[else_if_without_else]: Fix duplicate diagnostics

[CBSpeir]

Relates to: #12379

changelog: Fix duplicate lint diagnostic emission from [else_if_without_else]

[rustbot]

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[dswij]

I'm guessing this is to lint nested blocks (albeit weirdly?). Can you help to also add a test case for this?

[CBSpeir]

This caused the lint to emit on any if-else-if-else... chain that did not end with an else. The lint would flag at the top of the chain, and then also flag each time EarlyLintPass::check_expr was called on the subsequent expressions in the chain.

I added a test with 4 chains:

if bla1() {
     println!("if");
} else if bla2() {
     println!("else if 1");
} else if bla3() {
     println!("else if 2");
} else if bla4() {
     println!("else if 3");
} else if bla5() {
      //~^ ERROR: `if` expression with an `else if`, but without a final `else`
      println!("else if 4");
}

Before this fix, four duplicate lints are emitted for this if-else-if... chain.

Also added a passing case four chain test, and fail/pass cases which contain suppressed [collapsible_else_if] blocks.

[dswij]

Thank you! @bors r+

[bors]

📌 Commit 8d78cd1 has been approved by dswij

It is now in the queue for this repository.

[bors]

⌛ Testing commit 8d78cd1 with merge 59a5ad4...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: dswij
Pushing 59a5ad4 to master...