rustdoc: `rustdoc -W help` prints error unless a file argument is given

[lilyball]

Compiler lints can be printed with rustc -W help, with no filename arguments given. Unfortunately this doesn't work the same way for rustdoc:

❯ rustdoc -W help
error: missing file operand

If I pass it an arbitrary file, it will print any errors produced by parsing the file as rust. This includes errors from using async without passing --edition 2018. This part is actually the same as rustc -W help, in that passing arbitrary files to that produces the same behavior.

If I pass it a file that parses cleanly (either 2015 rust, or 2018 rust with --edition 2018 specified) then and only then will rustdoc -W help actually print the lint groups. In this way it behaves similarly to rustc -W help $filename in that rustc will also print the lint groups when passed a filename that parses cleanly. The difference is that rustc -W help doesn't require a filename at all.

The simplest workaround here is to type rustdoc -W help /dev/null as the empty /dev/null file does not trigger any parse errors.

Potential root cause

It's possible that the root cause here is that calling rustdoc with no args produces the same error:

❯ rustdoc
error: missing file operand

Compare this with rustc, which prints the help:

❯ rustc
Usage: rustc [OPTIONS] INPUT

Options:
    -h, --help          Display this message
        --cfg SPEC      Configure the compilation environment
    -L [KIND=]PATH      Add a directory to the library search path. The
︙

This suggests that rustdoc is checking for filename arguments and erroring out early on, whereas rustc skips that check and then later interprets "no filename args" as "fallback to help".

Meta

❯ rustdoc --version --verbose
rustdoc 1.54.0 (a178d0322 2021-07-26)
binary: rustdoc
commit-hash: a178d0322ce20e33eac124758e837cbd80a6f633
commit-date: 2021-07-26
host: x86_64-apple-darwin
release: 1.54.0
LLVM version: 12.0.1
❯ rustdoc +nightly --version --verbose
rustdoc 1.57.0-nightly (fdf65053e 2021-09-07)
binary: rustdoc
commit-hash: fdf65053e99e8966f9bd83b5a8491326cb33d638
commit-date: 2021-09-07
host: x86_64-apple-darwin
release: 1.57.0-nightly
LLVM version: 13.0.0

[jyn514]

Mentoring instructions: move

rust/src/librustdoc/config.rs

Line 674 in 97032a6

let (lint_opts, describe_lints, lint_cap) = get_cmd_lint_options(matches, error_format);

to before

rust/src/librustdoc/config.rs

Line 422 in 97032a6

diag.struct_err("missing file operand").emit();

[inashivb]

Hi @jyn514 !
May I claim this issue if no one else is working on it?

[jyn514]

@inashivb go for it :)

[inashivb]

@rustbot claim