rustdoc: Regression in display of `macro_rules!` matchers

[camelid]

Oddly enough, the two lines (and the areas around them) that show up when I grep for "macro_rules! in rustdoc don't appear to have changed at all in a while, so I'm not sure what caused this regression. Perhaps the span of the matcher changed? I wasn't able to reproduce this locally, even when I tried using a cross-crate re-export with a copy-pasted version of core::todo (modulo replacing $crate:: with ::std::). Also, the bug doesn't appear with, e.g., panic!, but does with compile_error!. So, this seems to be a subtle bug.

$ rg '"macro_rules!' src/librustdoc/
src/librustdoc/clean/mod.rs
2167:                "macro_rules! {} {{\n{}}}",

src/librustdoc/clean/inline.rs
541:                "macro_rules! {} {{\n{}}}",

beta (incorrect)

stable (correct)

[camelid]

This bug doesn't occur for core::todo! or core::compile_error!, which makes me think it's a bug with cross-crate spans (either in rustc or rustdoc). I think std::panic! rendering normally is further evidence of this, because std::panic! is currently not a re-export of core::panic!.

[camelid]

I wonder if #80339 is related?

[jyn514]

@camelid you can test by bisecting

[jyn514]

That line of code is

rust/src/librustdoc/clean/inline.rs

Line 545 in 0a8629b

.map(|span| { format!(" {} => {{ ... }};\n", span.to_src(cx)) })

which hasn't changed in 2 years. So either span or to_src has changed.

[camelid]

@camelid you can test by bisecting

But I don't have an MCVE to bisect with. Can you bisect std itself?

[jyn514]

@camelid sure, it will just take a while 😅

[camelid]

I meant in terms of getting bisect-rustc to work with rustbuild so we're not also rebuilding rustdoc every time and instead are using CI rustdocs. Unless you're suggesting git bisect? But, yeah, it would probably take a while.