rustdoc: Strip blank lines from start and end of code blocks

[camelid]

People will often use the # character to hide lines that aren't relevant to the example, but rustdoc will display blank lines you leave between #-prefixed lines and the visible lines. For example, this source:

//! ```
//! # #[cfg(any(feature = "alloc", feature = "std"))]
//! # mod test {
//!
//! use num::FromPrimitive;
//! use num::bigint::BigInt;
//! use num::rational::{Ratio, BigRational};
//!
//! # pub
//! fn approx_sqrt(number: u64, iterations: usize) -> BigRational {
//!     let start: Ratio<BigInt> = Ratio::from_integer(FromPrimitive::from_u64(number).unwrap());
//!     let mut approx = start.clone();
//!
//!     for _ in 0..iterations {
//!         approx = (&approx + (&start / &approx)) /
//!             Ratio::from_integer(FromPrimitive::from_u64(2).unwrap());
//!     }
//!
//!     approx
//! }
//! # }
//! # #[cfg(not(any(feature = "alloc", feature = "std")))]
//! # mod test { pub fn approx_sqrt(n: u64, _: usize) -> u64 { n } }
//! # use crate::test::approx_sqrt;
//!
//! fn main() {
//!     println!("{}", approx_sqrt(10, 4)); // prints 4057691201/1283082416
//! }
//!
//! ```

leads to this output:

I propose that rustdoc strips blank lines from the start and end of code blocks to avoid this not-great-looking output. This would happen after the # lines were stripped out.

[camelid]

I feel like I remember Guillaume or someone else working on something to fix this recently, but I can't find the PR. Am I mistaken or do you know which PR it is?

[ojeda]

The start/end behaves differently, which is confusing, e.g. this:

/// ```
/// # use c::f;
///
///
/// f()?;
///
///
/// # Ok::<(), i32>(())
/// ```

will render 2 blank lines on top, but only 1 on the bottom.

Edit: since this may be considered a bug (inconsistent number of blank lines), and this issue is a feature request (stripping all blank lines), I have created a separate issue: #102996.