Consider doing custom flow analysis during migration tool's EdgeBuilder stage.

[stereotype441]

Currently the migration tool re-uses the analyzer/CFE shared flow analysis stage to analyze the flow of nullabilities through the bodies of functions. This is not ideal for several reasons:

• Flow analysis makes certain approximations in order to be able to to full type promotion on loops in a single pass; these approximations are not necessary if we only need to consider nullability.
• Flow analysis's join operation only preserves a promotion if the promoted types are the same, but we can't reliably check "sameness" on decorated types, so we wind up losing promotion information when joining two control paths that both independently check that a variable is non-null.
• Flow analysis doesn't currently promote variables on assignment (non-nullable promotion on assignment not working #39562), and even when this is implemented, it will only promote in a few special circumstances. This means that the migration tool will fail to detect certain cases where a value is known to be non-null.
• Flow analysis emphasizes soundness over detecting user intent; for example it does not promote fields. It would be nice if the migration tool struck a better balance here (see Migration tool should take advantage of unsound promotions #38470 and Migration tool should assume assertions enabled. #44292).
• @MichaelRFairhurst has discovered some cases where flow analysis would need to be updated in order to support Migration: propagate non-null intent when all downstream nodes have non-null intent. #40509 (see Migration: propagate non-null intent when all downstream nodes have non-null intent. #40509 (comment)).

Rather than address all these issues through changes to the shared flow analysis stage, I think it would be better to have a separate nullability flow analysis stage that's specific to the migration tool.

[MichaelRFairhurst]

If we do this, it is worth taking a look at #42380 for other opportunities here.

The one that I see is self invocating function literals. If those could be treated as being guaranteed to execute (even in, in the case of that bug, an assert) then that could be a potential improvement as wel.

[stereotype441]

As of 1c7fe71, the null safety migration tool has been removed from active development and retired. No further work on the tool is planned.

If you still need help, or you believe this issue has been closed in error, please feel free to reopen.