Pre-allocate capacity for maps in json! macro.

Attempt to address issue serde-rs#810.

This implementation works by expanding the contents of the map twice,
first to increment a capacity variable for each element, then actually
inserting them.

In debug mode, this implementation does not appear to optimize out the incrementations
of the capacity variable, so it is likely not a good solution.

src/macros.rs

@@ -234,6 +234,112 @@ macro_rules! json_internal {
         json_internal!(@object $object ($($key)* $tt) ($($rest)*) ($($rest)*));
     };
 
+
+    //////////////////////////////////////////////////////////////////////////
+    // TT muncher for counting the elements inside of an object {...}.
+    // Each entry is replaced with `$capacity += 1;`
+    //
+    // Must be invoked as: json_internal!(@object_capacity $capacity () ($($tt)*) ($($tt)*))
+    //
+    // We require two copies of the input tokens so that we can match on one
+    // copy and trigger errors on the other copy.
+    //////////////////////////////////////////////////////////////////////////
+
+    // Done.
+    (@object_capacity $capacity:ident () () ()) => {};
+
+    // Current entry followed by trailing comma.
+    (@object_capacity $capacity:ident [$($key:tt)+] ($value:expr) , $($rest:tt)*) => {
+        $capacity += 1;
+        json_internal!(@object_capacity $capacity () ($($rest)*) ($($rest)*));
+    };
+
+    // Current entry followed by unexpected token.
+    (@object_capacity $capacity:ident [$($key:tt)+] ($value:expr) $unexpected:tt $($rest:tt)*) => {
+        json_unexpected!($unexpected);
+    };
+
+    // Insert the last entry without trailing comma.
+    (@object_capacity $capacity:ident [$($key:tt)+] ($value:expr)) => {
+        $capacity += 1;
+    };
+
+    // Next value is `null`.
+    (@object_capacity $capacity:ident ($($key:tt)+) (: null $($rest:tt)*) $copy:tt) => {
+        json_internal!(@object_capacity $capacity [$($key)+] (json_internal!(null)) $($rest)*)
+    };
+
+    // Next value is `true`.
+    (@object_capacity $capacity:ident ($($key:tt)+) (: true $($rest:tt)*) $copy:tt) => {
+        json_internal!(@object_capacity $capacity [$($key)+] (json_internal!(true)) $($rest)*)
+    };
+
+    // Next value is `false`.
+    (@object_capacity $capacity:ident ($($key:tt)+) (: false $($rest:tt)*) $copy:tt) => {
+        json_internal!(@object_capacity $capacity [$($key)+] (json_internal!(false)) $($rest)*)
+    };
+
+    // Next value is an array.
+    (@object_capacity $capacity:ident ($($key:tt)+) (: [$($array:tt)*] $($rest:tt)*) $copy:tt) => {
+        json_internal!(@object_capacity $capacity [$($key)+] (json_internal!([$($array)*])) $($rest)*)
+    };
+
+    // Next value is a map.
+    (@object_capacity $capacity:ident ($($key:tt)+) (: {$($map:tt)*} $($rest:tt)*) $copy:tt) => {
+        json_internal!(@object_capacity $capacity [$($key)+] (json_internal!({$($map)*})) $($rest)*)
+    };
+
+    // Next value is an expression followed by comma.
+    (@object_capacity $capacity:ident ($($key:tt)+) (: $value:expr , $($rest:tt)*) $copy:tt) => {
+        json_internal!(@object_capacity $capacity [$($key)+] (json_internal!($value)) , $($rest)*)
+    };
+
+    // Last value is an expression with no trailing comma.
+    (@object_capacity $capacity:ident ($($key:tt)+) (: $value:expr) $copy:tt) => {
+        json_internal!(@object_capacity $capacity [$($key)+] (json_internal!($value)))
+    };
+
+    // Missing value for last entry. Trigger a reasonable error message.
+    (@object_capacity $capacity:ident ($($key:tt)+) (:) $copy:tt) => {
+        // "unexpected end of macro invocation"
+        json_internal!()
+    };
+
+    // Missing colon and value for last entry. Trigger a reasonable error
+    // message.
+    (@object_capacity $capacity:ident ($($key:tt)+) () $copy:tt) => {
+        // "unexpected end of macro invocation"
+        json_internal!()
+    };
+
+    // Misplaced colon. Trigger a reasonable error message.
+    (@object_capacity $capacity:ident () (: $($rest:tt)*) ($colon:tt $($copy:tt)*)) => {
+        // Takes no arguments so "no rules expected the token `:`".
+        json_unexpected!($colon)
+    };
+
+    // Found a comma inside a key. Trigger a reasonable error message.
+    (@object_capacity $capacity:ident ($($key:tt)*) (, $($rest:tt)*) ($comma:tt $($copy:tt)*)) => {
+        // Takes no arguments so "no rules expected the token `,`".
+        json_unexpected!($comma)
+    };
+
+    // Key is fully parenthesized. This avoids clippy double_parens false
+    // positives because the parenthesization may be necessary here.
+    (@object_capacity $capacity:ident () (($key:expr) : $($rest:tt)*) $copy:tt) => {
+        json_internal!(@object_capacity $capacity ($key) (: $($rest)*) (: $($rest)*))
+    };
+
+    // Refuse to absorb colon token into key expression.
+    (@object_capacity $capacity:ident ($($key:tt)*) (: $($unexpected:tt)+) $copy:tt) => {
+        json_expect_expr_comma!($($unexpected)+)
+    };
+
+    // Munch a token into the current key.
+    (@object_capacity $capacity:ident ($($key:tt)*) ($tt:tt $($rest:tt)*) $copy:tt) => {
+        json_internal!(@object_capacity $capacity ($($key)* $tt) ($($rest)*) ($($rest)*))
+    };
+
     //////////////////////////////////////////////////////////////////////////
     // The main implementation.
     //
@@ -266,7 +372,9 @@ macro_rules! json_internal {
 
     ({ $($tt:tt)+ }) => {
         $crate::Value::Object({
-            let mut object = $crate::Map::new();
+            let mut capacity = 0;
+            json_internal!(@object_capacity capacity () ($($tt)+) ($($tt)+));
+            let mut object = $crate::Map::with_capacity(capacity);
             json_internal!(@object object () ($($tt)+) ($($tt)+));
             object
         })