Bitfield setter for enum-typed field emits invalid `as` cast (E0605), regression since 0.73.0

[raiko86]

bindgen 0.73.0/0.73.1 generates a bitfield setter that tries to as-cast an
enum-typed value to u32, which fails to compile with E0605: non-primitive cast. The corresponding getter for the same field correctly uses
mem::transmute instead of as, so the getter/setter pair is inconsistent.

This regressed between 0.72.1 (works) and 0.73.0 (broken). It breaks the Mesa
mesa-git build (rusticl / OpenCL frontend), which uses a C bitfield of enum
type:

enum pipe_resource_usage { PIPE_USAGE_DEFAULT, PIPE_USAGE_IMMUTABLE,
                            PIPE_USAGE_DYNAMIC, PIPE_USAGE_STREAM,
                            PIPE_USAGE_STAGING };

struct pipe_resource {
    ...
    unsigned compression_rate:4;
    enum pipe_resource_usage usage:4;
    ...
};

Environment

• bindgen / bindgen-cli: 0.73.1 (also reproduces on 0.73.0)
• Last known-good version: 0.72.1
• rustc: 1.98.1
• OS: Arch Linux x86_64
• libclang: system clang (Arch extra package)

Actual generated code (from Mesa, verified against bindgen 0.73.1)

#[inline]
pub fn usage(&self) -> pipe_resource_usage {
    unsafe { ::std::mem::transmute(self._bitfield_2.get_const::<4usize, 4u8>() as u32) }
}
#[inline]
pub fn set_usage(&mut self, val: pipe_resource_usage) {
    let val: u32 = val as _;   // <-- E0605 here
    self._bitfield_2.set_const::<4usize, 4u8>(val as u64)
}

Compiler error:

error[E0605]: non-primitive cast: `pipe_resource_usage` as `u32`
  --> rusticl_mesa_bindings.rs:41575:24
   |
41575 |         let val: u32 = val as _;
      |                        ^^^^^^^^ an `as` expression can only be used to
      |                                 convert between primitive types or to
      |                                 coerce to a specific trait object

The getter uses transmute (which is correct for a fieldless C-like enum),
but the setter and the _raw/new_bitfield_* constructor helpers use as _,
which does not work for enum types — only for values already known to be
primitive.

How to reproduce with the real project

git clone https://gitlab.freedesktop.org/mesa/mesa.git
cd mesa
git checkout c3b008c1ba01d455351b762253ef44c3ca19653f   # commit used when this was found
meson setup build -Dgallium-drivers=radeonsi -Dgallium-rusticl=true -Dllvm=enabled
ninja -C build src/gallium/frontends/rusticl/rusticl_mesa_bindings.rs
ninja -C build src/gallium/frontends/rusticl/libmesa_rust_gen.rlib

The rust.bindgen() invocation for this target
(src/gallium/frontends/rusticl/meson.build) uses:

--no-convert-floats
--default-enum-style rust
--with-derive-partialeq --with-derive-eq
--with-derive-partialord --with-derive-ord
--with-derive-hash --with-derive-default
--anon-fields-prefix anon_

What I tried (did NOT reproduce in isolation)

I tried to shrink this to a minimal standalone header:

enum Color { RED = 0, GREEN = 1, BLUE = 2 };
struct Foo {
    unsigned other : 4;
    enum Color color : 4;
};

Run with bindgen repro.h --default-enum-style rust (with and without all the
mesa --with-derive-* flags): this generates the same-looking
let val: u32 = val as _; setter code, but in this minimal case
Color is a plain fieldless enum and the as cast does compile
successfully — no error. So the bug seems to depend on something about the
larger translation unit / real header graph (Mesa's rusticl_mesa_bindings.h
pulls in many headers where pipe_resource_usage and pipe_map_flags are
also used as ordinary (non-bitfield) types in dozens of function signatures
across other headers, and are forward/re-referenced in several places). I did
not have time to narrow down the exact trigger further — possibly related to
enum type canonicalization changes in #3346 ("Do not use the canonical
declaration for enum type building"), but I can't confirm that without more
digging. Flagging this here so maintainers with more context on that change
can take a look.

Expected behavior

The setter (and _raw/new_bitfield_* helpers) for an enum-typed bitfield
should use the same conversion mechanism as the getter (e.g. a cast through
the underlying representation via transmute, or requiring the enum to
implement Into<u32>), not a bare as _ cast.

Impact

This currently breaks compilation of Mesa's rusticl (OpenCL) frontend
whenever it's built against bindgen >= 0.73.0, for anyone building
mesa-git-style packages (e.g. Arch Linux AUR mesa-amdonly-gaming-git,
and presumably any other rusticl build using a newer bindgen).

[emilio]

This reproduces with --default-enum-style newtype, can you confirm that somehow that enum ends up with a newtype representation (due to some of the bindgen flags or so?)

[emilio]

Regression from 01d65d9.

[emilio]

cc @armoha

Probably better to revert and just ignore the unnecessary_transmute warnings that trying to get codegen to be warning free? But open to other suggestions.

[emilio]

I put up the proposal above in #3455. I'll merge in the next couple days if there aren't better alternatives that don't make the bitfields code more complicated, I think just silencing the lint is fine tbh.

[freswa]

@emilio I'm maintaining rust-bindgen at Arch Linux. Right now we're stuck with 0.73.1 that has this bug. We can revert as you did, but that's a huge one. Could you create a new release to include the fix?

[emilio]

Yes, sure, that's the plan, will try to get to it today :)