extern "C" fn on mips64 targets does not respect repr(transparent)

[RalfJung]

This is the MIPS version of #115336: on the target mips64-unknown-linux-gnuabi64, all arrays and some tuple types like (i32, f32, i64, f64) have a different ABI when being wrapped in repr(transparent) wrappers, and hence this fails the test I added in #115372.

For instance for [u8; 2], the PassMode becomes

                   mode: Indirect {
                       attrs: ArgAttributes {
                           regular: NoAlias | NoCapture | NonNull | NoUndef,
                           arg_ext: None,
                           pointee_size: Size(2 bytes),
                           pointee_align: Some(
                               Align(1 bytes),
                           ),
                       },
                       extra_attrs: None,
                       on_stack: false,
                   },

But for Wrapper<[u8; 2]> it becomes

                   mode: Cast(
                       CastTarget {
                           prefix: [
                               None,
                               None,
                               None,
                               None,
                               None,
                               None,
                               None,
                               None,
                           ],
                           rest: Uniform {
                               unit: Reg {
                                   kind: Integer,
                                   size: Size(8 bytes),
                               },
                               total: Size(2 bytes),
                           },
                           attrs: ArgAttributes {
                               regular: (empty),
                               arg_ext: None,
                               pointee_size: Size(0 bytes),
                               pointee_align: None,
                           },
                       },
                       false,
                   ),

This also affects mips64el-unknown-linux-gnuabi64, mips64-openwrt-linux-musl, mips64-unknown-linux-muslabi64, mips64el-unknown-linux-muslabi64
Cc @Itus-Shield (listed as maintainer for the openwrt target)

[RalfJung]

As per #115238, all affected targets are tier 3 targets.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-low

[RalfJung]

Here's another example of a repr(transparent) violation on mips64, orthogonal to the first:

struct T(f64, f64, f64);
struct Wrap<T>(T);

#[rustc_abi(assert_eq)]
type Test = (extern "C" fn(T), extern "C" fn(Wrap<T>));

T gets passed in 3 float registers, but Wrap<T> is passed in 3 integer registers. The problem is that this search here doesn't recurse into fields of nested structs.

(I don't know what the C ABI says for this target, i.e. whether it is truly only double fields that appear directly in the struct type that should be passed in registers, and any doubles in nested fields-of-fields really should be ignored. So there might also be an ABI incompatibility with C here.)

[RalfJung]

I'm also seeing this target use PassMode::Cast for unsized types... that makes no sense, right?

[bjorn3]

Yeah, unsized args should always be passed as PassMode::Indirect with on_stack=false.