Overeager warning for clashing external declarations in different wasm import modules

[pchickey]

Rustc 1.48.0 gives an incorrect warning about an external symbol being redeclared with a different signature, when the external symbols are in different wasm_import_module namespaces.

I tried this code:

pub mod a {
    #[link(wasm_import_module = "a")]
    extern "C" {
        pub fn foo();
    }
}

pub mod b {
    #[link(wasm_import_module = "b")]
    extern "C" {
        pub fn foo() -> usize;
    }
}

I expected to see this happen: Compile without warnings

Instead, this happened:

warning: `foo` redeclared with a different signature
  --> src/lib.rs:11:9
   |
4  |         pub fn foo();
   |         ------------- `foo` previously declared here
...
11 |         pub fn foo() -> usize;
   |         ^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
   |
   = note: `#[warn(clashing_extern_declarations)]` on by default
   = note: expected `unsafe extern "C" fn()`
              found `unsafe extern "C" fn() -> usize`

Meta

rustc --version --verbose:

rustc 1.48.0 (7eac88abb 2020-11-16)
binary: rustc
commit-hash: 7eac88abb2e57e752f3302f02be5f3ce3d7adfb4
commit-date: 2020-11-16
host: x86_64-unknown-linux-gnu
release: 1.48.0
LLVM version: 11.0

The same behavior exists on nightly as well:

rustc 1.50.0-nightly (825637983 2020-11-18)
binary: rustc
commit-hash: 8256379832b5ecb7f71e8c5e2018446482223c12
commit-date: 2020-11-18
host: x86_64-unknown-linux-gnu
release: 1.50.0-nightly

Backtrace

<backtrace>

[pchickey]

cc @alexcrichton

[alexcrichton]

cc @jumbatm, would you be willing to help take a look at this? (it looks like you added this way-back-when in #70946)

On wasm targets there's special logic for symbol mangling which is why this should work but not actually generate a warning. Do you know if it's possible to call that logic from here perhaps? (would be able to consolidate "what's the name of this symbol" logic too!)

[jumbatm]

Sure thing! I'm currently in the process of moving, but will be able to get onto this on the weekend :)

@rustbot claim

[alexcrichton]

No worries, and thanks! If you run out of time/steam I'm happy to look into this as well