internal: Add preliminary SyntaxEditor functionality

[DropDemBits]

Related to #15710

Implements a SyntaxEditor interface to abstract over the details of modifying syntax trees, to both simplify creating new code fixes and code actions, as well as start on the path of getting rid of mutable syntax nodes.

SyntaxEditor relies on SyntaxMappingBuilders to feed in the correct information to map AST nodes created by make constructors, as make constructors do not guarantee that node identity is preserved. This is to paper over the fact that make constructors simply re-parse text input instead of building AST nodes from the ground up and re-using the provided syntax nodes.

SyntaxAnnotations are used to find where syntax elements have ended up after edits are applied. This is primarily useful for the add_{placeholder,tabstop} set of methods on SourceChangeBuilder, as that currently relies on the nodes provided being in the final syntax tree.

Eventually, the goal should be to move this into the rowan crate when we move away from mutable syntax nodes, but for now it'll stay in the syntax crate.

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Closes #14921 as SyntaxEditor ensures that all replace changes are disjoint
Closes #9649 by implementing SyntaxAnnotations

[Veykril]

Eventually, the goal should be to move this into the rowan crate when we move away from mutable syntax nodes, but for now it'll stay in the syntax crate.

We should also consider forking rowan in-tree as we will likely start changing things a bunch once we consider dropping mutable syntax trees. (and at some point feed the changes back to main rowan and adopt it into the monorepo)

[Veykril]

This is to paper over the fact that make constructors simply re-parse text input instead of building AST nodes from the ground up and re-using the provided syntax nodes.

Presumably, fixing this would simplify things here? Would be good to check if if thats a requirement we want for our next rowan and whether it has certain requirements on the implementation then

[DropDemBits]

Presumably, fixing this would simplify things here?

Thinking about it more, I think we'll still need SyntaxMapping especially for when we move to immutable syntax trees, since reusing the syntax nodes to preserve node identity that way would imply that we mutate that syntax node.

[DropDemBits]

I believe functionality-wise SyntaxEditor is complete, so marking as ready for review.

[davidbarsky]

Honestly, looks good to me: I'd like to land this and start moving assists over to this.

[davidbarsky]

@bors r+

[bors]

📌 Commit 12c6266 has been approved by davidbarsky

It is now in the queue for this repository.

[bors]

⌛ Testing commit 12c6266 with merge f13c776...

[bors]

☀️ Test successful - checks-actions
Approved by: davidbarsky
Pushing f13c776 to master...