Fix error[E0518] not displayed for #[inline] on structs inside fns

[nxnfufunezn]

Fixes : #31769
r? @eddyb

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[eddyb]

LGTM, but it needs a compile-fail test, something along the lines of:

fn main() {
    #[inline] struct Foo;
    #[repr(C)] fn foo() {}
}

Both of those compile right now and neither is valid.

[nxnfufunezn]

oops! I always forget about tests..

[nxnfufunezn]

@eddyb

[eddyb]

@bors r+ rollup Thanks!

[bors]

📌 Commit 27f8c53 has been approved by eddyb

[jonas-schievink]

Just noting that this is technically a [breaking-change] (with probably no impact)