Issue an error or warning when using no_mangle on language items

[alexandruradovici]

This pull requests adds the code to issue an error or a warning when using no_mangle on language items. This should detail why the undefined symbol error is issued for the code described in #139923.

The pull request adds two ui tests, one testing the error and the other one the warning.

I would love some feedback here, as I am not sure that the error and warning are issues using the right API.

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[alexandruradovici]

r? @bjorn3

[alexandruradovici]

This fails due to a test that defines #[panic_handler] in a wrong place that also defines #[no_mangle].

#[panic_handler] //~ ERROR `panic_impl` lang item must be applied to a function
#[no_mangle]
static X: u32 = 42;

This somehow seems to be an error priority issue, the error that this pull request issues is correct.

I am not sure exactly how to avoid this, as I don't think checking that the target is a function is correct, as some language items are not functions.

[Noratrieb]

This fails due to a test that defines #[panic_handler] in a wrong place that also defines #[no_mangle].

#[panic_handler] //~ ERROR `panic_impl` lang item must be applied to a function
#[no_mangle]
static X: u32 = 42;

This somehow seems to be an error priority issue, the error that this pull request issues is correct.

I am not sure exactly how to avoid this, as I don't think checking that the target is a function is correct, as some language items are not functions.

you can probably just fix the test if you look at the history of the test and ensure that what it's trying to test is still tested

[alexandruradovici]

Thank you @Noratrieb

I deleted the no_mangle attribute from the ui/panic-handler/panic-handler-wrong-location. I am not sure why that was placed there, but as it now is an error, I think this should be correct. I might be wrong though, so any feedback is welcome.

My understanding is that the test should verify that the panic_handler attribute is used on a function. The no_mangle attribute should impact it in any way. The test still issues the same error, panic_impl lang item must be applied to a function, when the no_mangle is deleted.