Merge #158: Remove default impl of hex_reserve_suggestion

908ce81 Remove default impl of hex_reserve_suggestion (Tobin C. Harding)

Pull request description:

  The current default impl returns 0, this reduces performance if implementors forget to implement this method and they know up front how many bytes are needed. The trait is sealed so we are the only implementors and it turns out we forgot in one place (well many places because its a macro).

  Remove the default implementation of `hex_reserve_suggestion` and implement it manually for arrays.

ACKs for top commit:
  Kixunil:
    ACK 908ce81
  apoelstra:
    ACK 908ce81; successfully ran local tests

Tree-SHA512: 36e4ea218ead9f01cc20a0050b8d02a574ba74f8cb8fbb8e83aa970eab11e3fb43c98d8117e04b94fcbaded333e16e69acc68ff2f10fe85298639794014afd69

src/display.rs

@@ -99,13 +99,11 @@ pub trait DisplayHex: Copy + sealed::IsRef + sealed::Sealed {
         })
     }
 
-    /// Hints how much bytes to reserve when creating a `String`.
-    ///
-    /// Implementors that know the number of produced bytes upfront should override this.
-    /// Defaults to 0.
+    /// Hints how many bytes to reserve when creating a `String`.
     ///
+    /// If you don't know you can just return 0 and take the perf hit.
     // We prefix the name with `hex_` to avoid potential collision with other methods.
-    fn hex_reserve_suggestion(self) -> usize { 0 }
+    fn hex_reserve_suggestion(self) -> usize;
 }
 
 fn internal_display(bytes: &[u8], f: &mut fmt::Formatter, case: Case) -> fmt::Result {
@@ -344,6 +342,8 @@ macro_rules! impl_array_as_hex {
                 fn as_hex(self) -> Self::Display {
                     DisplayArray::new(self)
                 }
+
+                fn hex_reserve_suggestion(self) -> usize { $len * 2 }
             }
         )*
     }