+            self.lint_id,
             span,
             format!(
                 "broken MIR in {:?} ({}) at {:?}:\n{}",
@@ -61,6 +73,7 @@ impl<'a, 'tcx> Lint<'a, 'tcx> {
                 location,
                 msg.as_ref()
             ),
+            |_| {},
         );
     }
 }

tests/mir-opt/reference_prop.rs

@@ -1,8 +1,7 @@
-// compile-flags: -Zlint-mir=no
 // unit-test: ReferencePropagation
 // needs-unwind
 
-#![feature(raw_ref_op)]
+#![feature(raw_ref_op, lint_reasons)]
 #![feature(core_intrinsics, custom_mir)]
 
 #[inline(never)]
@@ -680,6 +679,7 @@ 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);
@@ -706,6 +706,7 @@ fn multiple_storage() {
 }
 
 #[custom_mir(dialect = "runtime", phase = "post-cleanup")]
+#[expect(rustc::broken_mir)]
 fn dominate_storage() {
     // CHECK-LABEL: dominate_storage
     // CHECK: _5 = (*_2);
@@ -736,6 +737,7 @@ 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;