Add `--print=crate-root-lint-levels` to retrieve lints levels

[Urgau]

Proposal

Add a new unstable --print variant crate-root-lint-levels (formally proposed as lint-levels) (--print=crate-root-lint-levels -Zunstable-options) that would print out all the lints and their associated levels (allow, warn, deny, forbid) based on the regular Rust rules at crate root, that is:

• command line args (-W, -A, ...)
• crate root attributes (#![allow], #![warn], ...)
• the special warnings lint group
• the default lint level

The output format would be LINT_NAME=LINT_LEVEL, e.g.:

unknown_lint=warn
arithmetic_overflow=deny

Motivation

Exposing more stuff for external tools. 😄

In particular, exposing the actual lint level for lints could help external tools (like Cargo) extend some built-ins lints, e.g. unknown_lints for Cargo [lints] table¹, which would not need to try to compute the lint level, which require knowing the default, interpreting rustc args (and dealing with RUSTFLAGS), ... all of which are non-trivial.

Delegating all that work to the source of truth (rustc), makes it nearly "trivial" for those tools to know the lint level and re-use it.

Mentors or Reviewers

My-self for the work.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Footnotes

1. Cargo has not committed to using this new flag, it should be kept as an example, but it's still a good motivation to try out the flag ↩

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

@rfcbot concern reason-for-concern 
<description of the concern> 

Concerns can be lifted with:

@rfcbot resolve reason-for-concern 

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler

[epage]

In particular, exposing the actual lint level for lints could help external tools (like Cargo) extend some built-ins lints, e.g. unknown_lints for Cargo [lints] table, which would not need to try to compute the lint level, which require knowing the default, interpreting rustc args (and dealing with RUSTFLAGS), ... all of which are non-trivial.

Delegating all that work to the source of truth (rustc), makes it nearly "trivial" for those tools to know the lint level and re-use it.

Note that we've not committed to this being how we want to handle lints within Cargo.

[jieyouxu]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted