ensure unary minus propagates NaN payloads exactly

Author: RalfJung

compiler/rustc_const_eval/src/interpret/operator.rs

@@ -461,6 +461,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 Ok((ImmTy::from_bool(res, *self.tcx), false))
             }
             ty::Float(fty) => {
+                // No NaN adjustment here, `-` is a bitwise operation!
                 let res = match (un_op, fty) {
                     (Neg, FloatTy::F32) => Scalar::from_f32(-val.to_f32()?),
                     (Neg, FloatTy::F64) => Scalar::from_f64(-val.to_f64()?),

src/tools/miri/tests/pass/float_nan.rs

@@ -241,6 +241,14 @@ fn test_f32() {
         ]),
         || F32::from(just1 % all1_snan),
     );
+
+    // Unary `-` must preserve payloads exactly.
+    check_all_outcomes(HashSet::from_iter([F32::nan(Neg, Quiet, all1_payload)]), || {
+        F32::from(-all1)
+    });
+    check_all_outcomes(HashSet::from_iter([F32::nan(Neg, Signaling, all1_payload)]), || {
+        F32::from(-all1_snan)
+    });
 }
 
 fn test_f64() {