`cargo build -Z help` is missing options

[jyn514]

Problem
cargo build -Z help should show all available unstable options. Instead, it's missing at least -Z build-std and -Z build-std-features.

Steps

1. cargo build -Z help

Available unstable (nightly-only) flags:

    -Z avoid-dev-deps      -- Avoid installing dev-dependencies if possible
    -Z extra-link-arg      -- Allow `cargo:rustc-link-arg` in build scripts
    -Z minimal-versions    -- Install minimal dependency versions instead of maximum
    -Z no-index-update     -- Do not update the registry, avoids a network request for benchmarking
    -Z unstable-options    -- Allow the usage of unstable options
    -Z timings             -- Display concurrency information
    -Z doctest-xcompile    -- Compile and run doctests for non-host target using runner config
    -Z terminal-width      -- Provide a terminal width to rustc for error truncation
    -Z namespaced-features -- Allow features with `dep:` prefix
    -Z weak-dep-features   -- Allow `dep_name?/feature` feature syntax
    -Z patch-in-config     -- Allow `[patch]` sections in .cargo/config.toml files

Run with 'cargo -Z [FLAG] [SUBCOMMAND]'

See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html for more information about these flags.

Possible Solution(s)
Autogenerate -Z help from the unstable options somehow (maybe look how rustc does it?), so it isn't possible to forget to update it.

Notes

Output of cargo version: cargo 1.53.0-nightly (3c44c3c 2021-03-31)

[weihanglo]

Hmmm.... rustc uses its ad-hoc options macro to generate those flags.
https://github.com/rust-lang/rust/blob/c18c0ad2/compiler/rustc_session/src/options.rs#L886-L1221

[jyn514]

Sure, you could copy the macro from rustc and modify it for cargo.

[PicoJr]

It should be possible to generate most of cargo build -Z help content automatically.
IMO the help message still has to be written by a human at some point though.

Maybe the help message for each option can be derived in a similar way as thiserror derives error messages?

#[derive(Help)]
pub struct CliUnstable {
   // ...
   #[help("Allow *only* the listed unstable features")]
   pub allow_features: Option<BTreeSet<String>>,
    // ...

cf https://github.com/rust-lang/cargo/blob/master/src/cargo/core/features.rs#L548-L585

[PicoJr]

I'm working on it, it's time for me to learn about derive macros!

[ehuss]

I would suggest not using a derive or proc-macro. That would add some undesired weight for a relatively obscure part of the code. A regular macro_rules macro may be fine as long as it is not too complicated.

[ehuss]

Oh, and if you have any questions or need any guidance, let me know. (Here or on Zulip whichever you prefer.)

[PicoJr]

@ehuss thanks, you are probably right derive macros might be overkill for this situation, I'll write a macro_rules instead.
Yes I'd be happy to ask questions, I'm completely inexperienced when it comes to macros and It would be nice to share with you on Zulip (just registered a minute ago).

[PicoJr]

Here is what I've written so far: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=be40772212f24e9728e6adddcc388b4a

update0: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=847f63cccb9adf2f5fe47ebdc470eafe