Rust compilation issue on new version

[mattklein123]

With the upgrade to 25.12.19 I am seeing a huge amount of errors that look like this:

error[E0282]: type annotations needed
    --> bd-proto/src/flatbuffers/report_generated.rs:5258:49
     |
5258 |       let w: alloc::vec::Vec<_> = x.iter().map(|t| t.pack(_fbb)).collect();_fbb.create_vector(&w)
     |                                                 ^  - type must be known at this point
     |
help: consider giving this closure parameter an explicit type
     |
5258 |       let w: alloc::vec::Vec<_> = x.iter().map(|t: /* Type */| t.pack(_fbb)).collect();_fbb.create_vector(&w)
     |                                                  ++++++++++++

shared-core> flatc --version
flatc version 25.12.19
shared-core> rustc --version
rustc 1.92.0 (ded5c06cf 2025-12-08)

I haven't built up a self contained repro yet but let me know if it's not obvious to you and I can try to create one.

[jtdavis777]

Hey! I don't believe we have a rust expert maintainer active at the moment -- any additional support would be appreciated! A small repro and/or befor/after genned code which demonstrate the issue will never hurt.

[mattklein123]

I'm pretty sure the issue is this PR: #8828

The real problem I think is this:

error[E0433]: failed to resolve: use of unresolved module or unlinked crate `alloc`
   --> bd-proto/src/flatbuffers/common_generated.rs:115:14
    |
115 |   BinaryData(alloc::boxed::Box<BinaryDataT>),
    |              ^^^^^ use of unresolved module or unlinked crate `alloc`

The issue is basically that:

extern crate alloc;

Is put at the top of the generated file but not inside nested modules inside the file.

I don't know the full intent of the change but I would recommend reverting it.

[mattklein123]

cc @csmulhern

[mattklein123]

More specifically an example diff pretty clearly shows the issue:

index bc6fd715..e40ba3fd 100644
--- a/bd-proto/src/flatbuffers/common_generated.rs
+++ b/bd-proto/src/flatbuffers/common_generated.rs
@@ -1,46 +1,20 @@
 // automatically generated by the FlatBuffers compiler, do not modify
-
-
 // @generated
+extern crate alloc;
 
-use core::mem;
-use core::cmp::Ordering;
-
-extern crate flatbuffers;
-use self::flatbuffers::{EndianScalar, Follow};
 
 #[allow(unused_imports, dead_code)]
 pub mod bitdrift_public {
 
-  use core::mem;
-  use core::cmp::Ordering;
-
-  extern crate flatbuffers;
-  use self::flatbuffers::{EndianScalar, Follow};

[csmulhern]

@mattklein123 can you please include a simple example flatbufffer schema that reproduces this issue, and the flatc compiler flags you are using?

[csmulhern]

I have a fix up for review in #8877.

[mattklein123]

Thanks for the fix and looks like you figured it out. FWIW the code in question is public here: https://github.com/bitdriftlabs/shared-core/tree/main/bd-proto. The FBs in question are brought in by submodule and live here: https://github.com/bitdriftlabs/api/tree/8cdf0f8757299a4a13e13f2390f28ba28ef0f620/src/bitdrift_public/fbs

If possible it would be great to do a release with this as we will have to pin to the old version.