[rust] Add FlatBufferBuilder::force_defaults API (#5946)

* [rust] Add force_defaults method FlatBufferBuilder

This works just like the same method already available in other
languages.

* Add binary format test for force_defaults

rust/flatbuffers/src/builder.rs

@@ -52,6 +52,7 @@ pub struct FlatBufferBuilder<'fbb> {
     finished: bool,
 
     min_align: usize,
+    force_defaults: bool,
 
     _phantom: PhantomData<&'fbb ()>,
 }
@@ -85,6 +86,7 @@ impl<'fbb> FlatBufferBuilder<'fbb> {
             finished: false,
 
             min_align: 0,
+            force_defaults: false,
 
             _phantom: PhantomData,
         }
@@ -148,10 +150,9 @@ impl<'fbb> FlatBufferBuilder<'fbb> {
     #[inline]
     pub fn push_slot<X: Push + PartialEq>(&mut self, slotoff: VOffsetT, x: X, default: X) {
         self.assert_nested("push_slot");
-        if x == default {
-            return;
+        if x != default || self.force_defaults {
+            self.push_slot_always(slotoff, x);
         }
-        self.push_slot_always(slotoff, x);
     }
 
     /// Push a Push'able value onto the front of the in-progress data, and
@@ -327,6 +328,18 @@ impl<'fbb> FlatBufferBuilder<'fbb> {
         WIPOffset::new(self.push::<UOffsetT>(items.len() as UOffsetT).value())
     }
 
+    /// Set whether default values are stored.
+    ///
+    /// In order to save space, fields that are set to their default value
+    /// aren't stored in the buffer. Setting `force_defaults` to `true`
+    /// disables this optimization.
+    ///
+    /// By default, `force_defaults` is `false`.
+    #[inline]
+    pub fn force_defaults(&mut self, force_defaults: bool) {
+        self.force_defaults = force_defaults;
+    }
+
     /// Get the byte slice for the data that has been written, regardless of
     /// whether it has been finished.
     #[inline]

tests/rust_usage_test/tests/integration_test.rs

@@ -2460,6 +2460,25 @@ mod byte_layouts {
         ]);
     }
 
+    #[test]
+    fn layout_09b_vtable_with_one_default_bool_force_defaults() {
+        let mut b = flatbuffers::FlatBufferBuilder::new();
+        check(&b, &[]);
+        let off = b.start_table();
+        check(&b, &[]);
+        b.force_defaults(true);
+        b.push_slot(fi2fo(0), false, false);
+        b.end_table(off);
+        check(&b, &[
+            6, 0, // vtable bytes
+            8, 0, // length of object including vtable offset
+            7, 0, // start of bool value
+            6, 0, 0, 0, // offset for start of vtable (int32)
+            0, 0, 0, // padded to 4 bytes
+            0, // bool value
+      ]);
+    }
+
     #[test]
     fn layout_10_vtable_with_one_int16() {
         let mut b = flatbuffers::FlatBufferBuilder::new();