rustc panic on unwrap in rustc_codegen_llvm::llvm::diagnostic::Diagnostic::unpack when using custom target

[thepowersgang]

When using this custom target to compile libcore (and I'd assume any other crate), rustc ICEs on an unwrap in the above named function.

This unwrap at diagnostic.rs:88 seems to be only able to fail if there's a null reference in LLVM (or in the rustc shim)

[thepowersgang]

Correction: The original diagnosis was wrong, the crash is in InlineAsmDiagnostic::unpack. it's expecting instruction to be non-None/null, but LLVM appears to permit this to be nullptr

Reference:

• https://github.com/rust-lang/rust/blob/master/src/librustc_codegen_llvm/llvm/diagnostic.rs#L120
• https://github.com/llvm-mirror/llvm/blob/88ab670/include/llvm/IR/DiagnosticInfo.h#L162

[nikic]

There was an attempt to fix this at #55193, which ran into an assertion on Windows.