rustdoc incorrectly handles ##

[lcnr]

I tried this code:

/// ```rust
/// ##![no_std]
/// ```
pub struct A;

with cargo doc

I expected to see this happen: Either an error or #![no_std] gets hidden

What actually happend: The first # gets ignored.

[GuillaumeGomez]

A fix to this issue is present in this PR apparently (or maybe I understood it incorrectly?).

[lcnr]

Not sure, does #74094 change the way # is handled in code snippets?

[GuillaumeGomez]

I think it does: https://github.com/rust-lang/rust/pull/74094/files#diff-545d93b02fb448fc31d33a6b1e0e0c48R136-R153 At least it's changing the code handling it...

[lcnr]

So it seems like it was a conscious decision to print ##abc as #abc :/

Can we still change this/what was the reason for this decision?
Otherwise this can just be closed as it seems like this behavior is intended.

[GuillaumeGomez]

Well, I have to admit that it seems weird and I wonder if it isn't just a side-effect more than a desired behavior. What do you think @rust-lang/rustdoc ?

[Mark-Simulacrum]

See #41785 where the behavior for ## was introduced. If you want to hide #![no_std] you want to use # #![no_std], ##![no_std] is changed to #![no_std] I believe.

[GuillaumeGomez]

Should we maybe add a lint for it?

[Mark-Simulacrum]

I am confused -- the behavior may not be entirely intuitive, but linting would seem odd, as either case is entirely reasonable.

[GuillaumeGomez]

I completely agree but the confusion seems logical. So having a lint you can easily discard might be a good solution to help users when they have such a case, no?

[Mark-Simulacrum]

I disagree that we should have lints that are (not infrequently) allow'd in rustdoc/rustc; that seems more like clippy territory. It's also true that I don't think we have a good story for allowing lints that rustdoc emits today?

[GuillaumeGomez]

It's being improved currently, but as far as I can see, the rustdoc lints are fine.

[ollie27]

A lint would be overly complicated for this. The behavior is documented at https://doc.rust-lang.org/nightly/rustdoc/documentation-tests.html#hiding-portions-of-the-example but maybe it can be improved to make it clear that you need a space after the hash for a line to be hidden.