A shebang is displaced in HIR & expanded outputs

[fluiderson]

I tried this code:

#!/usr/bin/env rust

fn test() {}

After running rustc -Zunpretty=hir test.rs, I expected to see this happen:

#!/usr/bin/env rust
#[prelude_import]
use ::std::prelude::rust_2015::*;
#[macro_use]
extern crate std;

fn test() { }

Instead, this happened:

#[prelude_import]
use ::std::prelude::rust_2015::*;
#[macro_use]
extern crate std;
#!/usr/bin/env rust

fn test() { }

A shebang should remain at the beginning of an output but instead is placed in-between. The same happens with rustc -Zunpretty=expanded test.rs.

Meta

rustc --version --verbose:

rustc 1.85.0-nightly (5f23ef7d3 2024-12-20)
binary: rustc
commit-hash: 5f23ef7d3f7a8c3e0ca5c4e1978829c0448a3686
commit-date: 2024-12-20
host: x86_64-unknown-linux-gnu
release: 1.85.0-nightly
LLVM version: 19.1.6

[fmease]

Marking this as an enhancement instead of a bug because technically speaking we don't guarantee anything about the shape of the output produced by -Zunpretty. Of course, a PR fixing this would be very welcome regardless :)

Some breadcrumbs: Both -Zunpretty={expanded,hir} / rustc_{ast,hir}_pretty collect comments (which includes shebangs) via rustc_ast_pretty::pprust::state::Comments:

rust/compiler/rustc_ast_pretty/src/pprust/state.rs

Lines 59 to 63 in f5b6f93

	pub struct Comments<'a> {
	sm: &'a SourceMap,
	// Stored in reverse order so we can consume them by popping.
	reversed_comments: Vec<Comment>,
	}

And print them again via maybe_print_comment / print_remaining_comments.