Merge #4246

4246: Validate uses of self and super r=matklad a=djrenren

This change follows on the validation of the `crate` keyword in paths. It verifies the following things:

`super`:
 - May only be preceded by other `super` segments
 - If in a `UseItem` then all semantically preceding paths also consist only of `super`

`self`
 - May only be the start of a path


Just a note, a couple times while working on this I found myself really wanting a Visitor of some sort so that I could traverse descendants while skipping sub-trees that are unimportant. Iterators don't really work for this, so as you can see I reached for recursion. Considering paths are generally small a fancy debounced visitor probably isn't important but figured I'd say something in case we had something like this lying around and I wasn't using it.

Co-authored-by: John Renner <john@jrenner.net>

xtask/src/ast_src.rs

@@ -593,7 +593,7 @@ pub(crate) const AST_SRC: AstSrc = AstSrc {
             qualifier: Path,
         }
         struct PathSegment {
-            T![::], T![crate], T![<], NameRef, TypeArgList, ParamList, RetType, PathType, T![>]
+            T![::], T![crate], T![self], T![super], T![<], NameRef, TypeArgList, ParamList, RetType, PathType, T![>]
         }
         struct TypeArgList {
             T![::],