Fixes Rust code generation for single file output when using namespaces (#8877)

* Adds tests for Rust single file mode

All existing tests only compile Rust code using --rust-module-root-file.

* Adds standalone include tests for Rust

The imports for these tests have been moved to their own file, as the existing intergration_test.rs file hides compilation issues from code generation due to symbols brought into scope outside of the generated code (e.g. `extern crate alloc`).

* Declare alloc crate in every Rust namespace

When performing code generation within a single file, extern crate alloc needs to be delcared to bring alloc into scope within every inner namespace.

* Regenerates generated schemas

Author: csmulhern

goldens/rust/basic_generated.rs

@@ -1,14 +1,16 @@
 // automatically generated by the FlatBuffers compiler, do not modify
 // @generated
-extern crate alloc;
 
+extern crate alloc;
 
 #[allow(unused_imports, dead_code)]
 pub mod flatbuffers {
 
+extern crate alloc;
 #[allow(unused_imports, dead_code)]
 pub mod goldens {
 
+extern crate alloc;
 
 pub enum GalaxyOffset {}
 #[derive(Copy, Clone, PartialEq)]

scripts/generate_code.py

@@ -85,6 +85,11 @@ def glob(path, pattern):
     "--gen-name-strings",
     "--rust-module-root-file",
 ]
+RUST_STANDALONE_OPTS = BASE_OPTS + [
+    "--rust",
+    "--gen-all",
+    "--gen-name-strings",
+]
 RUST_SERIALIZE_OPTS = BASE_OPTS + [
     "--rust",
     "--gen-all",
@@ -288,13 +293,25 @@ def glob(path, pattern):
     schema="include_test/include_test1.fbs",
 )
 
+flatc(
+    RUST_STANDALONE_OPTS,
+    include="include_test",
+    schema="include_test/include_test1.fbs",
+)
+
 flatc(
     RUST_OPTS,
     prefix="include_test2",
     include="include_test",
     schema="include_test/sub/include_test2.fbs",
 )
 
+flatc(
+    RUST_STANDALONE_OPTS,
+    include="include_test",
+    schema="include_test/sub/include_test2.fbs",
+)
+
 flatc(
     BINARY_OPTS + ["--bfbs-filenames", str(tests_path)],
     include="include_test",

src/idl_gen_rust.cpp

@@ -424,7 +424,6 @@ class RustGenerator : public BaseGenerator {
     code_.Clear();
     code_ += "// " + std::string(FlatBuffersGeneratedWarning());
     code_ += "// @generated";
-    code_ += "extern crate alloc;";
 
     assert(!cur_name_space_);
 
@@ -3006,6 +3005,7 @@ class RustGenerator : public BaseGenerator {
           "use self::serde::ser::{Serialize, Serializer, SerializeStruct};";
       code_ += "";
     }
+    code_ += "extern crate alloc;";
   }
 
   // Set up the correct namespace. This opens a namespace if the current