[missing_asserts_for_indexing]: work with bodies instead of blocks separately

[y21]

Fixes #11856

Before this change, this lint would check blocks independently of each other, which means that it misses assert!()s from parent blocks.

// check_block
assert!(x.len() > 1);

{
  // check_block
  // no assert here
  let _ = x[0] + x[1];
}

This PR changes it to work with bodies rather than individual blocks. That means that a function will be checked in one go and we can remember if an assert! occurred anywhere.

Eventually it would be nice to have a more control flow-aware analysis, possibly by rewriting it as a MIR lint, but that's more complicated and I wanted this fixed first.

changelog: [missing_asserts_for_indexing]: accept assert!s from parent blocks

[rustbot]

r? @blyxyas

(rustbot has picked a reviewer for you, use r? to override)

[blyxyas]

LGTM, thanks! ❤️

[blyxyas]

@bors r+

[bors]

📌 Commit 553857b has been approved by blyxyas

It is now in the queue for this repository.

[bors]

⌛ Testing commit 553857b with merge 96eab06...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: blyxyas
Pushing 96eab06 to master...