cty: AsBigFloat must make a deep copy

Use Float.Copy in AsBigFloat to ensure that there is no shared data
between the mutable *big.Float and the cty.Value internal state.

big.Float includes some pointers, so a shallow copy is insufficient.

cty/value_ops.go

@@ -1283,9 +1283,7 @@ func (val Value) AsBigFloat() *big.Float {
 	}
 
 	// Copy the float so that callers can't mutate our internal state
-	ret := *(val.v.(*big.Float))
-
-	return &ret
+	return new(big.Float).Copy(val.v.(*big.Float))
 }
 
 // AsValueSlice returns a []cty.Value representation of a non-null, non-unknown

cty/value_ops_test.go

@@ -3442,3 +3442,16 @@ func TestHasWhollyKnownType(t *testing.T) {
 		})
 	}
 }
+
+func TestFloatCopy(t *testing.T) {
+	// ensure manipulating floats does not modify the cty.Value
+	v := NumberFloatVal(1.9)
+	vString := v.GoString()
+
+	// do something that will modify the internal big.Float mantissa
+	v.AsBigFloat().SetInt64(1)
+
+	if vString != v.GoString() {
+		t.Fatalf("original value changed from %s to %#v", vString, v)
+	}
+}