Within some AST context (at library granularity, for convenience), we
collect all the assignments to local vars:
- Declaration with no initializer amounts to `null` assignment
- Assignment ops are expanded naively: `x++` yield an assigned value of
`x + 1`, etc
We detect "trivially nullable" variables (e.g. `var x;`, `x =
breaking.out;`) by spotting assigned values that are nullable under the
optimistic assumption that all known variables are non-nullable.
Then we build a nullability dependency graph in linear time: whenever
we see `x = y;`, we know that "y is nullable" implies "x is nullable".
Finally, we propagate "trivial nullabilities" through that graph: any
variable that wasn't reached is deemed not-nullable.
(this is similar to mark and sweep garbage collection, where the roots
are the "trivially nullable" variables; credits to leafpetersen@ for
linear solution)
John Messerly