Prevent unrelated casts using `as`

[ahoppen]

Currently, SyntaxProtocol has an as method that allows casting to any other syntax node type. Because of this, it’s possible to e.g. cast a FunctionDeclSyntax to an IdentifierExprSyntax without any compiler errors or warnings, even though we know that the cast will always fail.

What we should do instead, is to only have a as(_: ExprSyntaxProtocol.Type) function on ExprSyntax (analogous for StmtSyntax and the other base nodes).

The as function on SyntaxProtocol can then be marked as deprecated and produce a warning that the cast will always fail.

is and cast will need to be updated accordingly.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://114330180

[Matejkob]

What does the term "as well as as(_: SyntaxProtocol.Type)" mean? Does it suggest that every base node should have this method? Would this be equivalent to having the as method directly on SyntaxProtocol?

[ahoppen]

Oh, that was an artifact of a sentence I started writing but then didn’t finish. Ignore it – and I removed it from the description

[grynspan]

If I have a Syntax, how do I then cast it to something else?

[Matejkob]

If I have a Syntax, how do I then cast it to something else?

I believe we should keep this part of the casting API. If you find a while you could review my pull request #2108, which contains a draft proposal on casting constraints?

[ahoppen]

If I have a Syntax, how do I then cast it to something else?

The proposal here is to remove the as methods on all types that are not base nodes, e.g. FunctionDeclSyntax.as. Syntax.as will continue to exist.

[grynspan]

Okay, so a Syntax can still be cast, but an arbitrary some SyntaxProtocol can't be? And if I find myself needing to cast in that case, I can do Syntax(foo).as(...)?

[ahoppen]

@Matejkob also noted that in #2108. The only case where you won’t be able to cast is on anything that’s statically known to be a leaf syntax node type. Casting will still be possible on some SyntaxProtocol, some DeclSyntaxProtocol, any SyntaxProtocol etc.

[grynspan]

Oh, I get it. Sorry, I misunderstood what was being described here. I'll be quiet. :)