JuliaLang / julia / #37954

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Commit Message

Make world-age increments explicit (#56509)

This PR introduces a new, toplevel-only, syntax form `:worldinc` that
semantically represents the effect of raising the current task's world
age to the latest world for the remainder of the current toplevel
evaluation (that context being an entry to `eval` or a module
expression). For detailed motivation on why this is desirable, see
#55145, which I won't repeat here, but the gist is that we never really
defined when world-age increments and worse are inconsistent about it.
This is something we need to figure out now, because the bindings
partition work will make world age even more observable via bindings.

Having created a mechanism for world age increments, the big question is
one of policy, i.e. when should these world age increments be inserted.

Several reasonable options exist:
1. After world-age affecting syntax constructs (as proprosed in #55145)
2. Option 1 + some reasonable additional cases that people rely on
3. Before any top level `call` expression
4. Before any expression at toplevel whatsover

As an example, case, consider `a == a` at toplevel. Depending on the
semantics that could either be the same as in local scope, or each of
the four world age dependent lookups (three binding lookups, one method
lookup) could (potentially) occur in a different world age.

The general tradeoff here is between the risk of exposing the user to
confusing world age errors and our ability to optimize top-level code
(in general, any `:worldinc` statement will require us to fully
pessimize or recompile all following code).

This PR basically implements option 2 with the following semantics:

1. The interpreter explicit raises the world age only at `:worldinc`
exprs or after `:module` exprs.
2. The frontend inserts `:worldinc` after all struct definitions, method
definitions, `using` and `import.
3. The `@eval` macro inserts a worldinc following the call to `eval` if
at toplev... (continued)

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124 existing lines in 8 files now uncovered.

65236 of 75064 relevant lines covered (86.91%)

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