Shrink the size of all Error types

This commit improves the in-memory size of `Error` from 7 pointers to 1 pointer.
Errors are in general relatively rare in applications and having a huge error
type ends up generating lots of instructions for moves and such, so this PR
optimizes for size and passing around errors rather than creating errors.

Author: alexcrichton

examples/size.rs

@@ -65,20 +65,13 @@ macro_rules! impl_error_chain_processed {
         ///   - a backtrace, generated when the error is created.
         ///   - an error chain, used for the implementation of `Error::cause()`.
         #[derive(Debug)]
-        pub struct $error_name(
-            // The members must be `pub` for `links`.
-            /// The kind of the error.
-            pub $error_kind_name,
-            /// Contains the error chain and the backtrace.
-            #[doc(hidden)]
-            pub $crate::State,
-        );
+        pub struct $error_name(pub Box<($error_kind_name, $crate::State)>);
 
         impl $crate::ChainedError for $error_name {
             type ErrorKind = $error_kind_name;
 
             fn new(kind: $error_kind_name, state: $crate::State) -> $error_name {
-                $error_name(kind, state)
+                $error_name(Box::new((kind, state)))
             }
 
             fn from_kind(kind: Self::ErrorKind) -> Self {
@@ -120,10 +113,10 @@ macro_rules! impl_error_chain_processed {
         impl $error_name {
             /// Constructs an error from a kind, and generates a backtrace.
             pub fn from_kind(kind: $error_kind_name) -> $error_name {
-                $error_name(
+                $error_name(Box::new((
                     kind,
                     $crate::State::default(),
-                )
+                )))
             }
 
             /// Constructs a chained error from another error and a kind, and generates a backtrace.
@@ -140,15 +133,15 @@ macro_rules! impl_error_chain_processed {
                 -> $error_name
                 where K: Into<$error_kind_name>
             {
-                $error_name(
+                $error_name(Box::new((
                     kind.into(),
                     $crate::State::new::<$error_name>(error, ),
-                )
+                )))
             }
 
             /// Returns the kind of the error.
             pub fn kind(&self) -> &$error_kind_name {
-                &self.0
+                &(self.0).0
             }
 
             /// Iterates over the error chain.
@@ -158,7 +151,7 @@ macro_rules! impl_error_chain_processed {
 
             /// Returns the backtrace associated with this error.
             pub fn backtrace(&self) -> Option<&$crate::Backtrace> {
-                self.1.backtrace()
+                (self.0).1.backtrace()
             }
 
             /// Extends the error chain with a new entry.
@@ -170,15 +163,15 @@ macro_rules! impl_error_chain_processed {
 
         impl ::std::error::Error for $error_name {
             fn description(&self) -> &str {
-                self.0.description()
+                (self.0).0.description()
             }
 
             #[allow(unknown_lints, unused_doc_comment)]
             fn cause(&self) -> Option<&::std::error::Error> {
-                match self.1.next_error {
+                match (self.0).1.next_error {
                     Some(ref c) => Some(&**c),
                     None => {
-                        match self.0 {
+                        match (self.0).0 {
                             $(
                                 $(#[$meta_foreign_links])*
                                 $error_kind_name::$foreign_link_variant(ref foreign_err) => {
@@ -194,18 +187,19 @@ macro_rules! impl_error_chain_processed {
 
         impl ::std::fmt::Display for $error_name {
             fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
-                ::std::fmt::Display::fmt(&self.0, f)
+                ::std::fmt::Display::fmt(&(self.0).0, f)
             }
         }
 
         $(
             $(#[$meta_links])*
             impl From<$link_error_path> for $error_name {
                 fn from(e: $link_error_path) -> Self {
-                    $error_name(
+                    let e = *e.0;
+                    $error_name(Box::new((
                         $error_kind_name::$link_variant(e.0),
                         e.1,
-                    )
+                    )))
                 }
             }
         ) *
@@ -243,7 +237,7 @@ macro_rules! impl_error_chain_processed {
             type Target = $error_kind_name;
 
             fn deref(&self) -> &Self::Target {
-                &self.0
+                &(self.0).0
             }
         }
 
@@ -305,7 +299,7 @@ macro_rules! impl_error_chain_processed {
 
         impl From<$error_name> for $error_kind_name {
             fn from(e: $error_name) -> Self {
-                e.0
+                (e.0).0
             }
         }
 
@@ -428,13 +422,13 @@ macro_rules! impl_extract_backtrace {
         fn extract_backtrace(e: &(::std::error::Error + Send + 'static))
             -> Option<::std::sync::Arc<$crate::Backtrace>> {
             if let Some(e) = e.downcast_ref::<$error_name>() {
-                return e.1.backtrace.clone();
+                return (e.0).1.backtrace.clone();
             }
             $(
                 $( #[$meta_links] )*
                 {
                     if let Some(e) = e.downcast_ref::<$link_error_path>() {
-                        return e.1.backtrace.clone();
+                        return (e.0).1.backtrace.clone();
                     }
                 }
             ) *