Fix #2640: Handle hidden crate attributes in fn main wrapping

[cobyfrombrooklyn-bot]

Problem

When a playground code block contains a hidden crate-level attribute (e.g. # #![feature(rustc_attrs)]) and no explicit fn main, the attribute gets placed inside the generated fn main() instead of before it. This causes the playground "play" button to produce compilation errors.

As @ehuss identified, the issue is in the regex used by partition_rust_source — it doesn't account for the # hidden line prefix, so # #![feature(...)] isn't recognized as a crate-level attribute.

Fix

Updated the HEADER_RE regex in partition_rust_source to optionally match the # (hash-space) hidden line prefix before crate-level inner attributes (#![...]).

Before: ^[ \t]*\#!\[
After: ^[ \t]*(?:\#\x20)?[ \t]*\#!\[

Test

• 3 unit tests for wrap_rust_main (basic wrapping, hidden feature attr, already-has-main)
• 1 unit test addition to partition_rust_source for hidden prefix case
• 1 integration test (playground_hidden_feature_attr) with a code block using # #![feature(rustc_attrs)]
• Full test suite passes on macOS ARM (Apple Silicon): 119 tests, 0 failures

[ehuss]

Why does this have \x20? This doesn't seem to have the same understanding of hidden lines as the rest of the code does.

Also, this seems to not also update the blank lines rule.

I would expect the following to work:

Input	Output	Hidden
#!feature(x)	#!feature(x)	Not hidden
##!feature(x)	#!feature(x)	Not hidden
# #![feature(x)	#!feature(x)	Hidden
# #![feature(x) # # #![feature(y)]	#![feature(x) #![feature(y)]	Hidden

Perhaps it would be best to extract the hide-lines logic from hide_lines_rust so that there could be an iterator over the rust source lines that indicates if a line is hidden or not, and then these different parts could reuse that logic. rustdoc does something like this here with an enum that represents whether a line is hidden.

[ehuss]

BTW, pull requests that fix an issue should use one of GitHub's keywords in the description (like Fixes #2640) so that the issue gets linked.