fix applying an error to infinities

Author: RalfJung

src/tools/miri/src/math.rs

@@ -15,6 +15,8 @@ pub(crate) fn apply_random_float_error<F: rustc_apfloat::Float>(
         || matches!(ecx.machine.float_rounding_error, FloatRoundingErrorMode::None)
         // relative errors don't do anything to zeros... avoid messing up the sign
         || val.is_zero()
+        // The logic below makes no sense if the input is already non-finite.
+        || !val.is_finite()
     {
         return val;
     }
@@ -54,6 +56,8 @@ pub(crate) fn apply_random_float_error_ulp<F: rustc_apfloat::Float>(
         // FIXME: also disturb zeros? That requires a lot more cases in `fixed_float_value`
         // and might make the std test suite quite unhappy.
         || val.is_zero()
+        // The logic below makes no sense if the input is already non-finite.
+        || !val.is_finite()
     {
         return val;
     }

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

@@ -1052,6 +1052,10 @@ pub fn libm() {
     assert_eq!(42f64.powf(0.0), 1.0);
     assert_eq!(f32::INFINITY.powf(0.0), 1.0);
     assert_eq!(f64::INFINITY.powf(0.0), 1.0);
+    assert_eq!(f32::NEG_INFINITY.powi(3), f32::NEG_INFINITY);
+    assert_eq!(f32::NEG_INFINITY.powi(2), f32::INFINITY);
+    assert_eq!(f64::INFINITY.powi(3), f64::INFINITY);
+    assert_eq!(f64::INFINITY.powi(2), f64::INFINITY);
 
     // f*::NAN is a quiet NAN and should return 1 as well.
     assert_eq!(f32::NAN.powi(0), 1.0);