Lint against unconditional recursion across function calls

[phil-opp]

Rust's unconditional_recursion lint only works when a function calls itself directly. Thus it misses many cases of infinite recursions, including simple ones that can easily happen by accident. For example:

struct Foo;

impl From<u64> for Foo {
    fn from(v: u64) -> Self {
        v.into()
    }
}

(playground)

There are more examples in the tracking issue at rust-lang/rust#57965. Ideally, this should be fixed in rustc itself of course, which was tried in rust-lang/rust#75067. Unfortunately, the PR was closed because of its performance regressions.

Given that stack overflow errors are always fatal and can be hard to debug, I thought that such a lint might be a good candidate for clippy, where performance doesn't matter that much.

[llogiq]

I don't doubt that it's a worthwhile extension, however there are two things to keep in mind:

1. Cycles can form across arbitrary methods. The lint should only fire if the cycle is unconditional though.
2. We may not have full information: Some functions may be in external crates, perhaps even in generic code, in FFI code (and thus outside Rust) or dispatched dynamically (and thus not known at compile time).