shelving comments and patch them at the end if needed

[iwusong]

• Start Date: 2022-06-22
• Target Major Version: 3.x
• Reference Issues: singleroot with comments is compiled as fragment on compilerOptions.comments = true
  fix(compiler-core): KeepAlive should ignore comments core#6026 HTML comment breaks automatic attribute inheritance core#5203 Root comment breaking the <Transition> without any warning core#6080 Attribute inheritance inconsistent across production and development core#5546 fix(compiler-core): fix patchFlag wrong on v-if in some cases core#5832

Summary

We should remove comments node in ast or transform, record them on render vnode or instance to add them in patch if needed.

Motivation

Comments should not affect PatchFlag and fall-through attribute inheritance in theory, it caused a lot of problems. and it will have more problems in app.config.compilerOptions.comments, dev_root_fragment, keep-alive, transition...
Even as long as we add a function, we must consider these.
Comments do not need to participate in diff or props update In fact，we can just patch them at the end if needed. This will solve the problem once and for all.

Detailed design

As said above
Remove comments node in ast or transform, record them on render vnode or instance to add them in patch if needed

Drawbacks

There may be involve multiple parts

Adoption strategy

It is not a breaking change.

[leopiccionia]

I can see that a component with one root tag + one or more comments becoming a fragment is confusing.

However, I don't see how silently ignoring comments in certain situations (even if compilerOptions.comment is explicitly set to true) is less confusing.

I think a better solution is:

1. Observing compilerOptions.comments during development/testing (not only in production);
2. Warning during development (if it doesn't already happen) when some built-in components like Transition or KeepAlive receives a fragment as children. Ideally, the warning message should have an actionable;
3. Warning during development if a component with fragment receives attrs not listed in props/emits (and inheritAttrs is not set to false).

I don't know if adding a comment is an idiom for forcing a template to be compiled as a fragment (for whatever reason), but changing it could be a breaking change.

[iwusong]

I think comment should not cause change of attribute inheritance behavior

[iwusong]

Comments may be deleted or added at any time，but PatchFlag shouldn't change

[theoephraim]

Ran into this recently and it was VERY confusing and took a long time to figure out. Please let's fix this!

Also see a related idea for being more explicit about preserving comments here - #469