THRIFT-6058: fix Rust codegen for list<union> and set<union> deserialization (#3545)

The Rust code generator produces incorrect deserialization code for
list<UnionType> and set<UnionType> fields. Two bugs:

1. Variable shadowing: the match binding Ok(val) shadows the outer
   list/set variable val, so val.push(val) calls push on the element
   instead of the collection.

2. Missing Box::new(): the collection type is Vec<Box<UnionType>> but
   the element is pushed without wrapping in Box::new().

Fix: rename the match binding to elem and wrap with Box::new().

The single-field deserialization path (render_struct_sync_read) already
handles this correctly; only the collection paths were affected.

Author: santiagomed

compiler/cpp/src/thrift/generate/t_rs_generator.cc

@@ -2013,7 +2013,7 @@ void t_rs_generator::render_list_sync_read(t_list* tlist, const string& list_var
     string read_call(resolved_type + "::read_from_in_protocol(i_prot)");
     f_gen_ << indent() << "match " << read_call << " {" << '\n';
     indent_up();
-    f_gen_ << indent() << "Ok(val) => { " << list_var << ".push(val); }," << '\n';
+    f_gen_ << indent() << "Ok(elem) => { " << list_var << ".push(Box::new(elem)); }," << '\n';
     f_gen_ << indent() << "Err(thrift::Error::Protocol(ref e)) if e.kind == ProtocolErrorKind::UnknownUnionVariant => { continue; }," << '\n';
     f_gen_ << indent() << "Err(e) => return Err(e)," << '\n';
     indent_down();
@@ -2048,7 +2048,7 @@ void t_rs_generator::render_set_sync_read(t_set* tset, const string& set_var) {
     string read_call(resolved_type + "::read_from_in_protocol(i_prot)");
     f_gen_ << indent() << "match " << read_call << " {" << '\n';
     indent_up();
-    f_gen_ << indent() << "Ok(val) => { " << set_var << ".insert(val); }," << '\n';
+    f_gen_ << indent() << "Ok(elem) => { " << set_var << ".insert(Box::new(elem)); }," << '\n';
     f_gen_ << indent() << "Err(thrift::Error::Protocol(ref e)) if e.kind == ProtocolErrorKind::UnknownUnionVariant => { continue; }," << '\n';
     f_gen_ << indent() << "Err(e) => return Err(e)," << '\n';
     indent_down();