gh-107901: make compiler inline basic blocks with no line number and no fallthrough

[iritkatriel]

This pattern shows up in code like this:

def f():         
    for e in seq:
        try:
            X = 3
        except OSError:
            try:
                if C3:
                    X = 4
            except OSError:
                pass
    return 42

We have a block with no line number, and two predecessors, which contains a jump to another block that has no line number. The two-predecessor block is not an exit block so it is not duplicated. As a result, the end block remains with no line number, even if it needs one. This change inlines the first block to each of its predecessors, so that the jump can get a line number.

• Issue: Traceback says line -1 when KeyboardInterrupt during minimal for-if-loop  #107901

[markshannon]

Two questions.

• Does this fix handle triply nested try-excepts?
• Is it feasible to add a test for this?

[iritkatriel]

• Does this fix handle triply nested try-excepts?

It does. The issue is not the nesting, but the multiple paths leading to the end of the except block. It needs to be nested in another except because otherwise the blocks get reordered and this impacts the situation. But with nesting all the blocks are cold, so they remain in this order. More nesting doesn't make a difference.

• Is it feasible to add a test for this?

I'll try to add a unit test.

This PR makes the build pass with the assertion that all eval break checks have line numbers (there are 3 more test failures then, but at least we get through the build).

[markshannon]

Looks good