Disallow duplicated extern declarations

[klutzy]

mod a {
    extern {
        fn func();
    }
}

mod b {
    extern {
        fn func(i: i8); // different signature
    }
}

Currently rustc accepts this, but I think it should be disallowed to reduce potential mistakes.

[brson]

This would disallow duplicate functions with different signatures, but still allow duplicate with the same?

[klutzy]

I thought identical duplicates are ok because make check-fast needs them. Some rpass tests use same extern fn then they are combined into one crate.

[klutzy]

Modified test case causing llvm assertion error on x64:

mod a {
    extern {
        fn func(); // declare void @func() unnamed_addr
    }
}

mod b {
    struct S {
        a: u64,
        b: u64,
        c: u64,
    }

    extern {
        fn func(s: S); // declare void @func(%"struct.b::S"* byval) unnamed_addr
    }
}

rustc: /media/a/lime/src/rust/src/llvm/include/llvm/Support/Casting.h:240:typename llvm::cast_retty<X, Y*>::ret_type llvm::cast(Y*) [with X = llvm::Argument; Y = llvm::Value; typename llvm::cast_retty<X, Y*>::ret_type = llvm::Argument*]: Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.

In the case, argument has byval attribute.

[klutzy]

cc #12762: it contains a patch to suppress llvm assertion error above. Not really.

[rprichard]

klutzy's test case is causing a different error now on 64-bit Linux:

Attribute after last parameter!
void ()* @func
LLVM ERROR: Broken module found, compilation aborted!

Are duplicate declarations useful/necessary for calling functions like objc_msgSend? IIRC, that function isn't really varargs -- the prototype varies depending upon which ObjC method is invoked. (I discovered this GitHub issue because I did something similar to objc_msgSend.)

[steveklabnik]

Traige: Still getting the same failure as @rprichard , but I'm also on x86-64. Does this fail everywhere now?

[Mark-Simulacrum]

On x86-64 as well, does not reproduce with --crate-type lib and rustc 1.18.0-nightly (bbdaad0dc 2017-04-14). Going to presume the assertions are fixed, but the original issue (multiple externs with the same name) isn't, so leaving open.

[steveklabnik]

Triage: no change

[BurgundyWillow]

The rustc compiler does throw warnings when we use two conflicting function signatures in extern declarations. So, shall we change this to disallow it and instead throw an error?

[bjorn3]

objc_msgSend is declared with multiple signatures in libstd. The signature of this function depends on the message sent. It forwards all arguments to the called method. Declaring it as a vararg function is not valid in AArch64 due to ABI differences.

[madsmtm]

objc_msgSend is declared with multiple signatures in libstd. The signature of this function depends on the message sent. It forwards all arguments to the called method. Declaring it as a vararg function is not valid in AArch64 due to ABI differences.

Since #117910, this is no longer true, instead we explicitly cast the function pointer (which is arguably the more correct behaviour, at least it's what the C headers also declare and expect you to do).