Patch flatbuffers to fix unused variable warning that occurs with some C++ compiler versions (#1322)

Author: AlmostMatt

cmake/external/flatbuffers.cmake

@@ -19,14 +19,17 @@ if(TARGET flatbuffers OR NOT DOWNLOAD_FLATBUFFERS)
 endif()
 
 set(version 99aa1ef21dd9dc3f9d4fb0eb82f4b59d0bb5e4c5)
+set(patch_file
+  ${CMAKE_CURRENT_LIST_DIR}/../../scripts/git/patches/flatbuffers/0001-remove-unused-var.patch)
 
 ExternalProject_Add(
   flatbuffers
 
-  DOWNLOAD_DIR ${FIREBASE_DOWNLOAD_DIR}
-  DOWNLOAD_NAME flatbuffers-${version}.tar.gz
-  URL https://github.com/google/flatbuffers/archive/${version}.tar.gz
+  DOWNLOAD_COMMAND
+    COMMAND git init flatbuffers
+    COMMAND cd flatbuffers && git fetch --depth=1 https://github.com/google/flatbuffers.git ${version} && git reset --hard FETCH_HEAD
 
+  PATCH_COMMAND git apply ${patch_file} && git gc --aggressive
   PREFIX ${PROJECT_BINARY_DIR}
 
   CONFIGURE_COMMAND ""

scripts/git/patches/flatbuffers/0001-remove-unused-var.patch

@@ -0,0 +1,32 @@
+From b08adb291f3ae0a2626d50eb8abecbc3a0de63d9 Mon Sep 17 00:00:00 2001
+From: "almostmatt@google.com" <almostmatt@google.com>
+Date: Fri, 21 Apr 2023 14:31:06 -0400
+Subject: [PATCH] remove unused var
+
+---
+ src/idl_gen_rust.cpp | 2 --
+ 1 file changed, 2 deletions(-)
+
+diff --git a/src/idl_gen_rust.cpp b/src/idl_gen_rust.cpp
+index 55b8439b..48539d52 100644
+--- a/src/idl_gen_rust.cpp
++++ b/src/idl_gen_rust.cpp
+@@ -406,7 +406,6 @@ class RustGenerator : public BaseGenerator {
+     // example: f(A, D::E)          -> super::D::E
+     // does not include leaf object (typically a struct type).
+ 
+-    size_t i = 0;
+     std::stringstream stream;
+ 
+     auto s = src->components.begin();
+@@ -417,7 +416,6 @@ class RustGenerator : public BaseGenerator {
+       if (*s != *d) { break; }
+       ++s;
+       ++d;
+-      ++i;
+     }
+ 
+     for (; s != src->components.end(); ++s) { stream << "super::"; }
+-- 
+2.40.1.606.ga4b1b128d6-goog
+