Fix corner case in expr_kind

[Aatch]

It looks like the expression isn't getting an entry in the node_types map, meaning that when it checks the type of the expression in expr_kind to see if it should use RvalueDatumExpr or RvalueDpsExpr, it cant find the type and panics.

As far as I can tell, this only comes up in this case shown in issue #19337, a unit struct on the left-hand side of an assignment expression. The only reason this happens is because a DefStruct is the only case where expr_kind checks the type of the expression, so falling back to RvalueDpsExpr when no type is found should be fine.

Fixes #19337

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

[emberian]

Needs a test (the one from the issue ought to be fine)

[ghost]

Will probably also fix #13407.

[Aatch]

@nikomatsakis hmm, I wasn't entirely comfortable with the fix myself, further investigation shows that this actually compiles:

struct Foo;

fn foo() {
  *&mut Foo = Foo;
}

Doesn't seem like it should, so I'll try to figure out a better fix that covers that case as well.

[Aatch]

Well I figured out why it's not in the table. That table is only updated at the end of the function. Until then, all the types are recorded in the function context, which isn't available to expr_kind.

As far as I can tell, the strange example I posted above is just a weird part of the language. It accepts it because there's no reason not to. It doesn't actually do anything (well, it assigns to a temporary, not exactly broken code...).

I've done some refactoring instead of this change though, now that I've looked at the issue properly. I'll open a new PR for it when I'm done.