Dart VM Support for NNBD

[aadilmaan]

Here is a list of items that need to get done for this:

Phase 1 will deal with supporting NNBD in weak mode (D28 milestone)

• Build the VM with the forked NNBD SDK Build the VM using the forked NNBD SDK #38663
• Migrate patch files in core libraries and dart:io to be NNBD clean Migrate patch files in core libraries and dart:io library to be NNBD clean #39754
• Late Fields and variables Late fields and variables in VM #38841
• Non-Null Assertions in VM Non-Null Assertions in VM #38840
• Implementation of runtime checks for weak mode support Implementation of NNBD runtime checks for weak mode support #39755
• Migrate dart:ffi library to NNBD [vm/ffi] Migrate to NNBD #39144

Phase 2 will deal with supporting NNBD with strong mode checks

• Implementation of runtime checks for strong mode support Implementation of NNBD strong checking mode in the VM #38845

Phase 3 will deal with NNBD performance/code size issues

Links:
NNBD specification
NNBD in Dart VM
dart-vm-nnbd-tracking

[mkustermann]

The CFE team has started working on non-nullable types and late fields. See the NNBD feature specification for more information. Both of those features will require VM support.

• There is already a DartType.nullability in the Kernel AST (though not serialized in the binary yet).

• The CFE issue Add the 'late' flag to fields and variables #37686 tracks adding the late bit.

@a-siva The VM work could probably start soon on this, even if the CFE is not done.

[johnniwinther]

Serialization of nullability just landed (https://dart-review.googlesource.com/c/sdk/+/112085). The values are still not added to types during parsing, though, so nothing is set yet (CFE issue #37811).

[mkustermann]

@a-siva okrs mention @aartbik / @liamappelbe might be looking at the new language features, is that still the plan?

/cc @leafpetersen

[a-siva]

@kustermann to answer your question above, @crelier is going to start working on the initial phase of this (accounting for non nullable types in the VM type system representation and type checks). A second phase of this would be to use the type information to potentially eliminate runtime null checks which generating code.

[a-siva]

Closing this as we have another Epic #41131 tracking the samething.