Exporting a nonexistent item does not cause compilation to fail

[bstrie]

Here's the revised test case from #1908. It used to just crash with a poor error message, but now it doesn't fail at all despite the lack of anything named output_styl.

export output_format;
export output_styl;
export config;

#[doc = "The type of document to output"]
enum output_format {
    #[doc = "Markdown"]
    markdown,
    #[doc = "HTML, via markdown and pandoc"]
    pandoc_html
}

#[doc = "How to organize the output"]
enum output_style {
    #[doc = "All in a single document"]
    one_doc,
    #[doc = "Each module in its own document"]
    doc_per_mod
}

#[doc = "The configuration for this rustdoc session"]
type config = {
    output_dir: &str,
    output_format: output_format,
    output_style: output_style,
    pandoc_cmd: option<&str>
};

Compiled with rustc --lib 1908.rs.

[jdm]

Traditionally that has simply triggered a warning message, not a failure as far as I know.

[catamorphism]

I'll look at this too. I know it has been just a warning in the past, but IMO it should be a failure.

[catamorphism]

Postponed because we're replacing exports with pub qualifiers on individual items, as per #1893.

[catamorphism]

Obsolete now that #1893 is implemented. You can't export a non-existent item, since an export is part of an item itself. What joy!