Warn if a `[patch.*]` section is used on a dependency of a library

What it does

If a [patch] section is used to override a dependency, it is only applicable to the current crate. Dependencies ignore this. If a library tries to patch a dependency (not a dev-dependency or build-dependency) this is useless since it won't apply to anything actually using this library, but it would appear to work in tests.

Advantage

• Prevents accidental use of the [patch] section when it won't work

Drawbacks

There might be some edge cases where this is wanted, but I'm not currently aware of any.

Example

[patch.crates-io]
foo = { git = 'https://github.com/example/foo.git' }

Would generate a clippy warning such as

The patch override for dependency `foo` will be ignored outside of this crate.

Start by locating Clippy's lint implementations and tests for Cargo manifest or dependency-related diagnostics. Reproduce the example with a library dependency and compare it with dev-dependency and build-dependency cases. Done means the requested warning is emitted only for patches that cannot affect downstream users, with tests covering the relevant dependency kinds.