Fix 10021

[smoelius]

This PR proposes a fix for #10021.

The problem is similar to the one that @mikerite described in #9505. The compiler is generating an empty substitution for a call, even though the type of Self seems to be needed for a predicate. In @mikerite's case, the call was to IntoFuture::into_future. In this case, the call is to Try::branch.

The proposed fix is to verify that the parameter whose type is changing has an index within the substitution. The strikes me as a reasonable approach, since if the check were to fail, the following code would be a no-op:

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

Lines 420 to 428 in 4c123a0

	let new_subst = cx.tcx.mk_substs(
	call_substs.iter()
	.enumerate()
	.map(|(i, t)|
	if i == (*param_index as usize) {
	GenericArg::from(ty)
	} else {
	t
	}));

Like @mikerite's original solution, this solution turns ICEs into false negatives.

changelog: fix unnecessary_to_owned false positive involving Try::branch

[rustbot]

r? @dswij

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[dswij]

Thanks for this!

[dswij]

@bors r+

[bors]

📌 Commit 2701a40 has been approved by dswij

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2701a40 with merge cb8df45...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: dswij
Pushing cb8df45 to master...

[ghost]

@rustbot label +beta-nominated

[rustbot]

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[flip1995]

rust-lang/rust#105470