config: merge lists in precedence order

[arlosi]

When merging configuration lists, the current order does not match the expected precedence. This makes merged lists follow precedence order, with higher precedence items merged later in lists.

When a list in configuration exists in multiple places, Cargo merges the lists together. The ordering of this merging is unexpected and does not follow the precedence rules that non-list configuration uses.

The current merging order appears to be:

• project-specific config.toml
• global config.toml
• command-line (--config)
• environment variable (CARGO_*)

This PR changes the order to follow the precedence rules with higher precedence configuration merging later in the lists.

• global config.toml
• project-specific config.toml
• environment variable (CARGO_*)
• command-line (--config)

This aligns with config such as build.rustflags where later flags take precedence over earlier ones.

Since --config is relatively new, it's unlikely to cause too much breakage by making it come after environment variables.

Switching global and project-specific ordering is more likely to cause breakage, since it's been around longer (reported as an issue in #8128). Projects relying on global configuration flags (in $CARGO_HOME\config.toml or in .cargo/config.toml further from the project) being merged first in lists will be broken.

For most uses of merged lists (such as build.rustflags), if the flags do not conflict with each other, there will be no impact.

Fixes #12506
Fixes #8128

[rustbot]

r? @weihanglo

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[arlosi]

Since this is a bug fix breaking change, let's make sure everyone is OK with it.

@rfcbot merge

[weihanglo]

You need
@rustbot label +T-cargo
to do an FCP

[arlosi]

Thanks @weihanglo. Let's try again.

@rfcbot fcp merge

[rfcbot]

Team member @arlosi has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Eh2406
• @Muscraft
• @arlosi
• @ehuss
• @epage
• @joshtriplett
• @weihanglo

Concerns:

• raise-awareness resolved by config: merge lists in precedence order #12515 (comment)

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[epage]

Could the PR re-summarize the situation, especially who will be broken and the level of impact of that?

[weihanglo]

Should we do a crater run for this?
I doubt it'll catch anything interesting with crater though. Usually this kind of configuration is deeply rooted in a user's or an enterprise environment.

[ehuss]

I don't think crater would be able to run in situations were there would be separate configuration files to merge.

[weihanglo]

I'll check my box here but want to raise the awareness to a wider audience. I knew some large cooperations do weird configurations around build.rustflags. We should have a way to solicit feedback before specifying an unspecified behavior. An FCP period sounds too short to me.

@rfcbot reviewed
@rfcbot concern raise-awareness

[weihanglo]

We've talked about this in the Cargo weekly meeting today. We'll have a blog post for this incoming change, and solicit feedback from users. Since beta is just branched off, we'll make this change available on nightly as soon as possible.

@rustbot resolve raise-awareness

We would like to make sure this will get into release note as a compatibility note, so

@rustbot label +relnotes

[weihanglo]

Oh no it's rfcbot

@rfcbot resolve raise-awareness

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[weihanglo]

I'll merge this today and update the submodule. Thanks!

[weihanglo]

The code looks correct. Just what to make sure we have test to verify this behavior.

[weihanglo]

@bors r+

[bors]

📌 Commit 66bda83 has been approved by weihanglo

It is now in the queue for this repository.

[bors]

⌛ Testing commit 66bda83 with merge 2cc50bc...

[bors]

☀️ Test successful - checks-actions
Approved by: weihanglo
Pushing 2cc50bc to master...