Fix create_vector_of_strings to use the stack, and test it. (#5074)

rust/flatbuffers/src/builder.rs

@@ -29,6 +29,8 @@ use vtable::{VTable, field_index_to_field_offset};
 use vtable_writer::VTableWriter;
 use vector::{SafeSliceAccess, Vector};
 
+pub const N_SMALLVEC_STRING_VECTOR_CAPACITY: usize = 16;
+
 #[derive(Clone, Copy, Debug)]
 struct FieldLoc {
     off: UOffsetT,
@@ -268,10 +270,12 @@ impl<'fbb> FlatBufferBuilder<'fbb> {
     #[inline]
     pub fn create_vector_of_strings<'a, 'b>(&'a mut self, xs: &'b [&'b str]) -> WIPOffset<Vector<'fbb, ForwardsUOffset<&'fbb str>>> {
         self.assert_not_nested("create_vector_of_strings can not be called when a table or vector is under construction");
-        // internally, smallvec can be a stack-allocated or heap-allocated vector.
-        // we expect it to usually be stack-allocated.
-        let mut offsets: smallvec::SmallVec<[WIPOffset<&str>; 0]> = smallvec::SmallVec::with_capacity(xs.len());
+        // internally, smallvec can be a stack-allocated or heap-allocated vector:
+        // if xs.len() > N_SMALLVEC_STRING_VECTOR_CAPACITY then it will overflow to the heap.
+        let mut offsets: smallvec::SmallVec<[WIPOffset<&str>; N_SMALLVEC_STRING_VECTOR_CAPACITY]> = smallvec::SmallVec::with_capacity(xs.len());
         unsafe { offsets.set_len(xs.len()); }
+
+        // note that this happens in reverse, because the buffer is built back-to-front:
         for (i, &s) in xs.iter().enumerate().rev() {
             let o = self.create_string(s);
             offsets[i] = o;

tests/rust_usage_test/bin/alloc_check.rs

@@ -35,6 +35,8 @@ pub use monster_test_generated::my_game;
 // verbatim from the test suite:
 fn create_serialized_example_with_generated_code(builder: &mut flatbuffers::FlatBufferBuilder) {
     let mon = {
+        let _ = builder.create_vector_of_strings(&["these", "unused", "strings", "check", "the", "create_vector_of_strings", "function"]);
+
         let s0 = builder.create_string("test1");
         let s1 = builder.create_string("test2");
         let fred_name = builder.create_string("Fred");
@@ -79,7 +81,7 @@ fn main() {
         create_serialized_example_with_generated_code(builder);
     }
 
-    // reset the builder, clearing its heap-allocted memory:
+    // reset the builder, clearing its heap-allocated memory:
     builder.reset();
 
     {