Tracking issue for config-cli

[ehuss]

Implementation: #7649
Original proposal: #6699
Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#config-cli

Summary

Adds --config CLI option to pass config options to any command.

Unresolved issues

• Make TOML restricted to KEY=VALUE. Will need updates to the toml crate to make that possible. DONE: Check --config for dotted keys only #10176
• Make --config PATH work has a shorthand for `--config 'include="path"' DONE: Support --config path_to_config.toml cli syntax. #7901

[jyn514]

A use case for this: docs.rs wants to pass RUSTFLAGS without having the spacing interpreted by the shell: rust-lang/docs.rs#1057

[Nemo157]

Is it not possible to specify registry sources via this? I wanted to migrate from using registry-index sources in Publishing crates to IPFS so that I could test it with #8890, but trying to use this flag to define the registries it doesn't seem to be picked up:

> cargo -Zunstable-options '--config=source.bs58_cli_0_1_0_anyhow.registry="http://127.0.0.1:8080/ifps/QmP6KmpjQrYpYyeXGqAvmrw45dPoKvmKeTqTanmSqJGJKz"' '--config=source.bs58_cli_0_1_0_bs58.registry="http://127.0.0.1:8080/ifps/Qmb2PJ9vMALRFmQjLDQbcNHYUvcFc96XzKMwAhBMEECxzb"' '--config=source.bs58_cli_0_1_0_paw.registry="http://127.0.0.1:8080/ifps/QmNkJbrmpjJErbX9fTpSSQYb7Ky1Qnpg47QptfMcVQUwBx"' '--config=source.bs58_cli_0_1_0_structopt.registry="http://127.0.0.1:8080/ifps/Qmbh5sZbKezvYq2ESVhbnymqs7VKMTaz7yXUaoMi9tBjJp"' update --manifest-path /tmp/ipfs-registry.bs58-cli-0.1.0.yIgXKp/bs58-cli-0.1.0/Cargo.toml
error: failed to parse manifest at `/tmp/ipfs-registry.bs58-cli-0.1.0.yIgXKp/bs58-cli-0.1.0/Cargo.toml`

Caused by:
  no index found for registry: `bs58_cli_0_1_0_anyhow`

[Nemo157]

🤦 ignore that, I was using the wrong config key.

[dancrossnyc]

Curious if there's any updated status on this? It would be really great to get stabilized!

[ehuss]

There's no update. There's still an unresolved issue noted above that needs someone to work on it.

[jonhoo]

I'll add what I think is a pretty compelling use-case for this: passing -Zinstrument-coverage for coverage testing (which is stabilizing in rust-lang/rust#90132). It requires leaving any existing rustflags intact since things like linker flags (-Clink-args) may be required for the crate to build in the first place, which using RUSTFLAGS doesn't do (#5376) but --config='build.rustflags = ["-Zinstrument-coverage"]' does (#5376 (comment)).

For the unresolved issue, some guidance on exactly what you're imagining would be helpful @ehuss. Specifically, why do you think that restriction is necessary in the first place? Alternatively, could we just parse into a toml::Value and then check that it only contains one actual entry?

[ehuss]

The restriction is to just constrain the format so that it has a straightforward syntax. That way, someone wouldn't be able to pass in comments or [bracketed.tables] or newlines or anything like that. It already parses to a toml::Value and checks for one value.