Fix ICE-133117: multiple never-pattern arm doesn't have false_edge_start_block

[Shunpoco]

Fixes#133117 , and close fixes#133063 , fixes#130779

In order to fix ICE-133117, at first I needed to tackle to ICE-133063 (this fixed 130779 as well).

ICE-133063 and ICE-130779

This ICE is caused by those steps:

1. An arm has or-pattern, and all of the sub-candidates are never-pattern
2. In that case, all sub-candidates are removed in remove_never_subcandidates(). So the arm (candidate) has no sub-candidate.
3. In the current implementation, if there is no sub-candidate, the function assigns pre_binding_block into the candidate (here). However, otherwise_block should be assigned to the candidate as well, because the otherwise_block is unwrapped in multiple place (like in lower_match_tree()). As a result, it causes the panic.

I simply added the same block as pre_binding_block into otherwise_block, but I'm wondering if there is a better block to assign to otherwise_block (is it ok to assign the same block into pre_binding and otherwise?)

ICE-133117

This is caused by those steps:

1. There are two arms, both are or-pattern and each has one match-pair (in the test code, both are (!|!)), and the second arm has a guard.
2. In match_candidate() for the first arm, it expands the second arm’s sub-candidates as well (here). As a result, the root candidate of the second arm is not evaluated/modified in match_candidate(). So a false_edge_start_block is not assigned to the candidate.
3. merge_trivial_subcandidates() is called against the candidate for the second arm. It just returns immediately because the candidate has a guard. So a flase_edge_start_block is not assigned to the candidate also in this function.
4. remove_never_subcandidates() is called against the candidate. Since all sub-candidates are never-pattern. they are removed.
5. In lower_match_tree(), since there is no sub-candidate for the candidate, the candidate itself is evaluated in visit_leave_rev (here). Because the candidate has no false_edge_start_block, it causes the panic.

So I modified the order of if blocks in merge_trivial_subcandidates() to assign a false_edge_start_block if the candidate doesn't have.

[rustbot]

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[compiler-errors]

cc @Nadrieril

[Nadrieril]

r? Nadrieril, thanks for doing this I'll have a look when I get a moment

[Shunpoco]

@rustbot review

[Nadrieril]

Looks good, thank you for investigating and fixing this :)

@bors r+

[bors]

📌 Commit 7275bdf has been approved by Nadrieril

It is now in the queue for this repository.

[Nadrieril]

@bors rollup