Highlight hidden lines in Rust code in documentation comments

[jonhoo]

Consider the following Rust code:

//! ```
//! # use std::sync::Arc;
//! let x = Arc::new(0);
//! ```

The code inside the code block is correctly highlighted, but the # character stands out by not being highlighted. Could it perhaps be highlighted as a doc comment too?

I think ideally lines leading with # in a doctest should be highlighted differently (perhaps with a different background or dim colors?), but that's probably much harder.

[chris-morgan]

Vim syntax matching knows about nested syntax structures; but Vim highlighting doesn’t—it only highlights based on the top item in the syntax stack.

Highlighting # as doc comment seems a reasonable course of action.