Regression in nightly-2025-06-28: `rustdoc --test -Zcrate-attr=feature(used_with_arg)`

[dtolnay]

#[used(linker)]
static REPRO: isize = 1;

$ rustdoc +nightly-2025-06-27 --test -Zcrate-attr='feature(used_with_arg)' lib.rs

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

$ rustdoc +nightly-2025-06-28 --test -Zcrate-attr='feature(used_with_arg)' lib.rs
error[E0658]: `#[used(linker)]` is currently unstable
 --> lib.rs:1:1
  |
1 | #[used(linker)]
  | ^^^^^^^^^^^^^^^
  |
  = note: see issue #93798 <https://github.com/rust-lang/rust/issues/93798> for more information
  = help: add `#![feature(used_with_arg)]` to the crate attributes to enable
  = note: this compiler was built on 2025-06-27; consider upgrading it if it is out of date

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0658`.

The most recent nightly (nightly-2025-10-02) remains affected.

Bisects to #142818. @JonathanBrouwer @jdonszelmann

[JonathanBrouwer]

@rustbot claim
Will fix

[Urgau]

Part of the issue is that rustdoc is not populating the unstable_opts (and thus -Zcrate-attr) when testing doctests.

rust/src/librustdoc/doctest.rs

Lines 162 to 177 in 5c7ae0c

	let sessopts = config::Options {
	sysroot: options.sysroot.clone(),
	search_paths: options.libs.clone(),
	crate_types,
	lint_opts,
	lint_cap: Some(options.lint_cap.unwrap_or(lint::Forbid)),
	cg: options.codegen_options.clone(),
	externs: options.externs.clone(),
	unstable_features: options.unstable_features,
	actually_rustdoc: true,
	edition: options.edition,
	target_triple: options.target.clone(),
	crate_name: options.crate_name.clone(),
	remap_path_prefix: options.remap_path_prefix.clone(),
	..config::Options::default()
	};

Adding unstable_opts: options.unstable_opts.clone() to sessopts initialization fixes the issue, but that's kind of surprising since the code around that area hasn't been touched in over a year, I wonder how that could have worked before.