Emit warning on non-FFI safe spine types

[IzaacMammadov]

The warning will look like this underlined on the type in the spine:

extern block uses type std::option::Option<flux::flux_timing::Nanos>, which is not FFI-safe
consider adding a #[repr(C)], #[repr(transparent)], or integer #[repr(...)] attribute to this enum
enum has no representation hint
#[warn(improper_ctypes)] on by default (rustc improper_ctypes)

In this case, it's pointing out that one of the fields in a type of the spine is an Option which is not FFI-safe.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.

[devin-ai-integration]

Devin Review found 1 new potential issue.

View 4 additional findings in Devin Review.

[devin-ai-integration]

🟡 Generated FFI check function names lack struct-name scoping, causing duplicate symbol errors when multiple #[from_spine] structs exist in the same module

The generated extern "C" function names use only the field name (_ffi_check_{field_ident}), not the struct name. If two #[from_spine] structs in the same module share any field name (e.g., every spine struct has tile_info: ShmemData<TileInfo>), the macro expansions produce duplicate extern function declarations (fn _ffi_check_tile_info(...)) in the module namespace, triggering a Rust E0428 duplicate definition error.

Example collision

Two structs in the same module:

#[from_spine("app1")]
struct Spine1 { pub tile_info: ShmemData<TileInfo>, pub qa: SpineQueue<MsgA> }
#[from_spine("app2")]
struct Spine2 { pub tile_info: ShmemData<TileInfo>, pub qb: SpineQueue<MsgB> }

Both expand to:

unsafe extern "C" { fn _ffi_check_tile_info(var: *const TileInfo); ... }
unsafe extern "C" { fn _ffi_check_tile_info(var: *const TileInfo); ... }

This is a duplicate definition error.

Prompt for agents

The generated FFI check function names at two locations (lines 172 and 322 in crates/spine-derive/src/lib.rs) use format_ident!("_ffi_check_{}", field_ident) which only incorporates the field name. To avoid duplicate symbol errors when multiple from_spine structs coexist in the same module, the struct name should be included in the generated function name. Change both occurrences to format_ident!("_ffi_check_{}_{}", struct_ident, field_ident). The struct_ident variable is already in scope at both locations.

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