Rustc doesn't emit `unused_doc_comments` warning for documentation on generic parameters

[jyn514]

I tried this code:

pub fn foo<#[doc = "bar"] A>(_baz: A) {}

I expected to see this happen: Either rustdoc adds "bar" somewhere in the documentation, or rustc gives a warning that the doc-comment does nothing.

Instead, this happened: The doc-comment is silently discarded.

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rustc --version --verbose: rustc 1.58.0-nightly (4961b10 2021-11-04)