Should unknown lint tools be a warning?

[ehuss]

Problem

When a new tool is introduced to the lints table, past versions of cargo will generate an error about an unknown tool. This can make it difficult to adopt new tools since it sets a floor on which Rust version is allowed. I'm wondering if it makes sense to treat unknown tools as a warning. Other tables in the manifest usually treat these as warnings (like [package.foo] is a warning).

Steps

Have a manifest with something like:

[package]
name = "foo"
version = "0.1.0"

[lints.new-cool-tool]
new-lint = "warn"

This generates an error:

error: failed to parse manifest at `/Users/eric/Temp/z20/Cargo.toml`

Caused by:
  unsupported `new-cool-tool` in `[lints]`, must be one of rust, clippy, rustdoc

Possible Solution(s)

Generate a warning instead of an error for unknown tools.

Notes

No response

Version

cargo 1.73.0-nightly (d78bbf4bd 2023-08-03)
release: 1.73.0-nightly
commit-hash: d78bbf4bde3c6b95caca7512f537c6f9721426ff
commit-date: 2023-08-03
host: aarch64-apple-darwin
libgit2: 1.6.4 (sys:0.17.2 vendored)
libcurl: 7.88.1 (sys:0.4.65+curl-8.2.1 system ssl:(SecureTransport) LibreSSL/3.3.6)
ssl: OpenSSL 1.1.1u  30 May 2023
os: Mac OS 13.4.1 [64-bit]

[epage]

Giving this a try with what rustc does:

#!/usr/bin/env -S cargo -Zscript

#![warn(new_cool_tool::new_lint)]

fn main() {}

$ ./cargo-12496.rs
warning: `package.edition` is unspecified, defaulting to `2021`
   Compiling cargo-12496 v0.0.0 (/home/epage/src/personal/dump)
error[E0710]: unknown tool name `new_cool_tool` found in scoped lint: `new_cool_tool::new_lint`
 --> cargo-12496.rs:3:9
  |
3 | #![warn(new_cool_tool::new_lint)]
  |         ^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0710`.
error: could not compile `cargo-12496` (bin "cargo-12496") due to previous error

[epage]

The reason I made this an error, instead of a warning, was to prevent abuse of the [lints] table by arbitrary tools.

[weihanglo]

If rustc gives an error, maybe cargo would be better follow.

[epage]

While there is room for improvement, I believe this specific issue was resolved by #13833.