Breaking change in wasm32-unknown-unknown's ABI on nightly

[alexcrichton]

Some tests in the wasm-bindgen project have started failing in nightly, and bisection points to #80594 as the culprit. The minimization of this looks like:

#[repr(C)]
pub struct Foo {
    pub a: u32,
    pub b: u32,
    pub c: u32,
}

#[no_mangle]
pub extern "C" fn foo(a: Foo) {}

where before this compiles as:

$ rustc +before foo.rs --crate-type cdylib --target wasm32-unknown-unknown -O && wasm2wat foo.wasm | grep 'func..foo'
  (func $foo (type 0) (param i32 i32 i32))
  (export "foo" (func $foo))
$ rustc +after foo.rs --crate-type cdylib --target wasm32-unknown-unknown -O && wasm2wat foo.wasm | grep 'func..foo'
  (func $foo (type 0) (param i32))
  (export "foo" (func $foo))

[bjorn3]

That must have been a93dace. This all types with Abi::Aggregate to become PassMode::Indirect. Later abi adjustments could then turn it into PassMode::Cast as necessary. I thought abi adjustments would never leave the dummy PassMode::Direct in place for Abi::Aggregate, but it seems not for wasm. I don't think this should ever happen as it will cause LLVM to do it's own abi adjustments. The whole point of that PR was to make it easier for Cranelift to match the abi of LLVM, which those abi adjustments would make harder.

I think there will need to be an arg.cast_to(...) call inserted in the abi handling of wasm. What kind of types does wasm-bindgen produce?

#[repr(C)]
pub struct Bar {
    pub foo: u8,
}

#[repr(C)]
pub struct Foo {
    pub a: Bar,
    pub b: u32,
    pub c: u32,
}

#[no_mangle]
pub extern "C" fn foo(a: Foo) {}

produces (func $foo (type 0) (param i32 i32 i32 i32 i32 i32)), so I can't think of a clear rule describing what is used by LLVM. The easiest fix is maybe to special case wasm32-unknown-unknown at

rust/compiler/rustc_middle/src/ty/layout.rs

Line 2754 in 84864bf

to use PassMode::Direct. This only affects wasm, which isn't supported by cg_clif anyway.

[bjorn3]

#81388 will hopefully fix this.

[hameerabbasi]

Assigning P-high as discussed on Zulip.

[Manishearth]

@alexcrichton do you remember which of the tests were failing here? I was under the impression wasm-bindgen didn't support passing structs as arguments, and instead passed them by-reference.

I'm working with passing structs by-value over Wasm and am dealing with similar ABI weirdnesses (rust-diplomat/diplomat#657), and was curious if wasm-bindgen had already solve the problem somewhere but I can't get it to ever pass any aggregate by-value.