error on empty export_name

[folkertdev]

fixes #155495

Using an empty string as the name makes LLVM make up a name. However this name can be inconsistent between compilation units, which is UB and can cause linking errors, and some parts of LLVM just crash on the empty name (see the linked issue).

As far as we know there is only one valid pattern that could use this, a #[used] static that is not referenced by the program at all. That is not UB, but the export_name is not required for that to work, just normal rust name mangling would do fine.

Technically this is a breaking change, but it seems unlikely that this actually breaks code in the wild that wasn't already broken. I'll leave it up to T-lang to determine what is required here (crater run, FCW, ...), but my gut feeling is that we could just merge this and nobody would notice.

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

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[Urgau]

Should we consider a symbol name with only white spaces as empty?

[folkertdev]

Hmm, I don't know that it's useful at all but it apparently does work https://godbolt.org/z/dMqhK6hbx

" ":
.Lfunc_begin0:
        .cfi_startproc
        .file   1 "/app" "example.rs"
        .loc    1 2 16 prologue_end