Add utils helper for filtering out type/lifetime parameters on generics

[Manishearth]

See https://github.com/rust-lang/rust-clippy/pull/3786/files , we have a lot of code that does something like params.args.iter().filter_map(|arg| match arg { GenericArg::Lifetime(lt) => Some(lt), _ => None }

We should probably have a helper function that returns an iterator for these, placed in utils. Probably something like:

fn types_of_generics(generics: &Generics) -> impl Iterator<Type> {...}
fn lifetimes_of_generics(generics: &Generics) -> impl Iterator<Lifetime> {...}

[oli-obk]

lol you read my mind. I was gonna add this to rustc directly, because they probably have the same problems. I think we can add this directly on the Generics type as a method. There's precedent for the Substs<'tcx> type.

[Manishearth]

IIRC this used to exist on Generics anyway, but got removed at some point. I recall doing a rustup where we added a bunch of these matches. But sure, adding this to rustc would be great, I'm sure it has a similar pattern.

[flip1995]

I'm in favor of implementing such a function in rustc:

rust-clippy/clippy_lints/src/methods/mod.rs

Lines 2471 to 2480 in db13e6f

	// FIXME(flip1995): messy, improve if there is a better option
	// in the compiler
	let types: Vec<_> = params
	.args
	.iter()
	.filter_map(|arg| match arg {
	hir::GenericArg::Type(ty) => Some(ty),
	_ => None,
	})
	.collect();

Git blame me: #2871

I'll open a PR in rustc in the next few days for these iterators.

[oli-obk]

Just found a case in a PR to rustc: https://github.com/rust-lang/rust/pull/58583/files#diff-9d22f4d35c9bd98748f3e1a7cda1b3e7R694