Lint to warn on use of `#[deny(lint)]` in code

What it does

Warn on use of #[deny(lint)] / #![deny(lint)]

Advantage

• Avoid builds breaking due to a new Rust toolchain changing the behavior of a lint
• Give developers flexibility to be sloppy during development, resolving out the lints before merging (generally being enforced in CI)

Drawbacks

• It becomes awkward to allow this warning. A parent scope can do it. Unsure if you could in the same scope with !. Cargo.toml [lints] could also

Example

#![deny(elided-lifetimes-in-paths)]

pub struct Foo<'a> {}

fn foo(f: Foo) {}

Could be written as:

#![warn(elided-lifetimes-in-paths)]

pub struct Foo<'a> {}

fn foo(f: Foo) {}

with the suggestion to turn warnings into errors where relevant (e.g. CI)

The issue names no implementation files or tests; start by locating Clippy's handling and tests for attribute-based lints, then review the examples for both inner and outer attributes. Done requires an agreed behavior for warning on deny attributes, including how the warning can be allowed, with coverage for the shown cases and the suggested replacement.