dataflow_const_prop: do not eval a ptr address in SwitchInt

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -534,8 +534,13 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             // This allows the set of visited edges to grow monotonically with the lattice.
             FlatSet::Bottom => TerminatorEdges::None,
             FlatSet::Elem(scalar) => {
-                let choice = scalar.assert_scalar_int().to_bits_unchecked();
-                TerminatorEdges::Single(targets.target_for_value(choice))
+                if let Ok(scalar_int) = scalar.try_to_scalar_int() {
+                    TerminatorEdges::Single(
+                        targets.target_for_value(scalar_int.to_bits_unchecked()),
+                    )
+                } else {
+                    TerminatorEdges::SwitchInt { discr, targets }
+                }
             }
             FlatSet::Top => TerminatorEdges::SwitchInt { discr, targets },
         }

tests/ui/dataflow_const_prop/ptr-in-switch-int-issue-131227.rs

@@ -0,0 +1,19 @@
+//! Issue: <https://github.com/rust-lang/rust/issues/131227>
+//! Test that constant propagation in SwitchInt does not crash
+//! when encountering a ptr-to-int transmute.
+
+//@ check-pass
+//@ compile-flags: -Zmir-enable-passes=+InstSimplify-before-inline,+DataflowConstProp
+
+#![crate_type = "lib"]
+
+static mut G: i32 = 0;
+
+pub fn myfunc() -> i32 {
+    let var = &raw mut G;
+    let u: usize = unsafe { std::mem::transmute(var) };
+    match u {
+        0 => 0,
+        _ => 1,
+    }
+}