Fix porting mistake found by fuzzing: -x should be `x.wrapping_neg(…

…)` to avoid overflow panics.

src/lib.rs

@@ -360,7 +360,7 @@ pub trait Float:
     fn from_bits(input: u128) -> Self;
     fn from_i128_r(input: i128, round: Round) -> StatusAnd<Self> {
         if input < 0 {
-            Self::from_u128_r(-input as u128, -round).map(|r| -r)
+            Self::from_u128_r(input.wrapping_neg() as u128, -round).map(|r| -r)
         } else {
             Self::from_u128_r(input as u128, round)
         }

tests/downstream.rs

@@ -0,0 +1,44 @@
+//! Tests added to `rustc_apfloat`, that were not ported from the C++ code.
+
+use rustc_apfloat::ieee::{Double, Single};
+use rustc_apfloat::Float;
+
+// `f32 -> i128 -> f32` previously-crashing bit-patterns (found by fuzzing).
+pub const FUZZ_IEEE32_ROUNDTRIP_THROUGH_I128_CASES: &[u32] = &[
+    0xff000000, // -1.7014118e+38
+    0xff00e203, // -1.713147e+38
+    0xff00e900, // -1.7135099e+38
+    0xff7fffff, // -3.4028235e+38
+    0xff800000, // -inf
+];
+
+// `f64 -> i128 -> f64` previously-crashing bit-patterns (found by fuzzing).
+pub const FUZZ_IEEE64_ROUNDTRIP_THROUGH_I128_CASES: &[u64] = &[
+    0xc7e5d58020ffedff, // -2.3217876724230413e+38
+    0xc7e7fffefefeff00, // -2.5521161229511617e+38
+    0xc7e8030000653636, // -2.5533639056589687e+38
+    0xea3501e2e8950007, // -4.116505897277026e+203
+    0xf3ff0620ca000600, // -5.553072340247723e+250
+    0xffc909842600d4ff, // -3.516340112093497e+307
+    0xfff0000000000000, // -inf
+];
+
+#[test]
+fn fuzz_roundtrip_through_i128() {
+    for &bits in FUZZ_IEEE32_ROUNDTRIP_THROUGH_I128_CASES {
+        assert_eq!(
+            Single::from_i128(Single::from_bits(bits.into()).to_i128(128).value)
+                .value
+                .to_bits(),
+            (f32::from_bits(bits) as i128 as f32).to_bits().into()
+        );
+    }
+    for &bits in FUZZ_IEEE64_ROUNDTRIP_THROUGH_I128_CASES {
+        assert_eq!(
+            Double::from_i128(Double::from_bits(bits.into()).to_i128(128).value)
+                .value
+                .to_bits(),
+            (f64::from_bits(bits) as i128 as f64).to_bits().into()
+        );
+    }
+}