rustdoc: Failed to parse rust code block with ## in 2024

[ehuss]

In 2024, rustdoc seems to fail to parse a rust code line that starts with ##.

//! Test
//!
//! ```
//! ##[allow(dead_code)]
//! println!("hello world");
//! ```

In 2021, this renders in a weird way where it strips the first #, but doesn't actually hide the line. I would expect the line to be hidden. I don't know if that is intentional.

In 2024, cargo doc generates a warning:

arning: could not parse code block as Rust code
 --> src/lib.rs:3:5
  |
3 |   //! ```
  |  _____^
4 | | //! ##[allow(dead_code)]
5 | | //! println!("hello world");
6 | | //! ```
  | |_______^
  |
  = note: error from rustc: reserved multi-hash token is forbidden
  = note: `#[warn(rustdoc::invalid_rust_codeblocks)]` on by default
help: mark blocks that do not contain Rust code as text
  |
3 | //! ```text
  |        ++++

I'm not sure what I expect here. I assume it is conflicting with the reserved syntax of 2024.

Is the ## form really intended to be supported as a way to hide a line? cc @GuillaumeGomez

Meta

rustc --version --verbose:

rustc 1.86.0-nightly (92bedea1c 2025-02-11)
binary: rustc
commit-hash: 92bedea1c51e3a969d60972be854506ffd8c5cb6
commit-date: 2025-02-11
host: aarch64-apple-darwin
release: 1.86.0-nightly
LLVM version: 19.1.7

[fmease]

So, an "isolated" # hides a line. This specifically excludes the sequence #[ for example which would likely start an attribute.

The sequence ## generally escapes # meaning it gets substituted with #. That's why ##[…] results in #[…]. I don't know why it's treated verbatim in R2024 tho, I haven't checked the source code.

[GuillaumeGomez]

Checking what's wrong.

[GuillaumeGomez]

I opened #136927 to fix it.