[unconditional_recursion]: compare by Tys instead of DefIds

[y21]

Fixes #12154
Fixes #12181 (this was later edited in, so the rest of the description refers to the first linked issue)

Before this change, the lint would work with DefIds and use those to compare types. This PR changes it to compare types directly. It fixes the linked issue, but also other false positives I found in a lintcheck run. For example, one of the issues is that some types don't have DefIds (primitives, references, etc., leading to possible FNs), and the helper function used to extract a DefId didn't handle type parameters.

Another issue was that the lint would use .peel_refs() in a few places where that could lead to false positives (one such FP was in the http crate). See the doc comment on one of the added functions and also the test case for what I mean.

The code in the linked issue was linted because the receiver type is T (a ty::Param), which was not handled in get_ty_def_id and returned None, so this wouldn't actually get to comparing self_arg != ty_id here, and skip the early-return:

rust-clippy/clippy_lints/src/unconditional_recursion.rs

Lines 171 to 178 in 70573af

	if let Some(ty) = cx.typeck_results().expr_ty_opt(receiver)
	&& let Some(ty_id) = get_ty_def_id(ty)
	&& self_arg != ty_id
	{
	// Since this called on a different type, the lint should not be
	// triggered here.
	return;
	}

This alone could be fixed by doing something like && get_ty_def_id(ty).map_or(true, |ty_id)| self_arg != ty_id), but we don't really need to work with DefIds in the first place, I don't think.

changelog: [unconditional_recursion]: avoid linting when the other comparison type is a type parameter

[rustbot]

r? @Jarcho

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[bors]

☔ The latest upstream changes (presumably #12200) made this pull request unmergeable. Please resolve the merge conflicts.

[Jarcho]

DefIds are actually unsuited to the task. They treat Foo<X> and Foo<Y> as the same thing.

Thank you. @bors r+

[bors]

📌 Commit 87a6300 has been approved by Jarcho

It is now in the queue for this repository.

[bors]

⌛ Testing commit 87a6300 with merge 62dcbd6...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Jarcho
Pushing 62dcbd6 to master...

[kpreid]

A user observed the unfixed issue in rustc 1.77.0-beta.7 (339fb6965 2024-03-06), so I think this deserves consideration for backport.

@rustbot label +beta-nominated

[flip1995]

rust-lang/rust#122510