Static UNSUPPORTED_FN_PTR_CALLING_CONVENTIONS

pub static UNSUPPORTED_FN_PTR_CALLING_CONVENTIONS: &'static Lint

The unsupported_fn_ptr_calling_conventions lint is output whenever there is a use of a target dependent calling convention on a target that does not support this calling convention on a function pointer.

For example stdcall does not make much sense for a x86_64 or, more apparently, powerpc code, because this calling convention was never specified for those targets.

Example

fn stdcall_ptr(f: extern "stdcall" fn ()) {
    f()
}

This will produce:

warning: the calling convention `"stdcall"` is not supported on this target
  --> $DIR/unsupported.rs:34:15
   |
LL | fn stdcall_ptr(f: extern "stdcall" fn()) {
   |               ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #130260 <https://github.com/rust-lang/rust/issues/130260>
   = note: `#[warn(unsupported_fn_ptr_calling_conventions)]` on by default

Explanation

On most of the targets the behaviour of stdcall and similar calling conventions is not defined at all, but was previously accepted due to a bug in the implementation of the compiler.