Use void to indicate that parsing will never return an Err

Cargo.toml

@@ -18,6 +18,7 @@ libc = "0.2"
 ord_subset = { optional = true, version = "3" }
 rustc-serialize = { optional = true, version = "0.3" }
 serde = { optional = true, version = "1" }
+void = "1.0"
 
 [features]
 default = ["ord_subset", "rustc-serialize", "serde"]

src/dec128.rs

@@ -24,6 +24,7 @@ use std::ops::{Add, AddAssign, Sub, SubAssign, Mul, MulAssign, Div, DivAssign, R
                ShlAssign, Shr, ShrAssign};
 use std::str::FromStr;
 use std::str::from_utf8_unchecked;
+use void::Void;
 
 thread_local!(static CTX: RefCell<Context> = RefCell::new(d128::default_context()));
 
@@ -185,10 +186,10 @@ impl AsRef<d128> for d128 {
 /// no limit to the coefficient length for finite inputs; NaN payloads must be integers with no
 /// more than 33 digits. Exponents may have up to nine significant digits. The syntax of the string
 /// is fully checked; if it is not valid, the result will be a quiet NaN and an error flag will be
-/// set.
+/// set - the `Result` will never be an `Err` variant.
 impl FromStr for d128 {
-    type Err = ();
-    fn from_str(s: &str) -> Result<Self, ()> {
+    type Err = Void;
+    fn from_str(s: &str) -> Result<Self, Void> {
         let cstr = match CString::new(s) {
             Err(..) => CString::new("qNaN").unwrap(),
             Ok(cstr) => cstr,

src/lib.rs

@@ -10,6 +10,7 @@ extern crate serde;
 #[cfg(feature = "serde")]
 #[cfg(test)]
 extern crate serde_json;
+extern crate void;
 
 #[macro_export]
 /// A macro to construct d128 literals.