From 5bd71df7c144b6de57713db5d1080b6505a9d5b4 Mon Sep 17 00:00:00 2001
From: root <root@ten.localdomain>
Date: Tue, 28 Apr 2026 01:51:40 +0800
Subject: [PATCH] Prevent ICE in SSA range propagation when CFG contains self- 
  loops

Replace self-loop assumptions in ssa_range_prop terminator
  handling with guards that skip range insertion when successor is
  in the same block. Add a UI regression test for issue #155836 to
  ensure optimized builds no longer ICE.

Fixes #155836

Constraint: Keep propagation behavior unchanged for non-
  self-loop edges

Rejected: Keep assert_ne! and special-case only Assert arm |
  still leaves SwitchInt self-loop paths vulnerable

Confidence: high

Scope-risk: narrow

Reversibility: clean

Directive: Do not reintroduce block-inequality assertions in
  terminator propagation without proving CFG invariants after
  simplification

Tested: ./x.py check compiler/rustc_mir_transform; ./x.py
  test tests/ui/mir/ssa-range-prop-self-loop-155836.rs

Not-tested: Broader SwitchInt multi-target stress cases
---
 .../rustc_mir_transform/src/ssa_range_prop.rs | 28 +++++++++++--------
 .../ui/mir/ssa-range-prop-self-loop-155836.rs | 13 +++++++++
 2 files changed, 29 insertions(+), 12 deletions(-)
 create mode 100644 tests/ui/mir/ssa-range-prop-self-loop-155836.rs

diff --git a/compiler/rustc_mir_transform/src/ssa_range_prop.rs b/compiler/rustc_mir_transform/src/ssa_range_prop.rs
index 97ba07d900960..9f9a74d4bd237 100644
--- a/compiler/rustc_mir_transform/src/ssa_range_prop.rs
+++ b/compiler/rustc_mir_transform/src/ssa_range_prop.rs
@@ -164,10 +164,11 @@ impl<'tcx> MutVisitor<'tcx> for RangeSet<'tcx, '_, '_> {
             {
                 let successor = Location { block: *target, statement_index: 0 };
                 if location.dominates(successor, &self.dominators) {
-                    assert_ne!(location.block, successor.block);
-                    let val = *expected as u128;
-                    let range = WrappingRange { start: val, end: val };
-                    self.insert_range(place, successor, range);
+                    if location.block != successor.block {
+                        let val = *expected as u128;
+                        let range = WrappingRange { start: val, end: val };
+                        self.insert_range(place, successor, range);
+                    }
                 }
             }
             TerminatorKind::SwitchInt { discr, targets }
@@ -188,7 +189,9 @@ impl<'tcx> MutVisitor<'tcx> for RangeSet<'tcx, '_, '_> {
                     }
                     let successor = Location { block: target, statement_index: 0 };
                     if self.unique_predecessors.contains(successor.block) {
-                        assert_ne!(location.block, successor.block);
+                        if location.block == successor.block {
+                            continue;
+                        }
                         let range = WrappingRange { start: val, end: val };
                         self.insert_range(place, successor, range);
                     }
@@ -201,13 +204,14 @@ impl<'tcx> MutVisitor<'tcx> for RangeSet<'tcx, '_, '_> {
                     && let [val] = targets.all_values()
                     && self.unique_predecessors.contains(otherwise.block)
                 {
-                    assert_ne!(location.block, otherwise.block);
-                    let range = if val.get() == 0 {
-                        WrappingRange { start: 1, end: 1 }
-                    } else {
-                        WrappingRange { start: 0, end: 0 }
-                    };
-                    self.insert_range(place, otherwise, range);
+                    if location.block != otherwise.block {
+                        let range = if val.get() == 0 {
+                            WrappingRange { start: 1, end: 1 }
+                        } else {
+                            WrappingRange { start: 0, end: 0 }
+                        };
+                        self.insert_range(place, otherwise, range);
+                    }
                 }
             }
             _ => {}
diff --git a/tests/ui/mir/ssa-range-prop-self-loop-155836.rs b/tests/ui/mir/ssa-range-prop-self-loop-155836.rs
new file mode 100644
index 0000000000000..9e7df9adca909
--- /dev/null
+++ b/tests/ui/mir/ssa-range-prop-self-loop-155836.rs
@@ -0,0 +1,13 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/155836>.
+// Ensure SSA range propagation does not ICE on self-loops.
+//@ compile-flags: -C opt-level=2
+//@ check-pass
+
+#![crate_type = "lib"]
+
+pub fn trigger(b: usize) -> usize {
+    while 0 != 2 {
+        b % b;
+    }
+    b
+}