ICE: `rustdoc`'s `--test` option may call `rustc` with too many arguments

[ojeda]

rustdoc's --test option may call rustc with too many arguments, e.g. due to --cfgs, ICE'ing with:

thread 'x.rs - S (line 1)' panicked at src/librustdoc/doctest.rs:407:38:
Failed to spawn rustc process: Os { code: 7, kind: ArgumentListTooLong, message: "Argument list too long" }

In particular, the original call to rustdoc (i.e. the user's call) may not suffer from this if they used @args to pass many arguments, but rustdoc will then try to spawn rustc passing them as parameters, and thus failing.

A reproducer may look like, for large enough N:

echo -e '//! ```\n//! ```' > x.rs
yes -- --cfg=A | head -n $N > cfgs
rustdoc --test @cfgs x.rs

Since different systems may have different limits, it may be best to have rustdoc use a temporary file for all arguments that come from unbounded lists. Or perhaps using the original @args, if that is possible, since users using too many e.g. --cfgs are probably using them already.

In the kernel we are close to hitting the limit: we have 28k+ --cfgs arguments in big configurations, and we may start soon passing --check-cfgs for those too. Passing our @cfgs twice already hits the limit.

Meta

Can be reproduced with both the latest stable (1.76.0) and the latest nightly (1.79.0-nightly (3c85e56 2024-03-18)).

[GuillaumeGomez]

Wow. That's... a lot. ^^'

Should be doable with a tempfile. Taking a look.

[ojeda]

Thanks Guillaume!