Splat is ignored in symbol mangling, leading to symbol clashes

[teor2345]

I tried this code:

#![allow(incomplete_features)]
#![feature(splat)]

fn main() {
    #[rustfmt::skip]
    let _x: fn(#[splat] (i32,)) = None.unwrap();

    let x: fn((i32,)) = None.unwrap();
    x((1,));
}

These directives seem to increase the chance of the error:

//@ incremental
//@ compile-flags: -C opt-level=0

I expected to see this happen:

No symbol clashes, either the symbol data is the same (and therefore merged), or it is different, and mangled differently.

Instead, this happened:

When I rebuild the file multiple times with small changes, I eventually get:

error: symbol `_RNvMNtCsCRATE_HASH_4core6optionINtB2_6OptionFTlEEuE6unwrapCsCRATE_HASH_14splat_mangling` is already defined

Meta

rustc version:

git main commit f46ec5218fe

In #153697, we decided not to include splat in symbol mangling. This is likely the cause of this error.

@rustbot label +C-bug +F-splat +T-compiler

cc @oli-obk

[teor2345]

@rustbot claim

[teor2345]

We'll also need a legacy symbol mangling variant for this test, which should pass without any fixes.
(The legacy encoding uses the hash of the FnSig, which already includes the splatted index. The v0 encoding currently ignores splatting.)

Destructuring the FnSig in the v0 mangling will make sure additional fields aren't missed in future. (Unless they're packed fields.)