rust: Allow for usage in no_std environment (#6989)

tests/RustTest.sh

@@ -35,6 +35,8 @@ cd ./rust_usage_test
 cargo test $TARGET_FLAG -- --quiet
 check_test_result "Rust tests"
 
+cargo test $TARGET_FLAG --no-default-features --features no_std -- --quiet
+check_test_result "Rust tests (no_std)"
 
 cargo run $TARGET_FLAG --bin=flatbuffers_alloc_check
 check_test_result "Rust flatbuffers heap alloc test"

tests/rust_usage_test/Cargo.toml

@@ -6,11 +6,16 @@ authors = ["Robert Winslow <hello@rwinslow.com>",
            "FlatBuffers Maintainers"]
 
 [dependencies]
-flatbuffers = { path = "../../rust/flatbuffers" }
+flatbuffers = { path = "../../rust/flatbuffers", default-features = false }
 flexbuffers = { path = "../../rust/flexbuffers" }
 serde_derive = "1.0"
 serde = "1.0"
 serde_bytes = "0.11"
+libc_alloc = { version = "1.0.3", optional = true }
+
+[features]
+default = ["flatbuffers/default"]
+no_std = ["flatbuffers/no_std", "libc_alloc"]
 
 [[bin]]
 name = "monster_example"

tests/rust_usage_test/tests/integration_test.rs

@@ -15,6 +15,10 @@
  * limitations under the License.
  */
 
+#[cfg(feature = "no_std")]
+#[global_allocator]
+static ALLOCATOR: libc_alloc::LibcAlloc = libc_alloc::LibcAlloc;
+
 #[macro_use]
 #[cfg(not(miri))] // slow.
 extern crate quickcheck;