Freeze messages during parsing to avoid freeze pass

[osa1]

Currently if I use a message in read-only way I need to first parse it, then do a pass to freeze it.

If we had a way of generating frozen (read-only) messages in parsers that would eliminate the extra pass for freezing.

Implementation should be trivial, but extending the generated message API in a backwards-compatible way will be sub-optimal because we can't mix optional positional and named arguments (dart-lang/language#1076).

If mixing positional and named optionals were allowed we could extend the fromBuffer and other parse methods with an optional named argument {bool freeze = false}. Since this is not possible yet we will need to add a new constructor like fromBufferReadOnly (or fromBufferFrozen, and another one for parsing JSON). With luck these extra constructors will be tree-shaken when not used and won't cause binary size issues in JS and AOT.

I'm looking at internal users to see if freezing right after parsing is a common pattern.

[osa1]

Currently if I use a message in read-only way I need to first parse it, then do a pass to freeze it.

Sometimes the freeze pass is implicit. A related use case I've noticed in the wild is this:

_defaultMessage.rebuild((copy) => copy..clear()..mergeFromBuffer(...))

where _defaultMessage has type T extends GeneratedMessage.

This is a convoluted way of parsing a buffer contents with a generic message type and generating a frozen message.

With something like mergeFromBufferFrozen this would be:

_defaultMessage.createEmptyInstance()..mergeFromBufferFrozen(...)

Which is easier to read because

• rebuild((msg) => msg.clear()) is an indirect way of creating an empty instance
  (Q: Does this assume that empty field set is the same as default message, and if it does, is it really the case?)
• It's difficult to see in the original message that the resulting message would be frozen

In terms of runtime, in addition to avoiding the freeze pass, the first code does a shallow copy that doesn't exist in the second code.

[alanrussian]

This would be useful to my team's project. I CCed you on an internal bug which has more details.